text: Despite Ca being the favored daughter nuclide, it is rarely useful dating as calcium is common in the crust, with Ca being the most abundant isotope.  Thus, the amount of calcium originally present is not known with enough accuracy to be able to measure the small increase produced by radioactive decay.

text: John Mitchell Nuttall (21 July 1890 – 28 January 1958) was an English physicist, born in Todmorden. He is best remembered for his work with the physicist Hans Geiger, which resulted in the Geiger–Nuttall law of radioactive decay.

text: John Mitchell Nuttall (21 July 1890 – 28 January 1958) was an English

entity_name: Physicist

text: physicist

text: , born in

entity_name: Todmorden

text: Todmorden

text: . He is best remembered for his work with the physicist

entity_name: Hans Geiger

text: Hans Geiger

text: , which resulted in the

entity_name: Geiger–Nuttall law

text: Geiger–Nuttall law

text: of

entity_name: Radioactive decay

text: radioactive decay

text: Unlike other radiometric dating techniques, those using the uranium decay series (except for those using the stable final isotopes Pb and Pb) compare the ratios of two radioactive unstable isotopes.  This complicates calculations as both the parent and daughter isotopes decay over time into other isotopes.

text: Unlike other radiometric dating techniques, those using the uranium decay series (except for those using the stable final isotopes Pb and Pb) compare the ratios of two

text: radioactive

text: unstable isotopes.  This complicates calculations as both the parent and daughter isotopes decay over time into other isotopes.

text: The decay scheme of a radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships. Examples are shown below.

text: The decay scheme of a

text: radioactive

text: substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships. Examples are shown below.

method: UNH-tfidf-stem

method: UNH-tfidf-ptsim

method: UNH-tfidf-lem

text: The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products, radioactive isotopes such as caesium-137, iodine-131, strontium-90 and other radionuclides, into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

text: The explosion and fire threw hot particles of the

entity_name: Nuclear fuel

text: nuclear fuel

text: and also far more dangerous

entity_name: Nuclear fission product

text: fission product

text: s, radioactive isotopes such as

text: and other

entity_name: Radionuclide

text: radionuclides

text: , into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

method: UNH-ecn

text: According to Jiri Hala's textbook, the radioactivity in the fission product mixture in an atom bomb is mostly caused by short-lived isotopes such as I-131 and Ba-140. After about four months Ce-141, Zr-95/Nb-95, and Sr-89 represent the largest share of radioactive material. After two to three years, Ce-144/Pr-144, Ru-106/Rh-106, and Promethium-147 are the bulk of the radioactivity. After a few years, the radiation is dominated by strontium-90 and caesium-137, whereas in the period between 10,000 and a million years it is technetium-99 that dominates.

text: According to Jiri Hala's textbook, the radioactivity in the fission product mixture in an

entity_name: Nuclear weapon

text: atom bomb

text: is mostly caused by short-lived isotopes such as

text: and

entity_name: Barium

text: Ba

text: -140. After about four months

entity_name: Cerium

text: Ce

entity_name: Zirconium

text: Zr

entity_name: Niobium

text: Nb

text: -95, and

entity_name: Strontium

text: Sr

text: -89 represent the largest share of radioactive material. After two to three years,

text: Ce

entity_name: Praseodymium

text: Pr

entity_name: Ruthenium

text: Ru

entity_name: Rhodium

text: Rh

text: -106, and

entity_name: Isotopes of promethium

text: are the bulk of the radioactivity. After a few years, the radiation is dominated by

text: and

text: , whereas in the period between 10,000 and a million years it is

entity_name: Technetium

text: technetium

text: -99 that dominates.

text: Curium metal is a radionuclide and emits alpa particles upon radioactive decay. Although it has a half life of 34 ms, many curium oxides, including curium sesquioxide, have half lives nearing thousands of years. Curium, in the form of curium sesquioxide, can be inhaled into the body, causing many biological defects. The LD50 of curium is 3 micro-Ci through ingestion and inhalation and 1 micro-Ci through absorption through the skin. In one experiment, rats were introduced to aerosolized particulates of curium(III) oxide. Although the experiment proved that inhaled CmO is half as carcinogenic as compared to inhaled PuO, the rats still suffered from many biological deformities, such as skin lesions, malignant tumors, and lung neoplasms. A small amount of the rat population was able to clear particulate curium sesquioxide from the lungs, suggesting that curium sesquioxide is partially soluble in lung fluid.

text: Curium metal is a

text: radionuclide

text: and emits

entity_name: Alpha particle

text: alpa particles

text: upon radioactive decay. Although it has a half life of 34 ms, many curium oxides, including curium sesquioxide, have half lives nearing thousands of years. Curium, in the form of curium sesquioxide, can be inhaled into the body, causing many biological defects. The LD50 of curium is 3 micro-Ci through ingestion and inhalation and 1 micro-Ci through absorption through the skin. In one experiment, rats were introduced to aerosolized particulates of curium(III) oxide. Although the experiment proved that inhaled CmO is half as carcinogenic as compared to inhaled PuO, the rats still suffered from many biological deformities, such as skin lesions, malignant tumors, and lung neoplasms. A small amount of the rat population was able to clear particulate curium sesquioxide from the lungs, suggesting that curium sesquioxide is partially soluble in lung fluid.

text: On Earth (and elsewhere), trace amounts of various elements continue to be produced from other elements as products of natural transmutation processes. These include some produced by cosmic rays or other nuclear reactions (see cosmogenic and nucleogenic nuclides), and others produced as decay products of long-lived primordial nuclides. For example, trace (but detectable) amounts of carbon-14 (C) are continually produced in the atmosphere by cosmic rays impacting nitrogen atoms, and argon-40 (Ar) is continually produced by the decay of primordially occurring but unstable potassium-40 (K). Also, three primordially occurring but radioactive actinides, thorium, uranium, and plutonium, decay through a series of recurrently produced but unstable radioactive elements such as radium and radon, which are transiently present in any sample of these metals or their ores or compounds. Three other radioactive elements, technetium, promethium, and neptunium, occur only incidentally in natural materials, produced as individual atoms by natural fission of the nuclei of various heavy elements or in other rare nuclear processes.

text: On Earth (and elsewhere), trace amounts of various elements continue to be produced from other elements as products of natural

entity_name: Nuclear transmutation

text: transmutation

text: processes. These include some produced by

entity_name: Cosmic ray

text: cosmic ray

text: s or other nuclear reactions (see

entity_name: Cosmogenic nuclide

text: cosmogenic

text: and

entity_name: Nucleogenic

text: nucleogenic

text: nuclides), and others produced as

entity_name: Decay product

text: decay product

text: s of long-lived

entity_name: Primordial nuclide

text: primordial nuclide

text: s. For example, trace (but detectable) amounts of

text: (C) are continually produced in the atmosphere by cosmic rays impacting

entity_name: Nitrogen

text: nitrogen

text: atoms, and argon-40 (Ar) is continually produced by the decay of primordially occurring but unstable potassium-40 (K). Also, three primordially occurring but radioactive

entity_name: Actinide

text: actinide

text: s,

entity_name: Thorium

text: thorium

entity_name: Uranium

text: uranium

text: , and

entity_name: Plutonium

text: plutonium

text: , decay through a series of recurrently produced but unstable radioactive elements such as radium and

entity_name: Radon

text: radon

text: , which are transiently present in any sample of these metals or their ores or compounds. Three other radioactive elements,

text: technetium

entity_name: Promethium

text: promethium

text: , and

entity_name: Neptunium

text: neptunium

text: , occur only incidentally in natural materials, produced as individual atoms by natural

entity_name: Nuclear fission

text: fission

text: of the

entity_name: Atomic nucleus

text: nuclei

text: of various heavy elements or in other rare nuclear processes.

text: In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate (e.g., due to decay of a parent isotope) is equal to its decay rate.

text: In

entity_name: Nuclear physics

text: nuclear physics

text: , secular equilibrium is a situation in which the quantity of a

text: radioactive

entity_name: Isotope

text: isotope

text: remains constant because its production rate (e.g., due to decay of a parent isotope) is equal to its decay rate.

text: Under the Tri-Party Agreement, lower-level hazardous wastes are buried in huge lined pits that will be sealed and monitored with sophisticated instruments for many years. Disposal of plutonium and other high-level wastes is a more difficult problem that continues to be a subject of intense debate. As an example, plutonium-239 has a half-life of 24,100 years, and a decay of ten half-lives is required before a sample is considered to cease its radioactivity. In 2000, the Department of Energy awarded a $4.3 billion contract to Bechtel, a San Francisco-based construction and engineering firm, to build a vitrification plant to combine the dangerous wastes with glass to render them stable. Construction began in 2002. The plant was originally scheduled to be operational by 2011, with vitrification completed by 2028. , according to a study by the General Accounting Office, there were a number of serious unresolved technical and managerial problems.  estimated costs were $13.4 billion with commencement of operations estimated to be in 2022 and about 3 decades of operation.

text: Under the Tri-Party Agreement, lower-level hazardous wastes are buried in huge lined pits that will be sealed and monitored with sophisticated instruments for many years. Disposal of plutonium and other high-level wastes is a more difficult problem that continues to be a subject of intense debate. As an example, plutonium-239 has a

entity_name: Half-life

text: half-life

text: of 24,100 years, and a decay of ten half-lives is required before a sample is considered to cease its radioactivity. In 2000, the Department of Energy awarded a $4.3 billion contract to

entity_name: Bechtel

text: Bechtel

text: , a San Francisco-based construction and engineering firm, to build a

entity_name: Radioactive waste

entity_section: Vitrification

text: vitrification

text: plant to combine the dangerous wastes with glass to render them stable. Construction began in 2002. The plant was originally scheduled to be operational by 2011, with vitrification completed by 2028. , according to a study by the

entity_name: Government Accountability Office

text: General Accounting Office

text: , there were a number of serious unresolved technical and managerial problems.  estimated costs were $13.4 billion with commencement of operations estimated to be in 2022 and about 3 decades of operation.

text: Radon has a short half-life (3.8 days) and decays into other solid particulate radium-series radioactive nuclides.Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.If the gas is inhaled, the radon atoms decay in the airways or the lungs, resulting in radioactive polonium and ultimately lead atoms attaching to the nearest tissue. If dust or aerosol is inhaled that already carries radon decay products, the deposition pattern of the decay products in the respiratory tract depends on the behaviour of the particles in the lungs. Smaller diameter particles diffuse further into the respiratory system, whereas the larger — tens to hundreds of micron-sized — particles often deposit higher in the airways and are cleared by the body's mucociliary staircase. Deposited radioactive atoms or dust or aerosol particles continue to decay, causing continued exposure by emitting energetic alpha radiation with some associated gamma radiation too, that can damage vital molecules in lung cells,by either creating free radicals or causing DNA breaks or damage,perhaps causing mutations that sometimes turn cancerous. In addition, through ingestion and blood transport, following crossing of the lung membrane by radon, radioactive progeny may also be transported to other parts of the body.

text: Radon has a short half-life (3.8 days) and decays into other solid particulate

entity_name: Decay chain

entity_section: Radium series

text: radium-series

text: radioactive nuclides.Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.If the gas is inhaled, the radon atoms decay in the airways or the lungs, resulting in radioactive polonium and ultimately lead atoms attaching to the nearest tissue. If dust or aerosol is inhaled that already carries radon decay products, the deposition pattern of the decay products in the respiratory tract depends on the behaviour of the particles in the lungs. Smaller diameter particles diffuse further into the respiratory system, whereas the larger — tens to hundreds of micron-sized — particles often deposit higher in the airways and are cleared by the body's mucociliary staircase. Deposited radioactive atoms or dust or aerosol particles continue to decay, causing continued exposure by emitting energetic

entity_name: Alpha decay

text: alpha radiation

text: with some associated gamma radiation too, that can damage vital molecules in lung cells,by either creating

entity_name: Radical (chemistry)

text: free radical

text: s or causing

entity_name: DNA

text: DNA

text: breaks or damage,perhaps causing mutations that sometimes turn cancerous. In addition, through ingestion and blood transport, following crossing of the lung membrane by radon, radioactive progeny may also be transported to other parts of the body.

text: Naturally occurring cadmium (Cd) is composed of 8 isotopes. For two of them, natural radioactivity was observed, and three others are predicted to be radioactive but their decays were never observed, due to extremely long half-life times. The two natural radioactive isotopes are Cd (beta decay, half-life is 8.04 × 10 years) and Cd (two-neutrino double beta decay, half-life is 2.8 × 10 years). The other three are Cd, Cd (double electron capture), and Cd (double beta decay); only lower limits on their half-life times have been set. At least three isotopes—Cd, Cd, and Cd—are absolutely stable (except, theoretically, to spontaneous fission). Among the isotopes absent in the natural cadmium, the most long-lived are Cd with a half-life of 462.6 days, and Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives that are less than 2.5 hours and the majority of these have half-lives that are less than 5 minutes. This element also has 8 known meta states with the most stable being Cd (t 14.1 years), Cd (t 44.6 days) and Cd (t 3.36 hours).

text: Naturally occurring

entity_name: Cadmium

text: cadmium

text: (Cd) is composed of 8

text: isotope

text: s. For two of them, natural

text: radioactivity

text: was observed, and three others are predicted to be radioactive but their decays were never observed, due to extremely long

text: half-life

text: times. The two natural radioactive isotopes are Cd (

entity_name: Beta decay

text: beta decay

text: , half-life is 8.04 × 10 years) and Cd (two-neutrino

entity_name: Double beta decay

text: double beta decay

text: , half-life is 2.8 × 10 years). The other three are Cd, Cd (

entity_name: Double electron capture

text: double electron capture

text: ), and Cd (double beta decay); only lower limits on their half-life times have been set. At least three isotopes—Cd, Cd, and Cd—are absolutely stable (except, theoretically, to

entity_name: Spontaneous fission

text: spontaneous fission

text: ). Among the isotopes absent in the natural cadmium, the most long-lived are Cd with a half-life of 462.6 days, and Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives that are less than 2.5 hours and the majority of these have half-lives that are less than 5 minutes. This element also has 8 known

entity_name: Nuclear isomer

text: meta state

text: s with the most stable being Cd (t 14.1 years), Cd (t 44.6 days) and Cd (t 3.36 hours).

text: For atoms with low atomic numbers, a nucleus that has more neutrons than protons tends to drop to a lower energy state through radioactive decay so that the neutron–proton ratio is closer to one. However, as the atomic number increases, a higher proportion of neutrons is required to offset the mutual repulsion of the protons. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number Z = 20 (calcium) and as Z increases, the neutron–proton ratio of stable isotopes increases. The stable isotope with the highest proton–neutron ratio is lead-208 (about 1.5).

text: For atoms with low atomic numbers, a nucleus that has more neutrons than protons tends to drop to a lower energy state through radioactive decay so that the

entity_name: Neutron–proton ratio

text: neutron–proton ratio

text: is closer to one. However, as the atomic number increases, a higher proportion of neutrons is required to offset the mutual repulsion of the protons. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number Z = 20 (calcium) and as Z increases, the neutron–proton ratio of stable isotopes increases. The stable isotope with the highest proton–neutron ratio is

entity_name: Isotopes of lead

text: (about 1.5).

text: Th decays very slowly (its half-life is comparable to the age of the universe) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than Th, though on a mass basis they are negligible.

text: Th decays very slowly (its

text: half-life

text: is comparable to the age of the universe) but other thorium

text: isotope

text: s occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than Th, though on a mass basis they are negligible.

text: The next group is the primordial radioactive nuclides. These have been measured to be radioactive, or decay products have been identified (Te-130, Ba-130). There are (currently) 32 of these, of which 28 have half-lives considerably longer than the age of the universe (13.8 billion years). With most of these 28, decay is difficult to observe and for most purposes they can be regarded as effectively stable. Bismuth-209 is notable as the only natural occurring isotope of an element long considered stable. A further 4 nuclides, thorium-232, uranium-238, potassium-40 and uranium-235 have half lives between 14 billion and 700 million years, which means they have experienced measurable decay since the formation of the solar system about 4.6 billion years ago, but still exist on earth in significant quantities. They are the primary source of radiogenic heat and radioactive decay products. Together, there are a total of 286 primordial nuclides.

text: The next group is the primordial radioactive nuclides. These have been measured to be radioactive, or decay products have been identified (Te-130, Ba-130). There are (currently) 32 of these, of which 28 have half-lives considerably longer than the age of the universe (13.8 billion years). With most of these 28, decay is difficult to observe and for most purposes they can be regarded as effectively stable.

text: is notable as the only natural occurring isotope of an element long considered stable. A further 4 nuclides,

entity_name: Isotopes of thorium

text: and

text: have half lives between 14 billion and 700 million years, which means they have experienced measurable decay since the formation of the solar system about 4.6 billion years ago, but still exist on earth in significant quantities. They are the primary source of

entity_name: Radiogenic nuclide

entity_section: Radiogenic heating

text: radiogenic heat

text: and radioactive decay products. Together, there are a total of 286

text: primordial nuclides

method: UvABottomUpChangeOrder

text: A typical simple example is the linear differential equation used to model radioactive decay. Let N(t) denote the number of radioactive atoms remaining in some sample of material  at time t. Then for some constant k > 0, the rate at which the radioactive atoms decay can be modelled by

text: Though Tono-Bungay is not a science-fiction novel, radioactive decay plays a small but consequential role in it. Radioactive decay plays a much larger role in The World Set Free (1914). This book contains what is surely his biggest prophetic "hit". Scientists of the day were well aware that the natural decay of radium releases energy at a slow rate over thousands of years. The rate of release is too slow to have practical utility, but the total amount released is huge. Wells's novel revolves around an (unspecified) invention that accelerates the process of radioactive decay, producing bombs that explode with no more than the force of ordinary high explosives—but which "continue to explode" for days on end. "Nothing could have been more obvious to the people of the earlier twentieth century", he wrote, "than the rapidity with which war was becoming impossible ... [but] they did not see it until the atomic bombs burst in their fumbling hands". In 1932, the physicist and conceiver of nuclear chain reaction Leó Szilárd read The World Set Free, a book which he said made a great impression on him.

text: Though Tono-Bungay is not a science-fiction novel, radioactive decay plays a small but consequential role in it. Radioactive decay plays a much larger role in

text: The World Set Free

text: (1914). This book contains what is surely his biggest prophetic "hit". Scientists of the day were well aware that the natural decay of

text: radium

text: releases energy at a slow rate over thousands of years. The rate of release is too slow to have practical utility, but the total amount released is huge. Wells's novel revolves around an (unspecified) invention that accelerates the process of radioactive decay, producing bombs that explode with no more than the force of ordinary high explosives—but which "continue to explode" for days on end. "Nothing could have been more obvious to the people of the earlier twentieth century", he wrote, "than the rapidity with which war was becoming impossible ... [but] they did not see it until the atomic bombs burst in their fumbling hands". In 1932, the physicist and conceiver of

text: nuclear chain reaction

text: Leó Szilárd

text: read The World Set Free, a book which he said made a great impression on him.

text: Radioactive cosmogenic Be is produced in the atmosphere of the Earth by the cosmic ray spallation of oxygen. Be accumulates at the soil surface, where its relatively long half-life (1.36 million years) permits a long residence time before decaying to boron-10. Thus, Be and its daughter products are used to examine natural soil erosion, soil formation and the development of lateritic soils, and as a proxy for measurement of the variations in solar activity and the age of ice cores. The production of Be is inversely proportional to solar activity, because increased solar wind during periods of high solar activity decreases the flux of galactic cosmic rays that reach the Earth. Nuclear explosions also form Be by the reaction of fast neutrons with C in the carbon dioxide in air. This is one of the indicators of past activity at nuclear weapon test sites.The isotope Be (half-life 53 days) is also cosmogenic, and shows an atmospheric abundance linked to sunspots, much like Be.

text: Radioactive

text: cosmogenic

text: Be is produced in the

text: atmosphere of the Earth

text: by the

text: cosmic ray spallation

text: of

text: oxygen

text: . Be accumulates at the

text: soil

text: surface, where its relatively long

text: half-life

text: (1.36 million years) permits a long

text: residence time

text: before decaying to

text: boron

text: -10. Thus, Be and its daughter products are used to examine natural

text: soil erosion

text: soil formation

text: and the development of

text: lateritic soils

text: , and as a

text: proxy

text: for measurement of the variations in

text: solar activity

text: and the age of

text: ice core

text: s. The production of Be is inversely proportional to solar activity, because increased

text: solar wind

text: during periods of high solar activity decreases the flux of

text: galactic cosmic ray

text: s that reach the Earth. Nuclear explosions also form Be by the reaction of fast neutrons with C in the carbon dioxide in air. This is one of the indicators of past activity at

text: nuclear weapon test

text: sites.The isotope Be (half-life 53 days) is also cosmogenic, and shows an atmospheric abundance linked to sunspots, much like Be.

text: By their chemical nature, rock minerals contain certain elements and not others; but in rocks containing radioactive isotopes, the process of radioactive decay generates exotic elements over time. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are argon from decay of potassium-40, and lead from decay of uranium and thorium. If the rock becomes molten, as happens in Earth's mantle, such nonradioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.

text: By their chemical nature,

text: rock

text: mineral

text: s contain certain

text: elements

text: and not others; but in rocks containing radioactive isotopes, the process of

text: radioactive decay

text: generates exotic elements over time. By measuring the

text: concentration

text: of the stable end product of the decay, coupled with knowledge of the

text: half life

text: and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are

text: argon

text: from decay of

text: potassium

text: -40, and

text: lead

text: from decay of

text: uranium

text: and

text: thorium

text: . If the rock becomes molten, as happens in Earth's

text: mantle

text: , such nonradioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.

text: Reports as spectacular as the above raise a lot of doubt. In March 2006, Nature published a special report that called into question the validity of the results of the Purdue experiments. The report quotes Brian Naranjo of the University of California, Los Angeles to the effect that neutron energy spectrum reported in the 2006 paper by Taleyarkhan, et al.  was statistically inconsistent with neutrons produced by the proposed fusion reaction and instead highly consistent with neutrons produced by the radioactive decay of Californium 252, an isotope commonly used as a laboratory neutron source .

text: Reports as spectacular as the above raise a lot of doubt. In March 2006,

text: Nature

text: published a special report that called into question the validity of the results of the Purdue experiments. The report quotes Brian Naranjo of the

text: University of California, Los Angeles

text: to the effect that neutron energy spectrum reported in the 2006 paper by Taleyarkhan, et al.  was statistically inconsistent with neutrons produced by the proposed fusion reaction and instead highly consistent with neutrons produced by the radioactive decay of

text: Californium 252

text: , an isotope commonly used as a laboratory

text: neutron source

text: Although the transitions are nominally forbidden, there is a small probability of their spontaneous occurrence, should an atomic nucleus, atom or molecule be raised to an excited state. More precisely, there is a certain probability that such an excited entity will make a forbidden transition to a lower energy state per unit time; by definition, this probability is much lower than that for any transition permitted or allowed by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so before any transition occurs via a forbidden route. Nevertheless, most forbidden transitions are only relatively unlikely: states that can only decay in this way (so-called meta-stable states) usually have lifetimes on the order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions. In some radioactive decay systems, multiple levels of forbiddenness can stretch life times by many orders of magnitude for each additional unit by which the system changes beyond what is most allowed under the selection rules. Such excited states can last years, or even for many billions of years (too long to have been measured).

text: Although the transitions are nominally forbidden, there is a small probability of their spontaneous occurrence, should an

text: atomic nucleus

text: atom

text: or

text: molecule

text: be raised to an excited state. More precisely, there is a certain probability that such an excited entity will make a forbidden transition to a lower energy state per unit time; by definition, this probability is much lower than that for any transition permitted or allowed by the selection rules. Therefore, if a state can de-excite via a permitted transition (or otherwise, e.g. via collisions) it will almost certainly do so before any transition occurs via a forbidden route. Nevertheless, most forbidden transitions are only relatively unlikely: states that can only decay in this way (so-called

text: meta-stable

text: states) usually have lifetimes on the order milliseconds to seconds, compared to less than a microsecond for decay via permitted transitions. In some radioactive decay systems, multiple levels of forbiddenness can stretch life times by many orders of magnitude for each additional unit by which the system changes beyond what is most allowed under the selection rules. Such excited states can last years, or even for many billions of years (too long to have been measured).

text: Naturally occurring chromium (Cr) is composed of four stable isotopes; Cr, Cr, Cr, and Cr with Cr being the most abundant (83.789% natural abundance). Cr is suspected of decaying by ββ to Ti with a half-life of (more than) 1.8x10 years. Twenty-two radioisotopes, all of which are entirely synthetic, have been characterized with the most stable being Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute, the least stable being Cr with a half-life of 10 milliseconds. This element also has 2 meta states, Cr, the more stable one, and Cr, the least stable isotope or isomer.

text: Naturally occurring

text: chromium

text: (Cr) is composed of four stable

text: isotope

text: s; Cr, Cr, Cr, and Cr with Cr being the most abundant (83.789%

text: natural abundance

text: ). Cr is suspected of decaying by

text: ββ

text: to Ti with a

text: half-life

text: of (more than) 1.8x10 years. Twenty-two

text: radioisotope

text: s, all of which are entirely synthetic, have been characterized with the most stable being Cr with a half-life of 27.7 days. All of the remaining

text: radioactive

text: isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute, the least stable being Cr with a half-life of 10 milliseconds. This element also has 2

text: meta state

text: s, Cr, the more stable one, and Cr, the least stable isotope or isomer.

text: All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including alpha decay (emission of alpha particles) and beta decay (electron emission, positron emission, or electron capture). Another possibility is spontaneous fission into two or more nuclides.

text: All ordinary

text: matter

text: is made up of combinations of

text: chemical element

text: s, each with its own

text: atomic number

text: , indicating the number of

text: proton

text: s in the

text: atomic nucleus

text: . Additionally, elements may exist in different

text: isotope

text: s, with each isotope of an element differing in the number of

text: neutron

text: s in the nucleus. A particular isotope of a particular element is called a

text: nuclide

text: . Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo

text: radioactive decay

text: and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including

text: alpha decay

text: (emission of

text: alpha particle

text: s) and

text: beta decay

text: electron

text: emission,

text: positron

text: emission, or

text: electron capture

text: ). Another possibility is

text: spontaneous fission

text: into two or more nuclides.

text: A radioactive tracer, or radioactive label, is a chemical compound in which one or more atoms have been replaced by a radioisotope so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling is thus the radioactive form of isotopic labeling.

text: A radioactive tracer, or radioactive label, is a

text: chemical compound

text: in which one or more atoms have been replaced by a

text: radioisotope

text: so by virtue of its

text: radioactive decay

text: it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products. Radiolabeling is thus the radioactive form of

text: isotopic labeling

text: Electronic devices decay in a way that is mathematically equivalent to radioactive decay processes for unstable atoms.

text: Electronic devices decay in a way that is mathematically equivalent to

text: radioactive decay

text: processes for unstable atoms.

text: Neutrons in unstable nuclei can decay by beta decay as described above. In this case, an energetically allowed quantum state is available for the proton resulting from the decay. One example of this decay is carbon-14 (6 protons, 8 neutrons) that decays to nitrogen-14 (7 protons, 7 neutrons) with a half-life of about 5,730 years.

text: Neutrons in unstable nuclei can decay by

text: beta decay

text: as described above. In this case, an energetically allowed quantum state is available for the proton resulting from the decay. One example of this decay is

text: (6 protons, 8 neutrons) that decays to

text: (7 protons, 7 neutrons) with a half-life of about 5,730 years.

text: Atoms of the same element have the same number of protons, called the atomic number. Within a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine the nuclide. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay.

text: Atoms of the same

text: element

text: have the same number of protons, called the

text: atomic number

text: . Within a single element, the number of neutrons may vary, determining the

text: isotope

text: of that element. The total number of protons and neutrons determine the

text: nuclide

text: . The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing

text: radioactive decay

text: In 1868, Janssen and Lockyer independently observed a yellow line in the solar spectrum that did not match that of any other element. In 1895, in each case at around the same time, Ramsay, Cleve, and Langlet independently observed helium trapped in cleveite. André-Marie Ampère predicted an element analogous to chlorine obtainable from hydrofluoric acid, and between 1812 and 1886 many researchers tried to obtain it. Fluorine was eventually isolated by Moissan, in 1886. In 1894, Lord Rayleigh and Ramsay discovered argon by comparing the molecular weights of nitrogen prepared by liquefaction from air and nitrogen prepared by chemical means. It was the first noble gas to be isolated. In 1898, within a period of three weeks, Ramsay and Travers successively separated krypton, neon and xenon from liquid argon by their differences in boiling points. In 1898, Dorn discovered a radioactive gas resulting from the radioactive decay of radium; Ramsay and Robert Whytlaw-Gray subsequently isolated radon in 1910.

text: In 1868,

text: Janssen

text: and

text: Lockyer

text: independently observed a yellow line in the solar spectrum that did not match that of any other element. In 1895, in each case at around the same time, Ramsay, Cleve, and Langlet independently observed

text: helium

text: trapped in

text: cleveite

text: André-Marie Ampère

text: predicted an element analogous to chlorine obtainable from

text: hydrofluoric acid

text: , and between 1812 and 1886 many researchers tried to obtain it. Fluorine was eventually isolated by

text: Moissan

text: , in 1886. In 1894,

text: Lord Rayleigh

text: and

text: Ramsay

text: discovered argon by comparing the molecular weights of nitrogen prepared by liquefaction from air and nitrogen prepared by chemical means. It was the first noble gas to be isolated. In 1898, within a period of three weeks, Ramsay and Travers successively separated krypton, neon and xenon from liquid argon by their differences in boiling points. In 1898,

text: Dorn

text: discovered a radioactive gas resulting from the radioactive decay of radium; Ramsay and

text: Robert Whytlaw-Gray

text: subsequently isolated radon in 1910.

text: In nuclear physics, a decay product (also known as a daughter product, daughter isotope or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps (decay chain).  For example, U decays to Th which decays to Pa which decays, and so on, to Pb (which is stable):

text: In

text: nuclear physics

text: , a decay product (also known as a daughter product, daughter isotope or daughter nuclide) is the remaining

text: nuclide

text: left over from

text: radioactive decay

text: . Radioactive decay often proceeds via a sequence of steps (

text: decay chain

text: ).  For example, U decays to Th which decays to Pa which decays, and so on, to Pb (which is stable):

text: For visualization purposes, the nucleic acid fragment is usually labelled with a radioactive, fluorescent or biotin label. Standard ethidium bromide staining is less sensitive than these methods and can lack the sensitivity to detect the nucleic acid if small amounts of nucleic acid or single-stranded nucleic acid(s) are used in these experiments. When using a biotin label, streptavidin conjugated to an enzyme such as horseradish peroxidase is used to detect the DNA fragment (Non-radioactive EMSA review). While isotopic DNA labeling has little or no effect on protein binding affinity, use of non-isotopic labels including flurophores or biotin can alter the affinity and/or stoichiometry of the protein interaction of interest. Competition between fluorophore- or biotin-labeled probe and unlabeled DNA of the same sequence can be used to determine whether the label alters binding affinity or stoichiometry.

text: For visualization purposes, the nucleic acid fragment is usually labelled with a

text: radioactive

text: fluorescent

text: or

text: biotin

text: label. Standard

text: ethidium bromide

text: staining is less sensitive than these methods and can lack the sensitivity to detect the nucleic acid if small amounts of nucleic acid or single-stranded nucleic acid(s) are used in these experiments. When using a biotin label,

text: streptavidin

text: conjugated to an enzyme such as horseradish peroxidase is used to detect the DNA fragment (Non-radioactive EMSA review). While isotopic DNA labeling has little or no effect on protein binding affinity, use of non-isotopic labels including flurophores or biotin can alter the affinity and/or stoichiometry of the protein interaction of interest. Competition between fluorophore- or biotin-labeled probe and unlabeled DNA of the same sequence can be used to determine whether the label alters binding affinity or stoichiometry.

text: For each of the 80 stable elements, the number of the stable isotopes is given. Only 90 isotopes are expected to be perfectly stable, and an additional 164 are energetically unstable, but have never been observed to decay. Thus, 254 isotopes (nuclides) are stable by definition (including tantalum-180m, for which no decay has yet been observed). Those that may in the future be found to be radioactive, are expected to have half-lives longer than 10 years (for example, xenon-134).

text: For each of the 80 stable elements, the number of the stable isotopes is given. Only 90 isotopes are expected to be perfectly stable, and an additional 164 are energetically unstable, but have never been observed to decay. Thus, 254 isotopes (

text: nuclide

text: s) are

text: stable

text: by definition (including tantalum-180m, for which no decay has yet been observed). Those that may in the future be found to be radioactive, are expected to have half-lives longer than 10 years (for example, xenon-134).

text: It is sometimes noted that the time of the decay of the nucleus cannot be controlled, and that the finite half-life invalidates the result.  This objection can be dispelled by sizing the hemispheres appropriately with regards to the half-life of the nucleus. The radii are chosen so that the more distant hemisphere is much farther away than the half-life of the decaying nucleus, times the flight-time of the alpha ray.

text: It is sometimes noted that the time of the decay of the nucleus cannot be controlled, and that the finite

text: half-life

text: invalidates the result.  This objection can be dispelled by sizing the hemispheres appropriately with regards to the half-life of the nucleus. The radii are chosen so that the more distant hemisphere is much farther away than the half-life of the decaying nucleus, times the flight-time of the alpha ray.

text: With the Very Large Telescope, astronomers have made an independent determination of the age of the universe and shed new light on the earliest stages of the Milky Way. For the first time, they measured the amount of the radioactive isotope uranium-238 in a star born when the Milky Way was still forming.

text: With the Very Large Telescope, astronomers have made an independent determination of the age of the universe and shed new light on the earliest stages of the Milky Way. For the first time, they measured the amount of the

text: radioactive isotope

text: uranium-238 in a star born when the Milky Way was still forming.

text: At first, it seemed as though the new radiation was similar to the then recently discovered X-rays. Further research by Becquerel, Ernest Rutherford, Paul Villard, Pierre Curie, Marie Curie, and others showed that this form of radioactivity was significantly more complicated. Rutherford was the first to realize that all such elements decay in accordance with the same mathematical exponential formula. Rutherford and his student Frederick Soddy were the first to realize that many decay processes resulted in the transmutation of one element to another. Subsequently, the radioactive displacement law of Fajans and Soddy was formulated to describe the products of alpha and beta decay.

text: At first, it seemed as though the new radiation was similar to the then recently discovered

text: X-rays

text: . Further research by Becquerel,

text: Ernest Rutherford

text: Paul Villard

text: Pierre Curie

text: Marie Curie

text: , and others showed that this form of radioactivity was significantly more complicated. Rutherford was the first to realize that all such elements decay in accordance with the same mathematical exponential formula. Rutherford and his student

text: Frederick Soddy

text: were the first to realize that many decay processes resulted in the

text: transmutation

text: of one element to another. Subsequently, the

entity_name: Radioactive displacement law of Fajans and Soddy

text: radioactive displacement law of Fajans and Soddy

text: was formulated to describe the products of

text: alpha

text: and

text: beta decay

text: Osmium has seven naturally occurring isotopes, six of which are stable: , , , , , and (most abundant) .  undergoes alpha decay with such a long half-life (2.0±1.1) years, approximately 140 000 times the age of the universe, that for practical purposes it can be considered stable. Alpha decay is predicted for all seven naturally occurring isotopes, but it has been observed only for , presumably due to very long half-lives. It is predicted that  and  can undergo double beta decay but this radioactivity has not been observed yet.

text: Osmium has seven naturally occurring

text: isotope

text: s, six of which are stable: , , , , , and (most abundant) .  undergoes

text: alpha decay

text: with such a long

text: half-life

text: (2.0±1.1) years, approximately 140 000 times the

text: age of the universe

text: , that for practical purposes it can be considered stable. Alpha decay is predicted for all seven naturally occurring isotopes, but it has been observed only for , presumably due to very long half-lives. It is predicted that  and  can undergo

text: double beta decay

text: but this radioactivity has not been observed yet.

text: Th is a radioactive isotope of thorium that decays by alpha emission with a half-life of 7340 years.Th is produced by the decay of uranium-233, and its principal use is for the production of the medical isotopes actinium-225 and bismuth-213.

text: Th is a

text: radioactive

text: isotope

text: of

text: thorium

text: that decays by

text: alpha

text: emission with a

text: half-life

text: of 7340 years.Th is produced by the decay of

text: , and its principal use is for the production of the

text: medical isotopes

text: and

text: If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. Dating of different minerals and/or isotope systems (with differing closure temperatures) within the same rock can therefore enable the tracking of the thermal history of the rock in question with time, and thus the history of metamorphic events may become known in detail. This field is known as thermochronology or thermochronometry.

text: If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through

text: diffusion

text: , setting the isotopic "clock" to zero. The temperature at which this happens is known as the

text: closure temperature

text: or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by

text: artificially resetting sample minerals

text: using a high-temperature furnace. As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. Dating of different minerals and/or isotope systems (with differing closure temperatures) within the same rock can therefore enable the tracking of the thermal history of the rock in question with time, and thus the history of metamorphic events may become known in detail. This field is known as

text: thermochronology

text: or thermochronometry.

text: It is known that the isotope responsible for the majority of the gamma exposure in fuel reprocessing plants (and the Chernobyl site in 2005) is Cs-137. Iodine-129 is one of the major radioactive elements released from reprocessing plants. In nuclear reactors both Cs-137 and strontium-90 are found in locations remote from the fuel.  This is because these isotopes are formed by the beta decay of noble gases (xenon-137 {halflife of 3.8 minutes} and krypton-90 {halflife 32 seconds}) which enable these isotopes to be deposited in locations remote from the fuel (e.g. on control rods).

text: It is known that the isotope responsible for the majority of the gamma exposure in

text: fuel reprocessing

text: plants (and the Chernobyl site in 2005) is

text: is one of the major radioactive elements released from reprocessing plants. In nuclear reactors both Cs-137 and

text: are found in locations remote from the

text: fuel

text: .  This is because these isotopes are formed by the

text: beta decay

text: of

text: noble gas

text: es (

text: {halflife of 3.8 minutes} and

text: {halflife 32 seconds}) which enable these isotopes to be deposited in locations remote from the fuel (e.g. on

text: control rod

text: s).

text: All of the alkaline earth metals except magnesium and strontium have at least one naturally occurring radioisotope: beryllium-7, beryllium-10, and calcium-41 are trace radioisotopes, calcium-48 and barium-130 have very long half-lives and, thus, occur naturally, and all isotopes of radium are radioactive. Calcium-48 is the lightest nuclide to undergo double beta decay.

text: All of the alkaline earth metals except magnesium and strontium have at least one naturally occurring

text: radioisotope

text: , and

text: are

text: trace radioisotope

text: s,

text: and

text: have very long

text: half-lives

text: and, thus, occur naturally, and all

text: isotopes of radium

text: are

text: radioactive

text: . Calcium-48 is the lightest nuclide to undergo

text: double beta decay

text: There are about 254 nuclides in nature that have never been observed to decay. They occur among the 80 different elements that have one or more stable isotopes. See stable isotope and primordial nuclide. Unstable nuclides are radioactive and are called radionuclides. Their decay products ('daughter' products) are called radiogenic nuclides. About 254 stable and about 85 unstable (radioactive) nuclides exist naturally on Earth, for a total of about 339 naturally occurring nuclides on Earth.

text: There are about 254 nuclides in nature that have never been observed to decay. They occur among the 80 different elements that have one or more stable isotopes. See

text: stable isotope

text: and

text: primordial nuclide

text: . Unstable nuclides are

text: radioactive

text: and are called

text: radionuclide

text: s. Their

text: decay product

text: s ('daughter' products) are called

text: radiogenic nuclide

text: s. About 254 stable and about 85 unstable (radioactive) nuclides exist naturally on Earth, for a total of about 339 naturally occurring nuclides on Earth.

text: In the early history of the Solar System, radioactive isotopes having a half-life on the order of a few million years (such as aluminium-26 and iron-60) were sufficiently abundant to produce enough heat to cause internal melting of some moons and even some asteroids, such as Vesta noted above.  After these radioactive isotopes had decayed to insignificant levels, the heat generated by longer-lived radioactive isotopes (such as potassium-40, thorium-232, and uranium-235 and uranium-238) was insufficient to keep these bodies molten unless they had an alternative source of internal heating, such as tidal heating.  Thus, Earth's Moon, which has no alternative source of internal heating is now geologically dead, whereas a moon as small as Enceladus that has sufficient tidal heating (or at least had it recently) and some remaining radioactive heating, is able to maintain an active and directly detectable cryovolcanism.

text: In the early history of the Solar System,

text: radioactive isotope

text: s having a

text: half-life

text: on the order of a few million years (such as

text: and

text: ) were sufficiently abundant to produce enough heat to cause internal melting of some moons and even some asteroids, such as Vesta noted above.  After these radioactive isotopes had decayed to insignificant levels, the heat generated by longer-lived radioactive isotopes (such as

text: , and

text: uranium-235 and uranium-238

text: ) was insufficient to keep these bodies molten unless they had an alternative source of internal heating, such as tidal heating.  Thus, Earth's

text: Moon

text: , which has no alternative source of internal heating is now geologically dead, whereas a moon as small as

text: Enceladus

text: that has sufficient tidal heating (or at least had it recently) and some remaining radioactive heating, is able to maintain an active and directly detectable

text: cryovolcanism

text: As an example, the radioactive decay of carbon-14 is exponential with a half-life of 5,730 years. A quantity of carbon-14 will decay to half of its original amount (on average) after 5,730 years, regardless of how big or small the original quantity was. After another 5,730 years, one-quarter of the original will remain. On the other hand, the time it will take a puddle to half-evaporate depends on how deep the puddle is. Perhaps a puddle of a certain size will evaporate down to half its original volume in one day. But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that. This is an example where the half-life reduces as time goes on. (In other non-exponential decays, it can increase instead.)

text: As an example, the radioactive decay of

text: is exponential with a half-life of 5,730 years. A quantity of carbon-14 will decay to half of its original amount (

text: on average

text: ) after 5,730 years, regardless of how big or small the original quantity was. After another 5,730 years, one-quarter of the original will remain. On the other hand, the time it will take a puddle to half-evaporate depends on how deep the puddle is. Perhaps a puddle of a certain size will evaporate down to half its original volume in one day. But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that. This is an example where the half-life reduces as time goes on. (In other non-exponential decays, it can increase instead.)

text: Naturally occurring europium (Eu) is composed of 2 isotopes, Eu and Eu, with Eu being the most abundant (52.2% natural abundance). While Eu is observationally stable, Eu was recently found to be unstable and to undergo alpha decay. The half-life is measured to be (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 10 y which corresponds to 1 alpha decay per two minutes in every kilogram of natural europium. Besides natural radioisotope Eu, 36 artificial radioisotopes have been characterized, with the most stable being Eu with a half-life of 36.9 years, Eu with a half-life of 13.516 years, and Eu with a half-life of 8.593 years. All of the remaining radioactive isotopes have half-lives that are less than 4.7612 years, and the majority of these have half-lives that are less than 12.2 seconds. This element also has 17 meta states, with the most stable being Eu (t 12.8 hours), Eu (t 9.3116 hours) and Eu (t 96 minutes).

text: Naturally occurring

text: europium

text: (Eu) is composed of 2

text: isotope

text: s, Eu and Eu, with Eu being the most abundant (52.2%

text: natural abundance

text: ). While Eu is observationally stable, Eu was recently found to be unstable and to undergo

text: alpha decay

text: . The

text: half-life

text: is measured to be (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 10 y which corresponds to 1 alpha decay per two minutes in every kilogram of natural europium. Besides natural radioisotope Eu, 36 artificial radioisotopes have been characterized, with the most stable being Eu with a

text: half-life

text: of 36.9 years, Eu with a half-life of 13.516 years, and Eu with a half-life of 8.593 years. All of the remaining

text: radioactive

text: isotopes have half-lives that are less than 4.7612 years, and the majority of these have half-lives that are less than 12.2 seconds. This element also has 17

text: meta state

text: s, with the most stable being Eu (t 12.8 hours), Eu (t 9.3116 hours) and Eu (t 96 minutes).

text: At Cambridge, Rutherford started to work with J. J. Thomson on the conductive effects of X-rays on gases, work which led to the discovery of the electron which Thomson presented to the world in 1897. Hearing of Becquerel's experience with uranium, Rutherford started to explore its radioactivity, discovering two types that differed from X-rays in their penetrating power. Continuing his research in Canada, he coined the terms alpha ray and beta ray in 1899 to describe the two distinct types of radiation. He then discovered that thorium gave off a gas which produced an emanation which was itself radioactive and would coat other substances. He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its "half-life" (11½ minutes in this case).

text: At Cambridge, Rutherford started to work with

text: J. J. Thomson

text: on the conductive effects of X-rays on gases, work which led to the discovery of the

text: electron

text: which Thomson presented to the world in 1897. Hearing of

text: Becquerel

text: 's experience with

text: uranium

text: , Rutherford started to explore its

text: radioactivity

text: , discovering two types that differed from X-rays in their penetrating power. Continuing his research in Canada, he coined the terms

text: alpha ray

text: and

text: beta ray

text: in 1899 to describe the two distinct types of

text: radiation

text: . He then discovered that

text: thorium

text: gave off a gas which produced an emanation which was itself radioactive and would coat other substances. He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its "

text: half-life

text: " (11½ minutes in this case).

text: Radioactive contamination is typically the result of a spill or accident during the production, or use of, radionuclides (radioisotopes); these have unstable nuclei which are subject to radioactive decay.

text: Radioactive contamination is typically the result of a spill or accident during the production, or use of,

text: radionuclide

text: s (radioisotopes); these have unstable nuclei which are subject to

text: radioactive decay

text: Radioactive decay accounts for about  of the Earth's internal heat, powering the geodynamo and plate tectonics. The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232.Radioactive elements are used for radiometric dating, the primary method for establishing an absolute time scale in geochronology.Unstable isotopes decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras. Radiometric mapping using ground and airborne gamma spectrometry can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.

text: Radioactive decay

text: accounts for about  of the Earth's

text: internal heat

text: , powering the

text: geodynamo

text: and

text: plate tectonics

text: . The main heat-producing

text: isotopes

text: are

text: , uranium-235, and

text: .Radioactive elements are used for

text: radiometric dating

text: , the primary method for establishing an absolute time scale in

text: geochronology

text: .Unstable

text: isotopes

text: decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past

text: geologic eras

text: . Radiometric mapping using ground and airborne

text: gamma spectrometry

text: can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.

text: Early researchers found that an electric or magnetic field could split radioactive emissions into three types of beams. The rays were given the names alpha, beta, and gamma, in order of their ability to penetrate matter. While alpha decay was observed only in heavier elements of atomic number 52 (tellurium) and greater, the other two types of decay were produced by all of the elements. Lead, atomic number 82, is the heaviest element to have any isotopes stable (to the limit of measurement) to radioactive decay. Radioactive decay is seen in all isotopes of all elements of atomic number 83 (bismuth) or greater. Bismuth, however, is only very slightly radioactive, with a half-life greater than the age of the universe; radioisotopes with extremely long half-lives are considered effectively stable for practical purposes.

text: Early researchers found that an

text: electric

text: or

text: magnetic field

text: could split radioactive emissions into three types of beams. The rays were given the names

text: alpha

text: beta

text: , and

text: gamma

text: , in order of their ability to penetrate matter. While alpha decay was observed only in heavier elements of atomic number 52 (

text: tellurium

text: ) and greater, the other two types of decay were produced by all of the elements. Lead,

text: atomic number

text: 82, is the heaviest element to have any isotopes stable (to the limit of measurement) to radioactive decay. Radioactive decay is seen in all isotopes of all elements of atomic number 83 (

text: bismuth

text: ) or greater. Bismuth, however, is only very slightly radioactive, with a half-life greater than the age of the universe; radioisotopes with extremely long half-lives are considered effectively stable for practical purposes.

text: Determining a nuclear material's age is critical to nuclear forensic investigations. Dating techniques can be utilized to identify a material's source as well as procedures performed on the material. This can aid in determining the information about the potential participant in the "age" of the material of interest. Nuclides, related through radioactive decay processes will have relative sample concentrations that can be predicted using parent-daughter in-growth equations and relevant half-lives. Because radioactive isotopes decay at a rate determined by the amount of the isotope in a sample and the half-life of the parent isotope, the relative amount of the decay products compared to the parent isotopes can be used to determine "age". Heavy element nuclides have a 4n+2 relationship, where the mass number divided by 4 leaves a remainder of two. The decay network begins with Pu and proceeds through the in-growth of long-lived U, Th, and Ra. If any member of the 4n+2 decay chain is purified it will immediately begin to produce descendant species. The time since a sample was last purified can be calculated from the ratio of any two concentrations among the decaying nuclides.

text: Determining a nuclear material's age is critical to nuclear forensic investigations. Dating techniques can be utilized to identify a material's source as well as procedures performed on the material. This can aid in determining the information about the potential participant in the "age" of the material of interest.

text: Nuclide

text: s, related through radioactive decay processes will have relative sample concentrations that can be predicted using parent-daughter in-growth equations and relevant half-lives. Because radioactive isotopes decay at a rate determined by the amount of the isotope in a sample and the half-life of the parent isotope, the relative amount of the decay products compared to the parent isotopes can be used to determine "age". Heavy element nuclides have a 4n+2 relationship, where the mass number divided by 4 leaves a remainder of two. The decay network begins with Pu and proceeds through the in-growth of long-lived U, Th, and Ra. If any member of the 4n+2 decay chain is purified it will immediately begin to produce descendant species. The time since a sample was last purified can be calculated from the ratio of any two concentrations among the decaying nuclides.

text: The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and glucose metabolism in the tissues of the working brain.  These measurements reflect the amount of brain activity in the various regions of the brain and allow to learn more about how the brain works.  PET scans were superior to all other metabolic imaging methods in terms of resolution and speed of completion (as little as 30 seconds), when they first became available.  The improved resolution permitted better study to be made as to the area of the brain activated by a particular task. The biggest drawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoring short tasks.  Before fMRI technology came online, PET scanning was the preferred method of functional (as opposed to structural) brain imaging, and it continues to make large contributions to neuroscience.

text: The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen and

text: glucose

text: metabolism

text: in the tissues of the working brain.  These measurements reflect the amount of brain activity in the various regions of the brain and allow to learn more about how the brain works.  PET scans were superior to all other metabolic imaging methods in terms of resolution and speed of completion (as little as 30 seconds), when they first became available.  The improved resolution permitted better study to be made as to the area of the brain activated by a particular task. The biggest drawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoring short tasks.  Before fMRI technology came online, PET scanning was the preferred method of functional (as opposed to structural) brain imaging, and it continues to make large contributions to

text: neuroscience

text: In the early 20th century a series of products claiming medicinal properties, which contained radioactive elements were marketed to the general public. This does not include certain medications that contain radioactive isotopes (e.g. iodine-131 for its oncological uses) but pertains to elixirs and other medications that made preposterous claims (see below) that were neither scientific nor verifiable.

text: In the early 20th century a series of products claiming medicinal properties, which contained radioactive elements were marketed to the general public. This does not include certain medications that contain radioactive isotopes (e.g.

text: for its oncological uses) but pertains to elixirs and other medications that made preposterous claims (see below) that were neither scientific nor verifiable.

text: For example, specific radioactivity of radium 226 with a half-life of 1600 years is obtained by

text: For example, specific radioactivity of

text: radium 226

text: with a half-life of 1600 years is obtained by

text: After several years of cooling, most radioactivity is from the fission products caesium-137 and strontium-90, which are each produced in about 6% of fissions, and have half-lives of about 30 years. Other fission products with similar half-lives have much lower fission product yields, lower decay energy, and several (Sm,Eu,Cd) are also quickly destroyed by neutron capture while still in the reactor, so are not responsible for more than a tiny fraction of the radiation production at any time. Therefore, in the period from several years to several hundred years after use, radioactivity of spent fuel can be modeled simply as exponential decay of the Cs and Sr. These are sometimes known as medium-lived fission products.

text: After several years of cooling, most radioactivity is from the fission products

text: and

text: , which are each produced in about 6% of fissions, and have half-lives of about 30 years. Other fission products with similar half-lives have much lower

text: fission product yield

text: s, lower

text: decay energy

text: , and several (Sm,Eu,Cd) are also quickly destroyed by

text: neutron capture

text: while still in the reactor, so are not responsible for more than a tiny fraction of the radiation production at any time. Therefore, in the period from several years to several hundred years after use, radioactivity of spent fuel can be modeled simply as

text: exponential decay

text: of the Cs and Sr. These are sometimes known as medium-lived fission products.

text: In the radioactive processes, the three unstable parent isotopes decay into their respective stable daughter isotopes of Pb. Each following a decay chain consisting of alpha and beta decays, parent isotopes U, U and Th, decay into a series of intermediate daughter isotopes, and finally lead to stable isotopes, Pb, Pb and Pb, respectively. Each decay chain has a unique half-life, which means the daughter isotopes are generated at different rates.

text: In the radioactive processes, the three unstable parent isotopes decay into their respective stable daughter isotopes of Pb. Each following a

text: decay chain

text: consisting of

text: alpha

text: and

text: beta decay

text: s, parent isotopes U, U and Th, decay into a series of intermediate daughter isotopes, and finally lead to stable isotopes, Pb, Pb and Pb, respectively. Each decay chain has a unique

text: half-life

text: , which means the daughter isotopes are generated at different rates.

text: Elements are composed of one or more naturally occurring isotopes. The unstable (radioactive) isotopes are either primordial or postprimordial. Primordial isotopes were a product of stellar nucleosynthesis or another type of nucleosynthesis such as cosmic ray spallation, and have persisted down to the present because their rate of decay is so slow (e.g. uranium-238 and potassium-40). Post-primordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g. uranium to radium). A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom.

text: Elements are composed of one or more naturally occurring isotopes. The unstable (radioactive) isotopes are either

text: primordial

text: or postprimordial. Primordial isotopes were a product of

text: stellar nucleosynthesis

text: or another type of nucleosynthesis such as

text: cosmic ray spallation

text: , and have persisted down to the present because their rate of decay is so slow (e.g.

text: and

text: ). Post-primordial isotopes were created by

text: cosmic ray

text: bombardment as

text: cosmogenic nuclide

text: s (e.g.,

text: tritium

text: ), or by the decay of a radioactive primordial isotope to a radioactive

text: radiogenic nuclide

text: daughter (e.g.

text: uranium

text: to

text: radium

text: ). A few isotopes are naturally synthesized as

text: nucleogenic

text: nuclides, by some other natural

text: nuclear reaction

text: , such as when neutrons from natural

text: nuclear fission

text: are absorbed by another atom.

text: For naturally occurring thorium, uranium-235, and uranium-238, spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms, alpha decay or beta decay occurs instead. Hence, the spontaneous fission of these isotopes is usually negligible, except in using the exact branching ratios when finding the radioactivity of a sample of these elements.

text: For naturally occurring thorium,

text: , and

text: , spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms,

text: alpha decay

text: or

text: beta decay

text: occurs instead. Hence, the spontaneous fission of these isotopes is usually negligible, except in using the exact branching ratios when finding the

text: radioactivity

text: of a sample of these elements.

text: Shortly before the Voyager 1 encounter, Stan Peale, Patrick Cassen, and R. T. Reynolds published a paper in the journal Science predicting a volcanically modified surface and a differentiated interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by radioactive isotope decay alone. This effect would be even greater with a differentiated Io.

text: Shortly before the Voyager 1 encounter,

text: Stan Peale

text: , Patrick Cassen, and R. T. Reynolds published a paper in the journal

text: Science

text: predicting a volcanically modified surface and a

text: differentiated

text: interior, with distinct rock types rather than a homogeneous blend. They based this prediction on models of Io's interior that took into account the massive amount of heat produced by the varying tidal pull of Jupiter on Io caused by its slightly eccentric orbit. Their calculations suggested that the amount of heat generated for an Io with a homogeneous interior would be three times greater than the amount of heat generated by

text: radioactive isotope decay

text: alone. This effect would be even greater with a differentiated Io.

text: Isotopic dating requires measuring the isotopic concentration of radioactive U and Th, and radiogenic Pb in monazite. By treating each decay chain in the U-Th-Pb system independently, three classic isochron equations can be obtained:

text: The second group of radionuclides that exist naturally consists of radiogenic nuclides such as  (t = ), an isotope of radium, which are formed by radioactive decay. They occur in the decay chains of primordial isotopes of uranium or thorium. Some of these nuclides are very short-lived, such as isotopes of francium. There exist about 51 of these daughter nuclides that have half-lives too short to be primordial, and which exist in nature solely due to decay from longer lived radioactive primordial nuclides.

text: The second group of radionuclides that exist naturally consists of

text: radiogenic nuclide

text: s such as  (t = ), an isotope of

text: radium

text: , which are formed by

text: radioactive decay

text: . They occur in the decay chains of primordial isotopes of uranium or thorium. Some of these nuclides are very short-lived, such as

text: isotopes of francium

text: . There exist about 51 of these daughter nuclides that have half-lives too short to be primordial, and which exist in nature solely due to decay from longer lived radioactive primordial nuclides.

text: The elements in group 13 are also capable of forming stable compounds with the halogens, usually with the formula MX (where M is a boron-group element and X is a halogen.) The only exception to this is thallium(III) iodide. Fluorine, the first halogen, is able to form stable compounds with every element that has been tested (except neon and helium), and the boron group is no exception. It is even hypothesized that nihonium could form a compound with fluorine, NhF, before spontaneously decaying due to nihonium's radioactivity. Chlorine also forms stable compounds with all of the elements in the boron group, including thallium, and is hypothesized to react with nihonium. All of the elements will react with bromine under the right conditions, as with the other halogens but less vigorously than either chlorine or fluorine. Iodine will react with all natural elements in the periodic table except for the noble gases, and is notable for its explosive reaction with aluminium to form 2AlI. Astatine, the heaviest halogen, has only formed a few compounds, due to its radioactivity and short half-life, and no reports of a compound with an At–B, –Al, –Ga, –In, –Tl, or –Nh bond have been seen, although scientists think that it should form salts with metals.

text: The elements in group 13 are also capable of forming stable compounds with the

text: halogens

text: , usually with the formula MX (where M is a boron-group element and X is a halogen.) The only exception to this is

text: thallium(III) iodide

text: Fluorine

text: , the first halogen, is able to form stable compounds with every element that has been tested (except

text: neon

text: and

text: helium

text: ), and the boron group is no exception. It is even hypothesized that nihonium could form a compound with fluorine, NhF, before spontaneously decaying due to nihonium's radioactivity.

text: Chlorine

text: also forms stable compounds with all of the elements in the boron group, including thallium, and is hypothesized to react with nihonium. All of the elements will react with

text: bromine

text: under the right conditions, as with the other halogens but less vigorously than either chlorine or fluorine.

text: Iodine

text: will react with all natural elements in the periodic table except for the noble gases, and is notable for its explosive reaction with aluminium to form 2AlI.

text: Astatine

text: , the heaviest halogen, has only formed a few compounds, due to its radioactivity and short half-life, and no reports of a compound with an At–B, –Al, –Ga, –In, –Tl, or –Nh bond have been seen, although scientists think that it should form salts with metals.

text: Since radioactive decay is exponential with a constant probability, each process could as easily be described with a different constant time period that (for example) gave its "(1/3)-life" (how long until only 1/3 is left) or "(1/10)-life" (a time period until only 10% is left), and so on. Thus, the choice of  and  for marker-times, are only for convenience, and from convention. They reflect a fundamental principle only in so much as they show that the same proportion of a given radioactive substance will decay, during any time-period that one chooses.

text: The danger of radiation from radioactive precipitation/"fallout" decreases with time, as radioactivity decays exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. Creating the following 7-10 rule-of-thumb after a typical nuclear detonation while under the conditions that all fallout that will fall on the land has done so completely and no further deposition in the area will occur - After 7 hours, the average dose rate outside is reduced by a factor of ten; after 49(7x7) hours, it is reduced by a further factor of ten (to a value of 1/100th of the initial dose rate); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared to the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.

text: The danger of radiation from radioactive precipitation/"

text: fallout

text: " decreases with time, as

text: radioactivity decays

text: exponentially with time, such that for each factor of seven increase in time, the radiation is reduced by a factor of ten. Creating the following

text: 7-10 rule-of-thumb

text: after a typical nuclear detonation while under the conditions that all fallout that will fall on the land has done so completely and no further deposition in the area will occur - After 7 hours, the average

text: dose rate

text: outside is reduced by a factor of ten; after 49(7x7) hours, it is reduced by a further factor of ten (to a value of 1/100th of the initial dose rate); after two weeks the radiation from the fallout will have reduced by a factor of 1000 compared to the initial level; and after 14 weeks the average dose rate will have reduced to 1/10,000th of the initial level.

text: Fourteen radioisotopes have been characterized, with the most stable being O with a half-life of 122.24 s and O with a half-life of 70.606 s. All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds (ms). For example, O has a half-life of 61 ms. The most common decay mode for isotopes lighter than the stable isotopes is β decay (to nitrogen) and the most common mode after is β decay (to fluorine).

text: Fourteen

text: radioisotope

text: s have been characterized, with the most stable being O with a

text: half-life

text: of 122.24 s and O with a half-life of 70.606 s. All of the remaining

text: radioactive

text: isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83

text: millisecond

text: s (ms). For example, O has a half-life of 61 ms. The most common

text: decay mode

text: for isotopes lighter than the stable isotopes is

text: β decay

text: (to

text: nitrogen

text: ) and the most common mode after is

text: β decay

text: (to

text: fluorine

text: For a summary table showing the number of stable and radioactive nuclides in each category, see radionuclide. There exist twenty-nine chemical elements on Earth that are radioactive. They are those that contain thirty-four radionuclides that date before the time of formation of the solar system, and are known as primordial nuclides. Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes such as potassium-40. Another fifty or so shorter-lived radionuclides, such as radium and radon, found on Earth, are the products of decay chains that began with the primordial nuclides, or are the product of ongoing cosmogenic processes, such as the production of carbon-14 from nitrogen-14 by cosmic rays. Radionuclides may also be produced artificially in particle accelerators or nuclear reactors, resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. See this list of nuclides for a list of these, sorted by half life.

text: For a summary table showing the number of stable and radioactive nuclides in each category, see

text: radionuclide

text: . There exist twenty-nine

text: chemical element

text: s on Earth that are radioactive. They are those that contain thirty-four radionuclides that date before the time of formation of the solar system, and are known as

text: primordial nuclide

text: s. Well-known examples are

text: uranium

text: and

text: thorium

text: , but also included are naturally occurring long-lived radioisotopes such as

text: . Another fifty or so shorter-lived radionuclides, such as

text: radium

text: and

text: radon

text: , found on Earth, are the products of

text: decay chain

text: s that began with the primordial nuclides, or are the product of ongoing

text: cosmogenic

text: processes, such as the production of

text: from

text: by

text: cosmic rays

text: . Radionuclides may also be

text: produced artificially

text: in

text: particle accelerators

text: or

text: nuclear reactors

text: , resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. See this

text: list of nuclides

text: for a list of these, sorted by half life.

text: Fourteen radioisotopes have been characterized. The most stable are O with a half-life of 122.24 seconds and O with a half-life of 70.606 seconds. All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds. The most common decay mode of the isotopes lighter than O is β decay to yield nitrogen, and the most common mode for the isotopes heavier than O is beta decay to yield fluorine.

text: Fourteen

text: radioisotope

text: s have been characterized. The most stable are O with a

text: half-life

text: of 122.24 seconds and O with a half-life of 70.606 seconds. All of the remaining

text: radioactive

text: isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds. The most common

text: decay mode

text: of the isotopes lighter than O is

text: β decay

text: to yield nitrogen, and the most common mode for the isotopes heavier than O is

text: beta decay

text: to yield

text: fluorine

text: The isotopes of cobalt range in atomic weight from 50 u (Co) to 73 u (Co). The primary decay mode for isotopes with atomic mass unit values less than that of the most abundant stable isotope, Co, is electron capture and the primary mode of decay for those of greater than 59 atomic mass units is beta decay. The primary decay products before Co are element 26 (iron) isotopes and the primary products after are element 28 (nickel) isotopes.

text: The isotopes of cobalt range in

text: atomic weight

text: from 50

text: u

text: (Co) to 73 u (Co). The primary

text: decay mode

text: for isotopes with atomic mass unit values less than that of the most abundant stable isotope, Co, is

text: electron capture

text: and the primary mode of decay for those of greater than 59 atomic mass units is

text: beta decay

text: . The primary

text: decay product

text: s before Co are element 26 (

text: iron

text: ) isotopes and the primary products after are element 28 (nickel) isotopes.

text: As discussed above, deuterium and protium are stable isotopes which never undergo radioactive decay.  Therefore, the D/H ratio of a pool containing hydrogen will remain constant as long as no hydrogen is added or removed from the system, a property known as conservation of mass.  When two pools of hydrogen A and B mix with molar amounts of hydrogen m and m, each with their own starting fractional abundance of deuterium (F and F), then the fractional abundance of the resulting mixture is given by the following exact equation:

text: As discussed above, deuterium and protium are stable isotopes which never undergo radioactive decay.  Therefore, the D/H ratio of a pool containing hydrogen will remain constant as long as no hydrogen is added or removed from the system, a property known as

text: conservation of mass

text: .  When two pools of hydrogen A and B mix with molar amounts of hydrogen m and m, each with their own starting fractional abundance of deuterium (F and F), then the fractional abundance of the resulting mixture is given by the following exact equation:

text: Carbon-14 goes through radioactive beta decay:

text: Carbon-14 goes through radioactive

text: beta decay

text: A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity. In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value (the "plasma half-life").

text: A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity. In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in

text: blood plasma

text: to reach one-half of its steady-state value (the "plasma half-life").

text: At first sight this problem may seem very trivial but its result is used in many fields like nuclear chemistry, medical field,etc. In nuclear chemistry many radioactive elements follow the above-mentioned differential equation. Hence, to know how much amount of substance will be left after particular amount of time, this result becomes very helpful. Also C is present in every living and non-living things and it is radioactive.Hence these result can be applied on it and by measuring its amount and rate of decay we can actually predict the age of fossils and the cosmic bodies that strike the earth. In biology, growth of some bacteria depend on this equation so their number can be calculated by this equation.This information can be helpful in development of drugs in medical field.

text: At first sight this problem may seem very trivial but its result is used in many fields like

text: nuclear chemistry

text: , medical field,etc. In nuclear chemistry many radioactive elements follow the above-mentioned differential equation. Hence, to know how much amount of substance will be left after particular amount of time, this result becomes very helpful. Also

text: C

text: is present in every living and non-living things and it is radioactive.Hence these result can be applied on it and by measuring its amount and rate of decay we can actually predict the age of fossils and the cosmic bodies that strike the earth. In biology, growth of some bacteria depend on this equation so their number can be calculated by this equation.This information can be helpful in development of drugs in medical field.

text: Historical geology is a discipline that uses the principles and techniques of geology to reconstruct and understand the geological history of Earth. It focuses on geologic processes that change the Earth's surface and subsurface; and the use of stratigraphy, structural geology and paleontology to tell the sequence of these events. It also focuses on the evolution of plants and animals during different time periods in the geological timescale.  The discovery of radioactivity and the development of several radiometric dating techniques in the first half of the 20th century provided a means of deriving absolute versus relative ages of geologic history.

text: Historical geology is a discipline that uses the principles and techniques of

text: geology

text: to reconstruct and understand the

text: geological history of Earth

text: . It focuses on geologic processes that change the

text: Earth

text: 's surface and subsurface; and the use of

text: stratigraphy

text: structural geology

text: and

text: paleontology

text: to tell the sequence of these events. It also focuses on the

text: evolution

text: of

text: plant

text: s and

text: animal

text: s during different time periods in the

text: geological timescale

text: .  The discovery of

text: radioactivity

text: and the development of several

text: radiometric dating

text: techniques in the first half of the 20th century provided a means of deriving absolute versus relative ages of geologic history.

text: The difference between short-lived high-level nuclear waste and long-lived low-level waste can be illustrated by the following example. As stated above, one mole of both I and I release 3x10 decays in a period equal to one half-life. I decays with the release of 970 keV whilst I decays with the release of 194 keV of energy. 131gm of I would therefore release 45 Gigajoules over eight days beginning at an initial rate of 600 EBq releasing 90 Kilowatts with the last radioactive decay occurring inside two years. In contrast, 129gm of I would therefore release 9 Gigajoules over 15.7 million years beginning at an initial rate of 850 MBq releasing 25 microwatts with the radioactivity decreasing by less than 1% in 100,000 years.

text: The difference between short-lived high-level nuclear waste and long-lived low-level waste can be illustrated by the following example. As stated above, one

text: mole

text: of both I and I release 3x10 decays in a period equal to one half-life. I decays with the release of 970

text: keV

text: whilst I decays with the release of 194

text: keV

text: of energy. 131gm of I would therefore release 45 Giga

text: joules

text: over eight days beginning at an initial rate of 600 E

text: Bq

text: releasing 90 Kilo

text: watt

text: s with the last radioactive decay occurring inside two years. In contrast, 129gm of I would therefore release 9 Giga

text: joules

text: over 15.7 million years beginning at an initial rate of 850 M

text: Bq

text: releasing 25 micro

text: watt

text: s with the radioactivity decreasing by less than 1% in 100,000 years.

text: Forgeries in which modern lead or white lead pigment has been used can be recognized by using a technique called Pb(Lead)-210-Dating. Pb-210 is a naturally occurring radioactive isotope of lead that is part of the uranium-238 Radioactive decay series, and has a half life of 22.3 years. To determine the amount of Pb-210, the alpha radiation emitted by another element, polonium-210 (Po-210), is measured. Thus it is possible to estimate the age of a painting, within a few years' span, by extrapolating the Pb-210 content present in the paint used to create the painting.

text: Forgeries in which modern lead or white lead pigment has been used can be recognized by using a technique called Pb(Lead)-210-Dating.

text: is a naturally occurring radioactive isotope of lead that is part of the

text: Radioactive decay series

text: , and has a

text: half life

text: of 22.3 years. To determine the amount of Pb-210, the

text: alpha radiation

text: emitted by another element,

text: (Po-210), is measured. Thus it is possible to estimate the age of a painting, within a few years' span, by extrapolating the Pb-210 content present in the paint used to create the painting.

text: Pu has a half-life of 14 years, corresponding to a decay of about 5% of Pu-241 nuclei over a one-year period. The longer spent nuclear fuel waits before reprocessing, the more Pu decays to americium-241, which is nonfissile (although fissionable by fast neutrons) and an alpha emitter with a halflife of 432 yearswhich is a major contributor to the radioactivity of nuclear waste on a scale of hundreds or thousands of years.

text: Pu has a

text: half-life

text: of 14 years, corresponding to a decay of about 5% of Pu-241 nuclei over a one-year period. The longer

text: spent nuclear fuel

text: waits before

text: reprocessing

text: , the more Pu decays to

text: , which is nonfissile (although

text: fissionable

text: by

text: fast neutron

text: s) and an

text: alpha

text: emitter with a

text: halflife

text: of 432 yearswhich is a major contributor to the

text: radioactivity

text: of

text: nuclear waste

text: on a scale of hundreds or thousands of years.

text: In exponential decay, such as of a radioactive isotope, the time constant can be interpreted as the mean lifetime. The half-life T is related to the exponential time constant

text: In

text: exponential decay

text: , such as of a

text: radioactive

text: isotope, the time constant can be interpreted as the

text: mean lifetime

text: . The

text: half-life

text: T is related to the exponential time constant

text: The radioactive Cs has a very long half-life of about 2.3 million years, longest of all radioactive isotopes of caesium. Cs and Cs have half-lives of 30 and two years, respectively. Cs decomposes to a short-lived Ba by beta decay, and then to nonradioactive barium, while Cs transforms into Ba directly. The isotopes with mass numbers of 129, 131, 132 and 136, have half-lives between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. At least 21 metastable nuclear isomers exist. Other than Cs (with a half-life of just under 3 hours), all are very unstable and decay with half-lives of a few minutes or less.

text: The radioactive

text: Cs

text: has a very long half-life of about 2.3 million years, longest of all radioactive isotopes of caesium.

text: Cs

text: and

text: Cs

text: have half-lives of 30 and two years, respectively. Cs decomposes to a short-lived

text: Ba

text: by

text: beta decay

text: , and then to nonradioactive barium, while Cs transforms into Ba directly. The isotopes with mass numbers of 129, 131, 132 and 136, have half-lives between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. At least 21 metastable

text: nuclear isomer

text: s exist. Other than Cs (with a half-life of just under 3 hours), all are very unstable and decay with half-lives of a few minutes or less.

text: Another example of short-lived extinct radionuclide dating is the Al – Mg chronometer, which can be used to estimate the relative ages of chondrules. Al decays to Mg with a half-life of 720 000 years. The dating is simply a question of finding the deviation from the natural abundance of Mg (the product of Al decay) in comparison with the ratio of the stable isotopes Al/Mg.

text: Another example of short-lived extinct radionuclide dating is the

text: Al

text: – Mg chronometer, which can be used to estimate the relative ages of

text: chondrules

text: . Al decays to Mg with a

text: half-life

text: of 720 000 years. The dating is simply a question of finding the deviation from the

text: natural abundance

text: of Mg (the product of Al decay) in comparison with the ratio of the stable isotopes Al/Mg.

text: A half life is the time it takes half of the radiation of a specific substance to decay. A large amount of short-lived isotopes such as Zr are present in bomb fallout. This isotope and other short-lived isotopes are constantly generated in a power reactor, but because the criticality occurs over a long length of time, the majority of these short lived isotopes decay before they can be released.

text: A

text: half life

text: is the time it takes half of the radiation of a specific substance to decay. A large amount of short-lived isotopes such as Zr are present in bomb fallout. This isotope and other short-lived isotopes are constantly generated in a power reactor, but because the

text: criticality

text: occurs over a long length of time, the majority of these short lived isotopes decay before they can be released.

text: For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay. Note that after one half-life there are not exactly one-half of the atoms remaining, only approximately, because of the random variation in the process. Nevertheless, when there are many identical atoms decaying (right boxes), the law of large numbers suggests that it is a very good approximation to say that half of the atoms remain after one half-life.

text: For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay. Note that after one half-life there are not exactly one-half of the atoms remaining, only approximately, because of the random variation in the process. Nevertheless, when there are many identical atoms decaying (right boxes), the

text: law of large numbers

text: suggests that it is a very good approximation to say that half of the atoms remain after one half-life.

text: The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. Rutherford applied the principle of a radioactive element's half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206.

text: The original term, half-life period, dating to

text: Ernest Rutherford

text: 's discovery of the principle in 1907, was shortened to half-life in the early 1950s. Rutherford applied the principle of a radioactive

text: element's

text: half-life to studies of age determination of

text: rock

text: s by measuring the decay period of

text: radium

text: to

text: Decades ago, radioluminescent and phosphorescent signs that require no electricity have also been developed. These have been around since the 1970s. Radioluminescence uses the radioactive decay of tritium to light the sign, while phosphorescence uses light-emitting phosphors to glow in the dark. While both of these signs meet California State Fire Marshall standards, where practical, electricity is used in the vast majority of signs.

text: Decades ago, radioluminescent and phosphorescent signs that require no electricity have also been developed. These have been around since the 1970s.

text: Radioluminescence

text: uses the radioactive decay of

text: tritium

text: to light the sign, while

text: phosphorescence

text: uses light-emitting

text: phosphors

text: to

text: glow in the dark

text: . While both of these signs meet California State

text: Fire Marshall

text: standards, where practical, electricity is used in the vast majority of signs.

text: is the time of reactor shutdown mesured from the time of startup (in seconds).  For an approach with a more direct physical basis, some models use the fundamental concept of radioactive decay.  Used nuclear fuel contains a large number of different isotopes that contribute to decay heat, which are all subject to the radioactive decay law, so some models consider decay heat to be a sum of exponential functions with different decay constants and initial contribution to the heat rate.  A more accurate model would consider the effects of precursors, since many isotopes follow several steps in their radioactive decay chain, and the decay of daughter products will have a greater effect longer after shutdown.

text: is the time of reactor shutdown mesured from the time of startup (in seconds).  For an approach with a more direct physical basis, some models use the fundamental concept of

text: radioactive decay

text: .  Used nuclear fuel contains a large number of different isotopes that contribute to decay heat, which are all subject to the radioactive decay law, so some models consider decay heat to be a sum of exponential functions with different decay constants and initial contribution to the heat rate.  A more accurate model would consider the effects of precursors, since many isotopes follow several steps in their radioactive

text: decay chain

text: , and the decay of daughter products will have a greater effect longer after shutdown.

text: Germanium (Ge) has five naturally occurring isotopes, Ge, Ge, Ge, Ge, and Ge. Of these, Ge is very slightly radioactive, decaying by double beta decay with a half-life of 1.78 × 10 years (130 billion times the age of the universe).

text: Germanium

text: (Ge) has five naturally occurring

text: isotope

text: s, Ge, Ge, Ge, Ge, and Ge. Of these, Ge is very slightly radioactive, decaying by

text: double beta decay

text: with a

text: half-life

text: of 1.78 × 10 years (130 billion times the age of the universe).

text: A half-life usually describes the decay of discrete entities, such as radioactive atoms. In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay". For example, if there are 3 radioactive atoms with a half-life of one second, there will not be "1.5 atoms" left after one second.

text: 23 isotopes of silicon have been discovered. Five of these are naturally occurring. The most common is stable silicon-28, followed by stable silicon-29 and stable silicon-30. Silicon-32 is a radioactive isotope that occurs naturally as a result of radioactive decay of actinides, and via spallation in the upper atmosphere. Silicon-34 also occurs naturally as the result of radioactive decay of actinides.

text: isotopes of silicon

text: have been discovered. Five of these are naturally occurring. The most common is stable silicon-28, followed by stable silicon-29 and stable silicon-30. Silicon-32 is a radioactive isotope that occurs naturally as a result of radioactive decay of

text: actinides

text: , and via

text: spallation

text: in the upper atmosphere. Silicon-34 also occurs naturally as the result of radioactive decay of actinides.

text: thus decays exponentially with time, similar to radioactive decay. After one lifetime, the number of excited states decays to 36.8% of its original value (

text: thus decays exponentially with time, similar to

text: radioactive decay

text: . After one lifetime, the number of excited states decays to 36.8% of its original value (

text: Ernest Rutherford and Frederick Soddy jointly  had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular isotope of a radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "half-life", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product".

text: Ernest Rutherford

text: and

text: Frederick Soddy

text: jointly  had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or

text: gamma

text: radiation

text: in the process. They also determined that a particular isotope of a radioactive element decays into another element at a distinctive rate. This rate is given in terms of a "

text: half-life

text: ", or the amount of time it takes half of a mass of that radioactive material to break down into its "decay product".

text: Geiger and Marsden then wanted to estimate the total number of alpha particles that were being reflected.  The previous setup was unsuitable for doing this because the tube contained several radioactive substances (radium plus its decay products) and thus the alpha particles emitted had varying ranges, and because it was difficult for them to ascertain at what rate the tube was emitting alpha particles.  This time, they placed a small quantity of radium C (bismuth-214) on the lead plate, which bounced off a platinum reflector (R) and onto the screen.  They found that only a tiny fraction of the alpha particles that struck the reflector bounced onto the screen (in this case, 1 in 8000).

text: Geiger and Marsden then wanted to estimate the total number of alpha particles that were being reflected.  The previous setup was unsuitable for doing this because the tube contained several radioactive substances (radium plus its decay products) and thus the alpha particles emitted had varying

text: ranges

text: , and because it was difficult for them to ascertain at what rate the tube was emitting alpha particles.  This time, they placed a small quantity of radium C (bismuth-214) on the lead plate, which bounced off a platinum reflector (R) and onto the screen.  They found that only a tiny fraction of the alpha particles that struck the reflector bounced onto the screen (in this case, 1 in 8000).

text: He was the leading scientist with the discovery experiments of the chemical elements darmstadtium (Ds, atomic number 110),  roentgenium (Rg, 111) and copernicium (Cn, 112). He made substantial contributions to the discovery experiments of the elements bohrium (Bh, 107), hassium (Hs, 108)  and meitnerium (Mt, 109). He participated in the discovery of the element flerovium (Fl, 114) at the Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia, and his research group confirmed data measured on the synthesis of the elements flerovium and livermorium (Lv, 116) at FLNR. He identified many new isotopes located at the proton drip-line, among those the isotope Lu, the first case of radioactive emission of protons from the ground-state of a nucleus. His speciality is nuclear spectroscopy and heavy ion reactions.

text: He was the leading scientist with the discovery experiments of the chemical elements

text: darmstadtium

text: Ds

text: atomic number

text: roentgenium

text: Rg

text: , 111) and

text: copernicium

text: Cn

text: , 112). He made substantial contributions to the discovery experiments of the elements

text: bohrium

text: Bh

text: , 107),

text: hassium

text: Hs

text: , 108)  and

text: meitnerium

text: Mt

text: , 109). He participated in the discovery of the element

text: flerovium

text: Fl

text: , 114) at the

text: Flerov Laboratory of Nuclear Reactions

text: (FLNR) in

text: Dubna

text: , Russia, and his research group confirmed data measured on the synthesis of the elements flerovium and

text: livermorium

text: Lv

text: , 116) at FLNR. He identified many new isotopes located at the

text: proton drip-line

text: , among those the isotope

text: Lu

text: , the first case of radioactive emission of protons from the ground-state of a nucleus. His speciality is nuclear spectroscopy and heavy ion reactions.

text: It is further claimed that if radioactive isotopes such as uranium are present, the cell enables the hydrogen nuclei to fuse with these isotopes, transforming them into stable elements and thus neutralizing the radioactivity; and this would be achieved without releasing any radiation to the environment and without expending any energy. A televised demonstration on June 11, 1997, on Good Morning America was not conclusive because there was no measurement of the radioactivity of the beads after the test, thus it cannot be discarded that the beads had simply absorbed the uranium ions and become radioactive themselves. In 2002, the neutralization of radioactive isotopes has only been achieved through intense neutron bombardment in a nuclear reactor or large scale high energy particle accelerator, and at a large expense of energy.

text: It is further claimed that if

text: radioactive

text: isotope

text: s such as

text: uranium

text: are present, the cell enables the hydrogen nuclei to fuse with these isotopes, transforming them into stable

text: elements

text: and thus neutralizing the radioactivity; and this would be achieved without releasing any radiation to the environment and without expending any energy. A televised demonstration on June 11, 1997, on

text: Good Morning America

text: was not conclusive because there was no measurement of the radioactivity of the beads after the test, thus it cannot be discarded that the beads had simply absorbed the uranium ions and become radioactive themselves. In 2002, the neutralization of radioactive isotopes has only been achieved through intense neutron bombardment in a

text: nuclear reactor

text: or large scale high energy

text: particle accelerator

text: , and at a large expense of energy.

text: Apart from the stable isotopes, which make up almost all lead that exists naturally, there are trace quantities of a few radioactive isotopes. One of them is lead-210; although it has a half-life of 22.3 years, a period too short to allow any primordial lead-210 to exist, some small non-primordial quantities of it occur in nature, because lead-210 is found in the uranium series: thus, even though it constantly decays away, it is constantly regenerated by decay of its parent, polonium-214, which, while also constantly decaying, is also supplied by decay of its parent, and so on, all the way up to original uranium-238, which has been present for billions of years on Earth. Lead-210 is particularly useful for helping to identify ages of samples containing it, which is performed by measuring lead-210 to lead-206 ratios (both isotopes are present in a single decay chain). Lead-214 is also present in the decay chain of natural uranium-238, lead-212 is present in that of natural thorium-232, and lead-211 is present in that of natural uranium-235; therefore, traces of all three of these isotopes exist naturally as well. Lastly, very minute traces of lead-209 are also present from the cluster decay of radium-223, one of the daughter products of natural uranium-235. Hence, natural lead consists of not only the four stable isotopes, but also minute traces of another five short-lived radioisotopes.

text: Apart from the stable isotopes, which make up almost all lead that exists naturally, there are

text: trace quantities

text: cluster decay

text: of radium-223, one of the daughter products of natural uranium-235. Hence, natural lead consists of not only the four stable isotopes, but also minute traces of another five short-lived radioisotopes.

text: Here, to the left, we now have an alpha decay. It is the decay of the element Polonium  discovered by Marie Curie, with mass number 210. The isotope Po is the penultimate member of the uranium-radium-decay series; it decays into a stable lead-isotope with a half-life of 138 days. In almost all cases, the decay is via the emission of an alpha particle of 5.305 MeV. Only in one case of 100000, an alpha particle of lower energy appears; in this case, the decay leads to an excited level of Pb, which then decays to the ground state via gamma radiation.

text: Here, to the left, we now have an

text: alpha decay

text: . It is the decay of the element

text: Polonium

text: discovered by

text: Marie Curie

text: , with

text: mass number

text: 210. The

text: isotope

text: Po is the penultimate member of the uranium-radium-

text: decay series

text: ; it decays into a stable

text: lead

text: -isotope with a half-life of 138 days. In almost all cases, the decay is via the emission of an alpha particle of 5.305

text: MeV

text: . Only in one case of 100000, an alpha particle of lower energy appears; in this case, the decay leads to an excited level of Pb, which then decays to the ground state via gamma radiation.

text: Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes.

text: Stable nuclides are

text: nuclide

text: s that are not radioactive and so (unlike

text: radionuclide

text: s) do not spontaneously undergo

text: radioactive decay

text: . When such nuclides are referred to in relation to specific elements, they are usually termed

text: stable isotope

text: s.

text: All methods based on the radioactive decay belong to this category. The principle at the base of radiometric dating is that natural unstable isotopes, called 'parent isotopes', decay to some isotope which is instead stable, called the 'daughter isotope'. Under the assumptions that (1) the initial amount of parent and daughter isotopes can be estimated, and (2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of radioactive decay.Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are:

text: All methods based on the

text: radioactive decay

text: belong to this category. The principle at the base of

text: radiometric dating

text: is that natural unstable

text: isotope

text: s, called 'parent isotopes', decay to some

text: isotope

text: which is instead stable, called the 'daughter isotope'. Under the assumptions that (1) the initial amount of parent and daughter isotopes can be estimated, and (2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of

text: radioactive decay

text: .Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are:

text: Alternatively, since the radioactive decay contributes to the "physical (i.e. radioactive)" half-life, while the metabolic elimination processes determines the "biological" half-life of the radionuclide, the two act as parallel paths for elimination of the radioactivity, the effective half-life could also be represented by the formula:

text: For example, in biodistribution studies, it is common to track the movement of a compound of interest through an animal.  To measure the compound it is chemically joined to a radionuclide. Thus the energy measured from the radioactive decay is related to the quantity of the compound of interest.

text: For example, in

text: biodistribution

text: studies, it is common to track the movement of a compound of interest through an animal.  To measure the compound it is chemically joined to a

text: radionuclide

text: . Thus the energy measured from the radioactive decay is related to the quantity of the compound of interest.

text: The decay correct might be used this way: a group of 20 animals is injected with a compound of interest on a Monday at 10:00 a.m.  The compound is chemically joined to the isotope copper-64, which has a known half-life of 12.7 hours, or 764 minutes.  After one hour, the 5 animals in the "one hour" group are killed, dissected, and organs of interest are placed in sealed containers to await measurement.  This is repeated for another 5 animals, at 2 hours, and again at 4 hours.  At this point, (say, 4:00 p.m., Monday) all the organs collected so far are measured for radioactivity (a proxy of the distribution of the compound of interest).  The next day (Tuesday), the "24 hour" group would be killed and dissected at 10:00 a.m., then measured for radioactivity, (say at 11:00 a.m.).  In order to compare ALL the groups together, the data from the "24 hour" must be decay corrected: the radioactivity measured on the second day must be "adjusted" in order to allow a comparison to measurements from an earlier time, but of the same original material.

text: Caesium-137 is a radioisotope commonly used as a gamma-emitter in industrial applications. Its advantages include a half-life of roughly 30 years, its availability from the nuclear fuel cycle, and having Ba as a stable end product. The high water solubility is a disadvantage which makes it incompatible with large pool irradiators for food and medical supplies. It has been used in agriculture, cancer treatment, and the sterilization of food, sewage sludge, and surgical equipment. Radioactive isotopes of caesium in radiation devices were used in the medical field to treat certain types of cancer, but emergence of better alternatives and the use of water-soluble caesium chloride in the sources, which could create wide-ranging contamination, gradually put some of these caesium sources out of use. Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, levelling, and thickness gauges. It has also been used in well logging devices for measuring the electron density of the rock formations, which is analogous to the bulk density of the formations.

text: is a

text: radioisotope

text: commonly used as a

text: gamma

text: -emitter in industrial applications. Its advantages include a half-life of roughly 30 years, its availability from the

text: nuclear fuel cycle

text: , and having

text: Ba

text: as a stable end product. The high water solubility is a disadvantage which makes it incompatible with large pool irradiators for food and medical supplies. It has been used in agriculture, cancer treatment, and the

text: sterilization

text: of food, sewage sludge, and surgical equipment. Radioactive

text: isotopes of caesium

text: in

text: radiation devices

text: were used in the medical field to treat certain types of cancer, but emergence of better alternatives and the use of water-soluble caesium chloride in the sources, which could create wide-ranging contamination, gradually put some of these caesium sources out of use. Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, levelling, and thickness gauges. It has also been used in

text: well logging

text: devices for measuring the

text: electron density

text: of the rock formations, which is analogous to the bulk density of the formations.

text: In nuclear physics, the valley of stability (also called the nuclear valley, energy valley, or beta stability valley) is a characterization of the stability of nuclides to radioactivity based on their binding energy.   Nuclides are composed of protons and neutrons.  The shape of the valley refers to the profile of binding energy as a function of the numbers of neutrons and protons, with the lowest part of the valley corresponding to the region of most stable nuclei.  The line of stable nuclides down the center of the valley of stability is known as the line of beta stability.  The sides of the valley correspond to increasing instability to beta decay (β or β). The decay of a nuclide becomes more energetically favorable the further it is from the line of beta stability.  The boundaries of the valley correspond to the nuclear drip lines, where nuclides become so unstable they emit single protons or single neutrons.  Regions of instability within the valley at high atomic number also include radioactive decay by alpha radiation or spontaneous fission.  The shape of the valley is roughly an elongated paraboloid corresponding to the nuclide binding energies as a function of neutron and atomic numbers.

text: In

text: nuclear physics

text: , the valley of stability (also called the nuclear valley, energy valley, or beta stability valley) is a characterization of the stability of

text: nuclide

text: s to

text: radioactivity

text: based on their binding energy.   Nuclides are composed of

text: proton

text: s and

text: neutron

text: s.  The shape of the valley refers to the profile of binding energy as a function of the numbers of neutrons and protons, with the lowest part of the valley corresponding to the region of most

text: stable nuclei

text: .  The line of stable nuclides down the center of the valley of stability is known as the

text: line of beta stability

text: .  The sides of the valley correspond to increasing instability to

text: beta decay

text: (β or β). The decay of a nuclide becomes more energetically favorable the further it is from the line of beta stability.  The boundaries of the valley correspond to the

text: nuclear drip line

text: s, where nuclides become so unstable they emit

text: single protons

text: or

text: single neutrons

text: .  Regions of instability within the valley at high atomic number also include radioactive decay by

text: alpha radiation

text: or

text: spontaneous fission

text: .  The shape of the valley is roughly an elongated

text: paraboloid

text: corresponding to the nuclide

text: binding energies

text: as a function of neutron and atomic numbers.

text: As the atom came to be better understood, the nature of radioactivity became clearer. Some larger atomic nuclei are unstable, and so decay (release matter or energy) after a random interval. The three forms of radiation that Becquerel and the Curies discovered are also more fully understood. Alpha decay is when a nucleus releases an alpha particle, which is two protons and two neutrons, equivalent to a helium nucleus. Beta decay is the release of a beta particle, a high-energy electron. Gamma decay releases gamma rays, which unlike alpha and beta radiation are not matter but electromagnetic radiation of very high frequency, and therefore energy. This type of radiation is the most dangerous and most difficult to block. All three types of radiation occur naturally in certain elements.

text: As the atom came to be better understood, the nature of radioactivity became clearer. Some larger atomic nuclei are unstable, and so

text: decay

text: (release matter or energy) after a random interval. The three forms of

text: radiation

text: that Becquerel and the Curies discovered are also more fully understood.

text: Alpha decay

text: is when a nucleus releases an

text: alpha particle

text: , which is two

text: proton

text: s and two

text: neutron

text: s, equivalent to a

text: helium

text: nucleus.

text: Beta decay

text: is the release of a

text: beta particle

text: , a high-energy

text: electron

text: Gamma decay

text: releases

text: gamma rays

text: , which unlike alpha and beta radiation are not matter but

text: electromagnetic radiation

text: of very high

text: frequency

text: , and therefore

text: energy

text: . This type of radiation is the most dangerous and most difficult to block. All three types of radiation occur naturally in

entity_name: List of elements by stability of isotopes

text: certain elements

text: Examples of radioactive isotopes that are used for efficiency calibration include Co, Co, Y, Cs and Eu.

text: Examples of

text: radioactive isotopes

text: that are used for efficiency calibration include Co, Co, Y, Cs and Eu.

text: This article discusses an important problem from application of view. Thedocument does not go into much depth but it has enough information to make youfamiliar with these problem.The result of this problem is used in many fields likenuclear chemistry, biology,etc. in one or another form. In our daily life, often we come across problems related with growth and decay like decay of organic matter, growth of plants,etc.But some growth and decay phenomenon can be described in terms of differential equations like decay of radioactive element like Uranium, growth of certain bacteria,etc. How this can be done, is described here.

text: For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, and zirconium-89, whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.

text: For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant. It is not affected by external factors such as

text: temperature

text: pressure

text: , chemical environment, or presence of a

text: magnetic

text: or

text: electric field

text: . The only exceptions are nuclides that decay by the process of

text: electron capture

text: , such as

text: , and

text: , whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a

text: clock

text: to measure the time from the incorporation of the original nuclides into a material to the present.

text: Using radioactive microspheres is an older method of measuring perfusion than the more recent imaging techniques. This process involves labeling microspheres with radioactive isotopes and injecting these into the test subject. Perfusion measurements are taken by comparing the radioactivity of selected regions within the body to radioactivity of blood samples withdrawn at the time of microsphere injection.

text: Using radioactive

text: microspheres

text: is an older method of measuring perfusion than the more recent imaging techniques. This process involves labeling microspheres with

text: radioactive isotopes

text: and injecting these into the test subject. Perfusion measurements are taken by comparing the radioactivity of selected regions within the body to radioactivity of blood samples withdrawn at the time of microsphere injection.

text: There are many different types of radioactive decay. A decay, or loss of energy from the nucleus, results when an atom with an initial type of nucleus, called the parent radionuclide (or parent radioisotope), transforms into a daughter nuclide. The transformation produces an atom in a different state (a nucleus containing a different number of protons and neutrons). In some decays, the parent and the daughter nuclides are different chemical elements, and thus the decay process results in the creation of an atom of a different element. This is known as a nuclear transmutation.

text: There are many different types of radioactive decay. A decay, or loss of energy from the nucleus, results when an atom with an initial type of nucleus, called the parent

text: radionuclide

text: (or parent radioisotope), transforms into a daughter nuclide. The transformation produces an atom in a different state (a nucleus containing a different number of

text: proton

text: s and

text: neutron

text: s). In some decays, the parent and the daughter nuclides are different

text: chemical element

text: s, and thus the decay process results in the creation of an atom of a different element. This is known as a

text: nuclear transmutation

text: The half-life of knowledge is the amount of time that has to elapse before half of the knowledge in a particular area is superseded or shown to be untrue. The concept is attributed to Fritz Machlup (1962). For example, Donald Hebb estimated the half-life of psychology to be five years.

text: The half-life of knowledge is the amount of time that has to elapse before half of the

text: knowledge

text: in a particular area is superseded or shown to be untrue. The concept is attributed to

text: Fritz Machlup

text: (1962). For example,

text: Donald Hebb

text: estimated the half-life of

text: psychology

text: to be five years.

text: All the known stable isotopes occur naturally on Earth; the other naturally occurring-isotopes are radioactive but occur on Earth due to their relatively long half-lives, or else due to other means of ongoing natural production. These include the afore-mentioned cosmogenic nuclides, the nucleogenic nuclides, and any radiogenic radioisotopes formed by ongoing decay of a primordial radioactive isotope, such as radon and radium from uranium.

text: All the known

text: stable isotopes

text: occur naturally on Earth; the other naturally occurring-isotopes are radioactive but occur on Earth due to their relatively long half-lives, or else due to other means of ongoing natural production. These include the afore-mentioned

text: cosmogenic nuclide

text: s, the

text: nucleogenic

text: nuclides, and any

text: radiogenic

text: radioisotopes formed by ongoing decay of a primordial radioactive isotope, such as

text: radon

text: and

text: radium

text: from uranium.

text: Richards was the first to show, by chemical analysis, that an element could have different atomic weights. He was asked to analyze samples of naturally occurring lead and lead produced by radioactive decay.  His measurements showed that the two samples had different atomic weights, supporting the concepts of isotopes.

text: Richards was the first to show, by chemical analysis, that an element could have different atomic weights. He was asked to analyze samples of naturally occurring

text: lead

text: and lead produced by radioactive decay.  His measurements showed that the two samples had different atomic weights, supporting the concepts of

text: isotopes

text: The relationship also shows that half-lives are exponentially dependent on decay energy, so that very large changes in half-life make comparatively small differences in decay energy, and thus alpha particle energy. In practice, this means that alpha particles from all alpha-emitting isotopes across many orders of magnitude of difference in half-life, all nevertheless have about the same decay energy.

text: Another standard is the use of 1950 as "present", in the sense that a calculation that shows that a sample's likely age is 500 years "before present" means that it is likely to have come from about the year 1450. This convention is necessary in order to keep published radiocarbon results comparable to each other; without this convention, a given radiocarbon result would be of no use unless the year it was measured was also known—an age of 500 years published in 2010 would indicate a likely sample date of 1510, for example. In order to allow measurements to be converted to the 1950 baseline, a standard activity level is defined for the radioactivity of wood in 1950. Because of the fossil fuel effect, this is not actually the activity level of wood from 1950; the activity would have been somewhat lower. The fossil fuel effect was eliminated from the standard value by measuring wood from 1890, and using the radioactive decay equations to determine what the activity would have been at the year of growth. The resulting standard value, A, is 226 becquerels per kilogram of carbon.

text: The test does not directly assert the presence of radioactive material. A successful test asserts that the claim of no radioactive material present is unlikely given the reading (and therefore ...). The double negative (disproving the null hypothesis) of the method is confusing, but using a counter-example to disprove is standard mathematical practice. The attraction of the method is its practicality. We know (from experience) the expected range of counts with only ambient radioactivity present, so we can say that a measurement is unusually large. Statistics just formalizes the intuitive by using numbers instead of adjectives. We probably do not know the characteristics of the radioactive suitcases; We just assumethat they produce larger readings.

text: The tritium in a warhead is continually undergoing radioactive decay, hence becoming unavailable for fusion. Furthermore its decay product, helium-3, absorbs neutrons if exposed to the ones emitted by nuclear fission. This potentially offsets or reverses the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated from the decay of tritium. Therefore, it is necessary to replenish tritium in boosted bombs periodically. The estimated quantity needed is 4 grams per warhead. To maintain constant levels of tritium, about 0.20 grams per warhead per year must be supplied to the bomb.

text: The tritium in a

text: warhead

text: is continually undergoing radioactive decay, hence becoming unavailable for fusion. Furthermore its

text: decay product

text: , helium-3, absorbs neutrons if exposed to the ones emitted by nuclear fission. This potentially offsets or reverses the intended effect of the tritium, which was to generate many free neutrons, if too much helium-3 has accumulated from the decay of tritium. Therefore, it is necessary to replenish tritium in boosted bombs periodically. The estimated quantity needed is 4 grams per warhead. To maintain constant levels of tritium, about 0.20 grams per warhead per year must be supplied to the bomb.

text: Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites. In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air. Time since recrystallization is calculated by measuring the ratio of the amount of Ar accumulated to the amount of K remaining. The long half-life of K allows the method to be used to calculate the absolute age of samples older than a few thousand years.

text: Potassium–argon dating, abbreviated K–Ar dating, is a

text: radiometric dating

text: method used in

text: geochronology

text: and

text: archaeology

text: . It is based on measurement of the product of the

text: radioactive

text: decay of an

text: isotope

text: of

text: potassium

text: (K) into

text: argon

text: (Ar). Potassium is a common element found in many materials, such as

text: mica

text: s,

text: clay minerals

text: tephra

text: , and

text: evaporites

text: . In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (

text: recrystallizes

text: ). The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air. Time since recrystallization is calculated by measuring the ratio of the amount of Ar accumulated to the amount of K remaining. The long

text: half-life

text: of K allows the method to be used to calculate the

text: absolute age

text: of samples older than a few thousand years.

text: Uranium (U) is a naturally occurring radioactive element that has no stable isotopes but two primordial isotopes (uranium-238 and uranium-235) that have long half-life and are found in appreciable quantity in the Earth's crust, along with the decay product uranium-234. The relative atomic mass of natural uranium is 238.02891(3). Other isotopes such as uranium-232 have been produced in breeder reactors.

text: Uranium

text: (U) is a naturally occurring

text: radioactive

text: element that has no

text: stable isotope

text: s but two

text: primordial isotope

text: s (

text: and

text: ) that have long

text: half-life

text: and are found in appreciable quantity in the

text: Earth's crust

text: , along with the

text: decay product

text: . The

text: relative atomic mass

text: of

text: natural uranium

text: is 238.02891(3). Other isotopes such as

text: have been produced in

text: breeder reactor

text: s.

text: Isotopic tracers are usually subdivided into two categories: stable isotope tracers and radiogenic isotope tracers. Stable isotope tracers involve only non-radiogenic isotopes and usually are mass-dependent. In theory, any element with two stable isotopes can be used as an isotopic tracer. However, the most commonly used stable isotope tracers involve relatively light isotopes, which readily undergo fractionation in natural systems. See also isotopic signature. A radiogenic isotope tracer  involves an isotope produced by radioactive decay, which is usually in a ratio with a non-radiogenic isotope (whose abundance in the earth does not vary due to radioactive decay).

text: Isotopic tracers are usually subdivided into two categories:

text: stable isotope

text: tracers and

text: radiogenic

text: isotope tracers. Stable isotope tracers involve only non-radiogenic isotopes and usually are mass-dependent. In theory, any element with two stable isotopes can be used as an isotopic tracer. However, the most commonly used stable isotope tracers involve relatively light isotopes, which readily undergo fractionation in natural systems. See also

text: isotopic signature

text: . A radiogenic isotope tracer  involves an isotope produced by

text: radioactive decay

text: , which is usually in a ratio with a non-radiogenic isotope (whose abundance in the earth does not vary due to radioactive decay).

text: There are situations in which a nuclide deposited on the CPAM filter decays into another nuclide, and that second nuclide remains on the filter. This "parent-progeny" or decay chain situation is especially relevant to so-called "radon-thoron" (RnTn) or natural airborne radioactivity. The mathematical treatment described in this article does not consider this situation, but it can be treated using matrix methods (see Ref [11]).

text: There are situations in which a nuclide deposited on the CPAM filter decays into another nuclide, and that second nuclide remains on the filter. This "parent-progeny" or decay chain situation is especially relevant to so-called

text: "radon-thoron"

text: (RnTn) or natural airborne radioactivity. The mathematical treatment described in this article does not consider this situation, but it can be treated using matrix methods (see Ref [11]).

text: The best known example is the "paradox" of the Schrödinger's cat.  A mechanism is arranged to kill a cat if a quantum event, such as the decay of a radioactive atom, occurs.  Thus the fate of a large scale object, the cat, is entangled with the fate of a quantum object, the atom.  Prior to observation, according to the Schrödinger equation, the cat is apparently evolving into a linear combination of states that can be characterized as an "alive cat" and states that can be characterized as a "dead cat". Each of these possibilities is associated with a specific nonzero probability amplitude; the cat seems to be in some kind of "combination" state called a "quantum superposition". However, a single, particular observation of the cat does not measure the probabilities: it always finds either a living cat, or a dead cat. After the measurement the cat is definitively alive or dead. The question is: How are the probabilities converted into an actual, sharply well-defined outcome?

text: The best known example is the "paradox" of the

text: Schrödinger's cat

text: .  A mechanism is arranged to kill a cat if a quantum event, such as the decay of a radioactive atom, occurs.  Thus the fate of a large scale object, the cat, is

text: entangled

text: with the fate of a quantum object, the atom.  Prior to observation, according to the Schrödinger equation, the cat is apparently evolving into a

text: linear combination

text: of states that can be characterized as an "alive cat" and states that can be characterized as a "dead cat". Each of these possibilities is associated with a specific nonzero

text: probability amplitude

text: ; the cat seems to be in some kind of "combination" state called a "

text: quantum superposition

text: ". However, a single, particular observation of the cat does not measure the probabilities: it always finds either a living cat, or a dead cat. After the measurement the cat is definitively alive or dead. The question is: How are the probabilities converted into an actual, sharply well-defined outcome?

text: Although in theory all F-FDG is metabolized as above with a radioactivity elimination half-life of 110 minutes (the same as that of fluorine-18), clinical studies have shown that the radioactivity of F-FDG partitions into two major fractions. About 75% of the fluorine-18 activity remains in tissues and is eliminated with a half-life of 110 minutes, presumably by decaying in place to O-18 to form O-glucose-6-phosphate, which is non-radioactive (this molecule can soon be metabolized to carbon dioxide and water, after nuclear transmutation of the fluorine to oxygen ceases to prevent metabolism). Another fraction of F-FDG, representing about 20% of the total fluorine-18 activity of an injection, is eliminated renally by two hours after a dose of F-FDG, with a rapid half-life of about 16 minutes (this portion makes the renal-collecting system and bladder prominent in a normal PET scan). This short biological half-life indicates that this 20% portion of the total fluorine-18 tracer activity is eliminated pharmacokinetically (through the renal system) much more quickly than the isotope itself can decay. The rapidity also suggests that some of this F is no longer attached to glucose, since low concentrations of glucose in the blood are retained by the normal kidney and not passed into the urine. Because of this rapidly excreted urine F, the urine of a patient undergoing a PET scan may therefore be especially radioactive for several hours after administration of the isotope.

text: Although in theory all F-FDG is metabolized as above with a radioactivity elimination half-life of 110 minutes (the same as that of fluorine-18), clinical studies have shown that the radioactivity of F-FDG partitions into two major fractions. About 75% of the fluorine-18 activity remains in tissues and is eliminated with a half-life of 110 minutes, presumably by decaying in place to O-18 to form O-glucose-6-phosphate, which is non-radioactive (this molecule can soon be metabolized to carbon dioxide and water, after

text: nuclear transmutation

text: of the fluorine to oxygen ceases to prevent metabolism). Another fraction of F-FDG, representing about 20% of the total fluorine-18 activity of an injection, is eliminated

text: renal

text: ly by two hours after a dose of F-FDG, with a rapid half-life of about 16 minutes (this portion makes the renal-collecting system and bladder prominent in a normal PET scan). This short biological half-life indicates that this 20% portion of the total fluorine-18 tracer activity is eliminated pharmacokinetically (through the renal system) much more quickly than the isotope itself can decay. The rapidity also suggests that some of this F is no longer attached to glucose, since low concentrations of glucose in the blood are retained by the normal kidney and not passed into the urine. Because of this rapidly excreted urine F, the urine of a patient undergoing a PET scan may therefore be especially radioactive for several hours after administration of the isotope.

text: A Light Weight Deflectometer (LWD) is a portable falling weight deflectometer. It is used primarily to test insitu base and subgrade moduli during construction. Light Weight Deflectometer (LWD) is quicker than the isotope measuring method and requires no reference measurements. The equipment has no radioactive sources requiring safety courses and can be operated by one operator, allowing for the analysis of collected data and printing out of data files on site.

text: The first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation (it is now understood that a nuclear isomeric transition, however, can produce inhibited gamma decay with a measurable and much longer half-life). Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as "radioactivity" by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type. Villard's radiation was recognized as being of a type fundamentally different from previously named rays, by Ernest Rutherford, who in 1903 named Villard's rays "gamma rays" by analogy with the beta and alpha rays that Rutherford had differentiated in 1899. The "rays" emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays.

text: The first gamma ray source to be discovered historically was the

text: radioactive decay

text: process called gamma decay. In this type of decay, an

text: excited

text: nucleus emits a gamma ray almost immediately upon formation (it is now understood that a nuclear

text: isomeric transition

text: , however, can produce inhibited gamma decay with a measurable and much longer half-life).

text: Paul Villard

text: , a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from

text: radium

text: . Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included

text: beta ray

text: s, first noted as "radioactivity" by

text: Henri Becquerel

text: in 1896, and

text: alpha ray

text: s, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type. Villard's radiation was recognized as being of a type fundamentally different from previously named rays, by

text: Ernest Rutherford

text: , who in 1903 named Villard's rays "gamma rays" by analogy with the beta and alpha rays that Rutherford had differentiated in 1899. The "rays" emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet:

text: alpha ray

text: s as the least penetrating, followed by

text: beta ray

text: s, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays.

text: The double beta decay half-life was also measured: T = 1.84·10 yr.

text: While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one type of atom at each position on the periodic table. The term isotope was coined by Margaret Todd as a suitable name for different atoms that belong to the same element. J.J. Thomson created a technique for separating atom types through his work on ionized gases, which subsequently led to the discovery of stable isotopes.

text: While experimenting with the products of

text: radioactive decay

text: , in 1913

text: radiochemist

text: Frederick Soddy

text: discovered that there appeared to be more than one type of atom at each position on the periodic table. The term

text: isotope

text: was coined by

text: Margaret Todd

text: as a suitable name for different atoms that belong to the same element. J.J. Thomson created a technique for separating atom types through his work on ionized gases, which subsequently led to the discovery of

text: stable isotope

text: s.

text: Lutetium occurs on the Earth in form of two isotopes: lutetium-175 and lutetium-176. Out of these two, only the former is stable, making the element monoisotopic. The latter one, lutetium-176, decays via beta decay with a half-life of 3.78×10 years; it makes up about 2.5% of natural lutetium.To date, 32 synthetic radioisotopes of the element have been characterized, ranging in mass from 149.973 (lutetium-150) to 183.961 (lutetium-184); the most stable such isotopes are lutetium-174 with a half-life of 3.31 years, and lutetium-173 with a half-life of 1.37 years. All of the remaining radioactive isotopes have half-lives that are less than 9 days, and the majority of these have half-lives that are less than half an hour. Isotopes lighter than the stable lutetium-175 decay via electron capture (to produce isotopes of ytterbium), with some alpha and positron emission; the heavier isotopes decay primarily via beta decay, producing hafnium isotopes.

text: Lutetium occurs on the Earth in form of two isotopes: lutetium-175 and lutetium-176. Out of these two, only the former is stable, making the element

text: monoisotopic

text: . The latter one, lutetium-176, decays via

text: beta decay

text: with a

text: half-life

text: of 3.78×10 years; it makes up about 2.5% of natural lutetium.To date, 32

text: synthetic radioisotope

text: s of the element have been characterized, ranging in mass from 149.973 (lutetium-150) to 183.961 (lutetium-184); the most stable such isotopes are lutetium-174 with a half-life of 3.31 years, and lutetium-173 with a half-life of 1.37 years. All of the remaining

text: radioactive

text: isotopes have half-lives that are less than 9 days, and the majority of these have half-lives that are less than half an hour. Isotopes lighter than the stable lutetium-175 decay via

text: electron capture

text: (to produce isotopes of

text: ytterbium

text: ), with some

text: alpha

text: and

text: positron emission

text: ; the heavier isotopes decay primarily via beta decay, producing hafnium isotopes.

text: Because the heavy hydrogen and oxygen isotopes used in the standard doubly labeled water measurement are non-radioactive, and also non-toxic in the doses used (see heavy water), the doubly labeled water measurement of mean metabolic rate has been used extensively in human volunteers, and even in infants and pregnant women. The technique has been used on over 200 species of wild animals (mostly birds, mammals and some reptiles). Applications of the method to animals have been reviewed.

text: Because the heavy hydrogen and oxygen isotopes used in the standard doubly labeled water measurement are non-radioactive, and also non-toxic in the doses used (see

text: heavy water

text: ), the doubly labeled water measurement of mean metabolic rate has been used extensively in human volunteers, and even in infants and pregnant women. The technique has been used on over 200 species of wild animals (mostly birds, mammals and some reptiles). Applications of the method to animals have been reviewed.

text: TRIUMF’s Ion Trap for Atomic and Nuclear Science (TITAN) measures the mass of short-lived isotopes with high precision.  Radioactive isotopes from ISAC are sent to TITAN to undergo cooling, charge-breeding and trapping.  The entire process occurs in about 10 milliseconds, allowing radioactive isotopes with short half lives to be studied.

text: List of the elements are available by name, atomic number, density, melting point, boiling point and by symbol, as well as ionization energies of the elements. The nuclides of stable and radioactive elements are also available as a list of nuclides, sorted by length of half-life for those that are unstable. One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the periodic table, which groups together elements with similar chemical properties (and usually also similar electronic structures).

text: List of the elements are available by name, atomic number, density, melting point, boiling point

text: and

text: by symbol

text: , as well as

entity_name: Molar ionization energies of the elements

text: ionization energies of the elements

text: . The nuclides of stable and radioactive elements are also available as a

text: list of nuclides

text: , sorted by length of half-life for those that are unstable. One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the

text: periodic table

text: , which groups together elements with similar chemical properties (and usually also similar electronic structures).

text: There has been extensive research into metallopharmaceuticals, including discussion of rhodium compounds in medicine. A substantial body of research has examined using metallocene derivatives of ruthenium and iron as metallopharmaceuticals. One area of such research has utilised metallocenes in place of the fluorophenyl group in haloperidol, which is a pharmaceutical classified as a typical antipsychotic. The ferrocenyl–haloperidol compound investigated has structure (CH)Fe(CH)–C(=O)–(CH)–N(CHCH)C(OH)–CHCl and can be converted to the ruthenium analog via a transmetalation reaction. Using the radioactive isotope Ru produces a ruthenocenyl–haloperidol radiopharmaceutical with a high affinity for lung but not brain tissue in mice and rats. Beta-decay of Ru produces the metastable isotope Rh in a rhodocenyl–haloperidol compound. This compound, like other rhodocene derivatives, has an unstable 19-valence electron configuration and rapidly oxidises to the expected cationic rhodocenium–haloperidol species. The separation of the ruthenocenyl–haloperidol and the rhodocenium–haloperidol species and the distributions of each amongst bodily organs has been studied. Rh has a half-life of 56 min and emits a gamma ray of energy 39.8 keV, so the gamma-decay of the rhodium isotope should follow soon after the beta-decay of the ruthenium isotope. Beta- and gamma-emitting radionuclides used medically include I, Fe, and Ca, and Rh has been proposed for use in radiotherapy for small tumours.

text: There has been extensive research into

text: metallopharmaceutical

text: s, including discussion of rhodium compounds in medicine. A substantial body of research has examined using metallocene derivatives of

text: ruthenium

text: and iron as metallopharmaceuticals. One area of such research has utilised metallocenes in place of the fluorophenyl group in

text: haloperidol

text: , which is a

text: pharmaceutical

text: classified as a

text: typical antipsychotic

text: . The ferrocenyl–haloperidol compound investigated has structure (CH)Fe(CH)–C(=O)–(CH)–N(CHCH)C(OH)–CHCl and can be converted to the ruthenium analog via a transmetalation reaction. Using the

text: radioactive

text: isotope

text: Ru

text: produces a ruthenocenyl–haloperidol radiopharmaceutical with a high affinity for

text: lung

text: but not

text: brain

text: tissue

text: in

text: mice

text: and

text: rat

text: s.

text: Beta-decay

text: of Ru produces the

text: metastable isotope

text: Rh

text: in a rhodocenyl–haloperidol compound. This compound, like other rhodocene derivatives, has an unstable 19-valence electron configuration and rapidly oxidises to the expected cationic rhodocenium–haloperidol species. The separation of the ruthenocenyl–haloperidol and the rhodocenium–haloperidol species and the distributions of each amongst bodily organs has been studied. Rh has a

text: half-life

text: of 56 min and emits a

text: gamma ray

text: of energy 39.8

text: keV

text: , so the

text: gamma-decay

text: of the rhodium isotope should follow soon after the beta-decay of the ruthenium isotope. Beta- and gamma-emitting

text: radionuclide

text: s used medically include

text: I

text: Fe

text: , and

text: Ca

text: , and Rh has been proposed for use in

text: radiotherapy

text: for small tumours.

text: If a gamma ray is emitted from a radioactive element within the human body due to radioactive decay, and its energy is sufficient to escape then it can be detected. This would be by means of either a scintillation detector or a semiconductor detector placed in close proximity to the body. Radioactive decay may give rise to gamma radiation which cannot escape the body due to being absorbed or other interaction whereby it can lose energy; so account must be taken of this in any measurement analysis.

text: If a gamma ray is emitted from a radioactive element within the human body due to

text: radioactive decay

text: , and its energy is sufficient to escape then it can be detected. This would be by means of either a

text: scintillation detector

text: or a

text: semiconductor detector

text: placed in close proximity to the body. Radioactive decay may give rise to gamma radiation which cannot escape the body due to being absorbed or other interaction whereby it can lose energy; so account must be taken of this in any measurement analysis.

text: Rainwater can be intensely radioactive due to high levels of radon and its decay progenies Bi and Pb; the concentrations of these radioisotopes can be high enough to seriously disrupt radiation monitoring at nuclear power plants. The highest levels of radon in rainwater occurs during thunderstorms, and it is hypothesized that radon is concentrated in thunderstorms on account of the atom's positive electrical charge. Estimates of the age of rain drops have been obtained from measuring the isotopic abundance of radon's short-lived decay progeny in rainwater.

text: Rainwater can be intensely radioactive due to high levels of radon and its decay progenies Bi and Pb; the concentrations of these

text: radioisotope

text: s can be high enough to seriously disrupt radiation monitoring at nuclear power plants. The highest levels of radon in rainwater occurs during thunderstorms, and it is hypothesized that radon is concentrated in thunderstorms on account of the atom's positive electrical charge. Estimates of the age of rain drops have been obtained from measuring the isotopic abundance of radon's short-lived decay progeny in rainwater.

text: Bismuth was long considered the element with the highest atomic mass that is stable. However, in 2003 it was discovered to be weakly radioactive: its only primordial isotope, bismuth-209, decays via alpha decay with a half life more than a billion times the estimated age of the universe. Because of its tremendously long half-life, bismuth may still be considered stable for almost all purposes.

text: Bismuth was long considered the element with the highest atomic mass that is stable. However, in 2003 it was discovered to be weakly radioactive: its only

text: primordial isotope

text: , decays via

text: alpha decay

text: with a

text: half life

text: more than a

text: billion

text: times the estimated

text: age of the universe

text: . Because of its tremendously long half-life, bismuth may still be considered stable for almost all purposes.

text: Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope (except for technetium, element 43 and promethium, element 61, which have no stable isotopes). Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected. Some of these elements, notably bismuth (atomic number 83), thorium (atomic number 90), and uranium (atomic number 92), have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy metals before the formation of our Solar System. At over 1.9 years, over a billion times longer than the current estimated age of the universe, bismuth-209 (atomic number 83) has the longest known alpha decay half-life of any naturally occurring element, and is almost always considered on par with the 80 stable elements. The very heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized.

text: Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one

text: stable isotope

text: (except for

text: technetium

text: , element 43 and

text: promethium

text: , element 61, which have no stable isotopes). Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are

text: unstable

text: to the point that radioactive decay of all isotopes can be detected. Some of these elements, notably

text: bismuth

text: (atomic number 83),

text: thorium

text: (atomic number 90), and

text: uranium

text: (atomic number 92), have one or more isotopes with half-lives long enough to survive as remnants of the explosive

text: stellar nucleosynthesis

text: that produced the

text: heavy metals

text: before the formation of our

text: Solar System

text: . At over 1.9 years, over a billion times longer than the current estimated age of the universe,

text: (atomic number 83) has the longest known

text: alpha decay

text: half-life of any naturally occurring element, and is almost always considered on par with the 80 stable elements. The very heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with

text: half-lives

text: so short that they are not found in nature and must be

text: synthesized

text: For these purposes, a particularly useful type of radioactive decay is positron emission. When a positron collides with an electron, it releases two high-energy photons traveling in diametrically opposite directions. If the positron is produced within a solid object, it is likely to do this before traveling more than a millimeter. If both of these photons can be detected, the location of the decay event can be determined very precisely.

text: For these purposes, a particularly useful type of radioactive decay is

text: positron emission

text: . When a positron collides with an electron, it releases two high-energy

text: photon

text: s traveling in diametrically opposite directions. If the positron is produced within a solid object, it is likely to do this before traveling more than a millimeter. If both of these photons can be detected, the location of the decay event can be determined very precisely.

text: The radioactive isotope used for the fallout material would be a high intensity gamma ray emitter, with a half-life long enough that it remains lethal for an extended period. It would also have to have a chemistry that causes it to return to earth as fallout, rather than stay in the atmosphere after being vaporized in the explosion.  Another consideration is biological: radioactive isotopes of elements normally taken up by animals as nutrition would pose a special threat to organisms that absorbed them, as their radiation would be delivered from within the body of the organism.

text: The radioactive isotope used for the fallout material would be a high intensity

text: gamma ray

text: emitter, with a

text: half-life

text: long enough that it remains lethal for an extended period. It would also have to have a chemistry that causes it to return to earth as fallout, rather than stay in the

text: atmosphere

text: after being vaporized in the explosion.  Another consideration is biological: radioactive isotopes of elements normally taken up by animals as nutrition would pose a special threat to organisms that absorbed them, as their radiation would be delivered from within the body of the organism.

text: Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their  decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.

text: Radiometric dating is based on the known and constant rate of decay of

text: radioactive isotopes

text: into their

text: radiogenic daughter isotopes

text: . Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their  decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.

text: Radon is formed as part of the normal radioactive decay chain of uranium and thorium. Uranium and thorium have been around since the earth was formed and their most common isotope has a very long half-life (14.05 billion years). Uranium and thorium, radium, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now. As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.

text: Radon is formed as part of the normal radioactive

text: decay chain

text: of uranium and thorium. Uranium and thorium have been around since the earth was formed and their

text: most common isotope

text: has a very long half-life (14.05 billion years). Uranium and thorium,

text: radium

text: , and thus radon, will continue to occur for millions of years at about the same concentrations as they do now. As the radioactive gas of radon decays, it produces new radioactive elements called radon daughters or decay products. Radon daughters are solids and stick to surfaces such as dust particles in the air. If contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.

text: Of the first 82 elements in the periodic table, 80 have isotopes considered to be stable. The 83rd element, bismuth, was traditionally regarded as having the heaviest stable nuclide, bismuth-209, but in 2003 researchers in Orsay, France, measured the half-life of  to be . Technetium, promethium (atomic numbers 43 and 61, respectively) and all the elements with an atomic number over 82 only have isotopes that are known to decompose through radioactive decay. No undiscovered elements are expected to be stable, therefore lead is considered the heaviest stable element. However, it is possible that some isotopes that are now considered stable will be revealed to decay with extremely long half-lives (as with ). This list depicts what is agreed upon by the consensus of the scientific community as of 2008.

text: Of the first 82 elements in the

text: periodic table

text: , 80 have

text: isotope

text: s considered to be stable. The 83rd element, bismuth, was traditionally regarded as having the heaviest stable nuclide,

text: , but in 2003 researchers in

text: Orsay

text: , France, measured the

text: half-life

text: of  to be .

text: Technetium

text: promethium

text: atomic number

text: s 43 and 61, respectively) and all the elements with an atomic number over 82 only have isotopes that are known to decompose through

text: radioactive decay

text: . No undiscovered elements are expected to be stable, therefore

text: lead

text: is considered the heaviest stable element. However, it is possible that some isotopes that are now considered stable will be revealed to decay with extremely long half-lives (as with ). This list depicts what is agreed upon by the consensus of the scientific community as of 2008.

text: Each radioactive isotope has a characteristic decay time period—the half-life—that is determined by the amount of time needed for half of a sample to decay. This is an exponential decay process that steadily decreases the proportion of the remaining isotope by 50% every half-life. Hence after two half-lives have passed only 25% of the isotope is present, and so forth.

text: Each

text: radioactive isotope

text: has a characteristic decay time period—the

text: half-life

text: —that is determined by the amount of time needed for half of a sample to decay. This is an

text: exponential decay

text: process that steadily decreases the proportion of the remaining isotope by 50% every half-life. Hence after two half-lives have passed only 25% of the isotope is present, and so forth.

text: In a neuroimaging application the radioligand is injected into the pertinent tissue, or infused into the bloodstream. It binds to its receptor.When the radioactive isotope in the ligand decays it can be measured by positron emission tomography (PET) or single photon emission computed tomography (SPECT). In in vivo systems it is often used to quantify the binding of a test molecule to the binding site of radioligand. The higher the affinity of the molecule the more radioligand is displaced from the binding site and the increasing radioactive decay can be measured by scintillography. This assay is commonly used to calculate binding constant of molecules to receptors.

text: In a

text: neuroimaging

text: application the radioligand is injected into the pertinent tissue, or infused into the bloodstream. It binds to its

text: receptor

text: .When the radioactive isotope in the ligand decays it can be measured by

text: positron emission tomography

text: (PET) or

entity_name: Single-photon emission computed tomography

text: single photon emission computed tomography

text: (SPECT). In in vivo systems it is often used to quantify the binding of a test molecule to the binding site of radioligand. The higher the affinity of the molecule the more radioligand is displaced from the binding site and the increasing radioactive decay can be measured by

text: scintillography

text: . This assay is commonly used to calculate

text: binding constant

text: of molecules to receptors.

text: Radioactive decay is a stochastic (i.e. random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. For a collection of atoms however, the collection's decay rate can be calculated from their measured decay constants or half-lives. This is the basis of radiometric dating. The half-lives of radioactive atoms have no known lower or upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe.A radioactive source emits its decay products isotropically (all directions and without bias) in the absence of external influence.

text: Radioactive decay is a

text: stochastic

text: (i.e. random) process at the level of single atoms, in that, according to

text: quantum theory

text: , it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. For a collection of atoms however, the collection's decay rate can be calculated from their measured

text: decay constant

text: s or

text: half-lives

text: . This is the basis of

text: radiometric dating

text: . The half-lives of radioactive atoms have no known lower or upper limit, spanning a time range of over 55

text: orders of magnitude

text: , from nearly instantaneous to far longer than the

text: age of the universe

text: .A radioactive source emits its decay products

text: isotropically

text: (all directions and without bias) in the absence of external influence.

text: For each element with atomic number Z small enough to occupy only the first three  nuclear shells, that is up to that of calcium (Z = 20), there exists a stable isotope with N/Z ratio of one, with the exception of beryllium (N/Z = 1.25) and every element with odd atomic number between 9 and 19 inclusive (N = Z+1). Hydrogen-1 (N/Z ratio = 0) and helium-3 (N/Z ratio = 0.5) are the only stable isotopes with neutron–proton ratio under one. Uranium-238 and plutonium-244 have the highest N/Z ratios of any primordial nuclide at 1.587 and 1.596, respectively, while lead-208 has the highest N/Z ratio of any known stable isotope at 1.537. Radioactive decay generally proceeds so as to change the N/Z ratio to increase stability.  If the N/Z ratio is greater than 1, alpha decay increases the N/Z ratio, and hence provides a common pathway towards stability for decays involving large nuclei with too few neutrons. Positron emission and electron capture also increase the ratio, while beta decay will decrease the ratio.

text: For each element with atomic number Z small enough to occupy only the first three

text: nuclear shells

text: , that is up to that of

text: calcium

text: (Z = 20), there exists a stable isotope with N/Z ratio of one, with the exception of

text: beryllium

text: (N/Z = 1.25) and every element with odd atomic number between 9 and 19 inclusive (N = Z+1).

text: (N/Z ratio = 0) and

text: (N/Z ratio = 0.5) are the only stable isotopes with neutron–proton ratio under one.

text: and

text: have the highest N/Z ratios of any

text: primordial nuclide

text: at 1.587 and 1.596, respectively, while

text: has the highest N/Z ratio of any known stable isotope at 1.537.

text: Radioactive decay

text: generally proceeds so as to change the N/Z ratio to increase stability.  If the N/Z ratio is greater than 1,

text: alpha decay

text: increases the N/Z ratio, and hence provides a common pathway towards stability for decays involving large nuclei with too few neutrons.

text: Positron emission

text: and

text: electron capture

text: also increase the ratio, while

text: beta decay

text: will decrease the ratio.

text: Plutonium isotopes undergo radioactive decay, which produces decay heat. Different isotopes produce different amounts of heat per mass. The decay heat is usually listed as watt/kilogram, or milliwatt/gram. In larger pieces of plutonium (e.g. a weapon pit) and inadequate heat removal the resulting self-heating may be significant. All isotopes produce weak gamma on decay.

text: Plutonium isotopes undergo radioactive decay, which produces

text: decay heat

text: . Different isotopes produce different amounts of heat per mass. The decay heat is usually listed as watt/kilogram, or milliwatt/gram. In larger pieces of plutonium (e.g. a weapon pit) and inadequate heat removal the resulting self-heating may be significant. All isotopes produce weak gamma on decay.

text: All of the remaining radioactive isotopes have half-lives that are less than an hour and the majority of these have half-lives that are less than 3 minutes. This element has numerous meta states with the most stable being Ag (t 418 years), Ag (t 249.79 days) and Ag (t 8.28 days).

text: All of the remaining

text: radioactive

text: isotopes have half-lives that are less than an hour and the majority of these have half-lives that are less than 3 minutes. This element has numerous

text: meta state

text: s with the most stable being Ag (t 418 years), Ag (t 249.79 days) and Ag (t 8.28 days).

text: Radium ( ) is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1601 years and decays into radon gas. Because of such instability, radium is luminescent, glowing a faint blue. Radium, in the form of radium chloride, was discovered by Marie Skłodowska-Curie and Pierre Curie in 1898. They extracted the radium compound from uraninite and published the discovery at the French Academy of Sciences five days later. Radium was isolated in its metallic state by Marie Curie and André-Louis Debierne through the electrolysis of radium chloride in 1910. Since its discovery, it has given names likeradium A and radium C to several isotopes of other elements that are decay products of radium-226. In nature, radium is found in uranium ores in trace amounts as small as a seventh of a gram per ton of uraninite. Radium is not necessary for living organisms, and adverse health effects are likely when it is incorporated into biochemical processes because of its radioactivity and chemical reactivity.

text: Radium ( ) is a

text: chemical element

text: with

text: atomic number

text: 88, represented by the symbol Ra. Radium is an almost pure-white

text: alkaline earth metal

text: , but it readily

text: oxidize

text: s on exposure to air, becoming black in color. All isotopes of radium are highly

text: radioactive

text: , with the most stable

text: isotope

text: being

text: , which has a

text: half-life

text: of 1601 years and

text: decays

text: into

text: radon

text: gas. Because of such instability, radium is

text: luminescent

text: , glowing a faint blue. Radium, in the form of

text: radium chloride

text: , was

entity_name: Timeline of chemical element discoveries

text: discovered

text: by

text: Marie Skłodowska-Curie

text: and

text: Pierre Curie

text: in 1898. They extracted the radium compound from

text: uraninite

text: and published the discovery at the

text: French Academy of Sciences

text: five days later. Radium was isolated in its

text: metal

text: lic state by Marie Curie and

text: André-Louis Debierne

text: through the

text: electrolysis

text: of radium chloride in 1910. Since its discovery, it has given names like

text: radium A

text: and

text: radium C

text: to several isotopes of other elements that are

text: decay product

text: s of radium-226. In nature, radium is found in

text: uranium

text: ores in trace amounts as small as a seventh of a gram per ton of

text: uraninite

text: . Radium is not necessary for living organisms, and adverse health effects are likely when it is incorporated into biochemical processes because of its radioactivity and chemical reactivity.

text: Twenty-eight artificial isotopes of zirconium have been synthesized, ranging in atomic mass from 78 to 110. Zr is the longest-lived artificial isotope, with a half-life of 1.53×10 years. Zr, the heaviest isotope of zirconium, is the most radioactive, with an estimated half-life of 30 milliseconds. Radioactive isotopes at or above mass number 93 decay by electron emission, whereas those at or below 89 decay by positron emission. The only exception is Zr, which decays by electron capture.

text: Twenty-eight artificial isotopes of zirconium have been synthesized, ranging in atomic mass from 78 to 110.

text: Zr

text: is the longest-lived artificial isotope, with a half-life of 1.53×10 years. Zr, the heaviest isotope of zirconium, is the most radioactive, with an estimated half-life of 30 milliseconds. Radioactive isotopes at or above mass number 93 decay by

text: electron emission

text: , whereas those at or below 89 decay by

text: positron emission

text: . The only exception is Zr, which decays by

text: electron capture

text: Natural uranium consists of three major isotopes: uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%). All three are radioactive, emitting alpha particles, with the exception that all three of these isotopes have small probabilities of undergoing spontaneous fission, rather than alpha emission. There are also five other trace isotopes: uranium-239, which is formed when U undergoes spontaneous fission, releasing neutrons that are captured by another U atom; uranium-237, which is formed when U captures a neutron but emits two more, which then decays to neptunium-237; and finally, uranium-233, which is formed in the decay chain of that neptunium-237. It is also expected that thorium-232 should be able to undergo double beta decay, which would produce uranium-232, but this has not yet been observed experimentally.

text: Natural uranium

text: consists of three major

text: isotope

text: s:

text: natural abundance

text: ), uranium-235 (0.71%), and

text: (0.0054%). All three are

text: radioactive

text: , emitting

text: alpha particle

text: s, with the exception that all three of these isotopes have small probabilities of undergoing

text: spontaneous fission

text: , rather than

text: alpha emission

text: . There are also five other trace isotopes: uranium-239, which is formed when U undergoes spontaneous fission, releasing neutrons that are captured by another U atom; uranium-237, which is formed when U captures a neutron but emits two more, which then decays to

text: ; and finally, uranium-233, which is formed in the

text: decay chain

text: of that neptunium-237. It is also expected that

text: should be able to undergo

text: double beta decay

text: , which would produce uranium-232, but this has not yet been observed experimentally.

text: The dose rate from a single uptake decays over time due to both radioactive decay, and biological decay (i.e. excretion from the body). The combined radioactive and biological half-life, called the effective half-life of the material, may range from hours for medical radioisotopes to decades for transuranic waste. Committed dose is the integral of this decaying dose rate over the presumed remaining lifespan of the organism. Most regulations require this integral to be taken over 50 years for uptakes during adulthood or over 70 years for uptakes during childhood. In dosimetry accounting, the entire committed dose is conservatively assigned to the year of uptake, even though it may take many years for the tissues to actually accumulate this dose.

text: The dose rate from a single uptake decays over time due to both

text: radioactive decay

text: , and biological decay (i.e. excretion from the body). The combined radioactive and

text: biological half-life

text: , called the

text: effective half-life

text: of the material, may range from hours for medical radioisotopes to decades for transuranic waste. Committed dose is the integral of this decaying dose rate over the presumed remaining lifespan of the organism. Most regulations require this integral to be taken over 50 years for uptakes during adulthood or over 70 years for uptakes during childhood. In

text: dosimetry

text: accounting, the entire committed dose is conservatively assigned to the year of uptake, even though it may take many years for the tissues to actually accumulate this dose.

text: Examples of non-radioactive medical isotopes are:

text: The main column shows times in seconds (31,556,926 seconds = 1 year), a second column showing half life in more usual units (year,days) is also provided.Entries starting with a ">" indicates that no decay has ever been observed, with null experiments establishing lower limits for the half-life. Such elements are considered stable unless a decay can be observed (establishing an actual estimate for the half-life). Note half lives may be imprecise estimates and can be subject to significant revision.

text: Naturally occurring decay heat is a significant source of the heat in the interior of the Earth.  Radioactive isotopes of uranium, thorium and potassium are the primary contributors to this decay heat, and this radioactive decay is the primary source of heat from which geothermal energy derives.

text: Naturally occurring decay heat is a significant source of the heat in the interior of the

text: Earth

text: .  Radioactive isotopes of

text: uranium

text: thorium

text: and

text: potassium

text: are the primary contributors to this decay heat, and this

text: radioactive decay

text: is the primary source of heat from which

text: geothermal energy

text: derives.

text: Radon is formed as one intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead. Thorium and uranium are the two most common radioactive elements on earth; they have been around since the earth was formed. Their naturally occurring isotopes have very long half-lives, on the order of billions of years. Thorium and uranium, their decay product radium, and its decay product radon, will therefore continue to occur for tens of millions of years at almost the same concentrations as they do now. As radon itself decays, it produces other radioactive elements called radon progeny (also known as radon daughters) or decay products. Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as dust particles in the air. If such contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.

text: Radon is formed as one intermediate step in the normal radioactive

text: decay chain

text: s through which

text: thorium

text: and

text: uranium

text: slowly decay into

text: lead

text: . Thorium and uranium are the two most common radioactive elements on earth; they have been around since the earth was formed. Their naturally occurring isotopes have very long half-lives, on the order of billions of years. Thorium and uranium, their decay product

text: radium

text: , and its decay product radon, will therefore continue to occur for tens of millions of years at almost the same concentrations as they do now. As radon itself decays, it produces other radioactive elements called radon progeny (also known as radon daughters) or decay products. Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as dust particles in the air. If such contaminated dust is inhaled, these particles can stick to the airways of the lung and increase the risk of developing lung cancer.

text: Einstein's equation is in no way an explanation of the large energies released in radioactive decay (this comes from the powerful nuclear forces involved; forces that were still unknown in 1905). In any case, the enormous energy released from radioactive decay (which had been measured by Rutherford) was much more easily measured than the (still small) change in the gross mass of materials, as a result. Einstein's equation, by theory, can give these energies by measuring mass differences before and after reactions, but in practice, these mass differences in 1905 were still too small to be measured in bulk. Prior to this, the ease of measuring radioactive decay energies with a calorimeter was thought possibly likely to allow measurement of changes in mass difference, as a check on Einstein's equation itself. Einstein mentions in his 1905 paper that mass–energy equivalence might perhaps be tested with radioactive decay, which releases enough energy (the quantitative amount known roughly by 1905) to possibly be "weighed," when missing from the system (having been given off as heat). However, radioactivity seemed to proceed at its own unalterable (and quite slow, for radioactives known then) pace, and even when simple nuclear reactions became possible using proton bombardment, the idea that these great amounts of usable energy could be liberated at will with any practicality, proved difficult to substantiate. Rutherford was reported in 1933 to have declared that this energy could not be exploited efficiently: "Anyone who expects a source of power from the transformation of the atom is talking moonshine."

text: Einstein's equation is in no way an explanation of the large energies released in radioactive decay (this comes from the powerful

text: nuclear force

text: moonshine

text: This technique has two different applications in the earth and environmental sciences. The analysis of 'stable isotopes' is normally concerned with measuring isotopic variations arising from mass-dependent isotopic fractionation in natural systems. On the other hand, radiogenic isotope analysis involves measuring the abundances of decay-products of natural radioactivity, and is used in most long-lived radiometric dating methods.

text: This technique has two different applications in the earth and environmental sciences. The analysis of '

text: stable isotopes

text: ' is normally concerned with measuring isotopic variations arising from mass-dependent

text: isotopic fractionation

text: in natural systems. On the other hand,

text: radiogenic

text: isotope analysis involves measuring the abundances of decay-products of natural radioactivity, and is used in most long-lived

text: radiometric dating

text: methods.

text: Suitable gamma-ray sources consist of a radioactive parent that decays to the desired isotope. For example, the source for Fe consists of Co, which decays by electron capture to an excited state of Fe, which in turn decays to a ground state emitting a gamma-ray of the appropriate energy. The radioactive cobalt is prepared on a foil, often of rhodium. Ideally the parent isotope will have a convenient half-life. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction resulting in a poor signal-to-noise ratio and requiring long collection times. The periodic table below indicates those elements having an isotope suitable for Mössbauer spectroscopy. Of these, Fe is by far the most common element studied using the technique, although I, Sn, and Sb are also frequently studied.

text: Suitable gamma-ray sources consist of a radioactive parent that decays to the desired isotope. For example, the source for

text: Fe

text: consists of

text: Co

text: , which decays by

text: electron capture

text: to an

text: excited state

text: of Fe, which in turn decays to a

text: ground state

text: emitting a gamma-ray of the appropriate energy. The radioactive cobalt is prepared on a foil, often of rhodium. Ideally the

text: parent

text: isotope will have a convenient half-life. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction resulting in a poor

text: signal-to-noise ratio

text: and requiring long collection times. The periodic table below indicates those elements having an isotope suitable for Mössbauer spectroscopy. Of these,

text: Fe

text: is by far the most common element studied using the technique, although

text: I

text: Sn

text: , and

text: Sb

text: are also frequently studied.

text: The above calculations make several assumptions, such as that the level of  in the atmosphere has remained constant over time. In fact, the level of  in the atmosphere has varied significantly and as a result the values provided by the equation above have to be corrected by using data from other sources.  This is done by calibration curves, which convert a measurement of  in a sample into an estimated calendar age.  The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: an age quoted in radiocarbon years means that no calibration curve has been used − the calculations for radiocarbon years assume that the / ratio has not changed over time.  Calculating radiocarbon ages also requires the value of the half-life for , which for more than a decade after Libby's initial work was thought to be 5,568 years.  This was revised in the early 1960s to 5,730 years, which meant that many calculated dates in papers published prior to this were incorrect (the error in the half-life is about 3%).  For consistency with these early papers, and to avoid the risk of a double correction for the incorrect half-life, radiocarbon ages are still calculated using the incorrect half-life value.  A correction for the half-life is incorporated into calibration curves, so even though radiocarbon ages are calculated using a half-life value that is known to be incorrect, the final reported calibrated date, in calendar years, is accurate.  When a date is quoted, the reader should be aware that if it is an uncalibrated date (a term used for dates given in radiocarbon years) it may differ substantially from the best estimate of the actual calendar date, both because it uses the wrong value for the half-life of , and because no correction (calibration) has been applied for the historical variation of  in the atmosphere over time.

text: Atomic nuclei consist of protons and neutrons, which attract each other through the nuclear force, while protons repel each other via the electric force due to their positive charge. These two forces compete, leading to some combinations of neutrons and protons being more stable than others. Neutrons stabilize the nucleus, because they attract protons, which helps offset the electrical repulsion between protons. As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus; if too many or too few neutrons are present with regard to the optimum ratio, the nucleus becomes unstable and subject to certain types of nuclear decay. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture. Many other rare types of decay, such as spontaneous fission or cluster decay are known. (See radioactive decay for details.)

text: Atomic

text: nuclei

text: consist of

text: proton

text: s and

text: neutron

text: s, which attract each other through the

text: nuclear force

text: , while protons repel each other via the

text: electric force

text: due to their positive

text: charge

text: . These two forces compete, leading to some combinations of neutrons and protons being more stable than others. Neutrons stabilize the nucleus, because they attract protons, which helps offset the electrical repulsion between protons. As a result, as the number of protons increases, an increasing

text: ratio of neutrons to protons

text: is needed to form a stable nucleus; if too many or too few neutrons are present with regard to the optimum ratio, the nucleus becomes unstable and subject to certain types of

text: nuclear decay

text: . Unstable isotopes decay through various

text: radioactive decay pathways

text: , most commonly

text: alpha decay

text: beta decay

text: , or

text: electron capture

text: . Many other rare types of decay, such as

text: spontaneous fission

text: or

text: cluster decay

text: are known. (See

text: radioactive decay

text: for details.)

text: Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation. The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.

text: Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a

text: characteristic unit

text: for the exponential decay equation. The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed.

text: There are no stable isotopes of astatine. However, there are three naturally occurring radioactive isotopes of astatine produced via radioactive decay of uranium, neptunium, and plutonium. These isotopes are astatine-215, astatine-217, and astatine-219. A total of 31 isotopes of astatine have been discovered, with atomic masses ranging from 193 to 223.

text: There are no stable

text: isotopes of astatine

text: . However, there are three naturally occurring radioactive isotopes of astatine produced via radioactive decay of

text: uranium

text: neptunium

text: , and

text: plutonium

text: . These isotopes are astatine-215, astatine-217, and astatine-219. A total of 31 isotopes of astatine have been discovered, with atomic masses ranging from 193 to 223.

text: In practice, the most commonly used small laboratory sources of neutrons use radioactive decay to power neutron production. One noted neutron-producing radioisotope, californium-252 decays (half-life 2.65 years) by spontaneous fission 3% of the time with production of 3.7 neutrons per fission, and is used alone as a neutron source from this process. Nuclear reaction sources (that involve two materials) powered by radioisotopes use an alpha decay source plus a beryllium target, or else a source of high-energy gamma radiation from a source that undergoes beta decay followed by gamma decay, which produces photoneutrons on interaction of the high energy gamma ray with ordinary stable beryllium, or else with the deuterium in heavy water. A popular source of the latter type is radioactive antimony-124 plus beryllium, a system with a half-life of 60.9 days, which can be constructed from natural antimony (which is 42.8% stable antimony-123) by activating it with neutrons in a nuclear reactor, then transported to where the neutron source is needed.

text: In practice, the most commonly used small laboratory sources of neutrons use radioactive decay to power neutron production. One noted neutron-producing

text: radioisotope

text: californium

text: -252 decays (half-life 2.65 years) by

text: spontaneous fission

text: 3% of the time with production of 3.7 neutrons per fission, and is used alone as a neutron source from this process.

text: Nuclear reaction

text: sources (that involve two materials) powered by radioisotopes use an

text: alpha decay

text: source plus a beryllium target, or else a source of high-energy gamma radiation from a source that undergoes

text: beta decay

text: followed by

text: gamma decay

text: , which produces photoneutrons on interaction of the high energy

text: gamma ray

text: with ordinary stable beryllium, or else with the

text: deuterium

text: in

text: heavy water

text: . A popular source of the latter type is radioactive

text: plus beryllium, a system with a half-life of 60.9 days, which can be constructed from natural antimony (which is 42.8% stable antimony-123) by activating it with neutrons in a nuclear reactor, then transported to where the neutron source is needed.

text: The radioactive decay of elements in the Earth's mantle and crust results in production of daughter isotopes and release of particles and heat energy, or radiogenic heat. Four radioactive isotopes are responsible for the majority of radiogenic heat, uranium-238 (U), uranium-235 (U), thorium-232 (Th), and potassium-40 (K). Due to a lack of rock samples from below 200 km depth, it is not possible to do a simple radiogenic heat estimate of known radioactive isotope concentrations in rock throughout the whole mantle. For the Earth's core, geochemical studies indicate that it would not be a significant source of radiogenic heat due to an expected low concentration of radioactive elements. Radiogenic heat production in the mantle is linked to the structure of mantle convection, a topic of much debate, and it is thought that the mantle may either have a layered structure with a higher concentration of radioactive heat-producing elements in the lower mantle, or small reservoirs enriched in radioactive elements dispersed throughout the whole mantle.

text: The

text: radioactive decay

text: of elements in the Earth's mantle and crust results in production of daughter

text: isotope

text: s and release of particles and heat energy, or

text: radiogenic heat

text: . Four radioactive isotopes are responsible for the majority of radiogenic heat,

text: (U),

text: (U),

text: (Th), and

text: (K). Due to a lack of rock samples from below 200 km depth, it is not possible to do a simple radiogenic heat estimate of known radioactive isotope concentrations in rock throughout the whole mantle. For the Earth's core,

text: geochemical

text: studies indicate that it would not be a significant source of radiogenic heat due to an expected low concentration of radioactive elements. Radiogenic heat production in the mantle is linked to the structure of

text: mantle convection

text: , a topic of much debate, and it is thought that the mantle may either have a layered structure with a higher concentration of radioactive heat-producing elements in the lower mantle, or small reservoirs enriched in radioactive elements dispersed throughout the whole mantle.

text: This is a summary table of decay class for the ~986 nuclides with half-lives longer than one hour, given in this list of nuclides. Numbers may change slightly in the future, as nuclides radioactive only in theory are observed to be radioactive, half-life measurements improve, or new isotopes are discovered.

text: Clocks, watches, and instruments dating from the first half of the 20th century, often in military applications, may have been painted with radioactive luminous paint. They are usually no longer luminous; however, this is not due to radioactive decay of the radium (which has a half-life of 1600 years) but to the fluorescence of the zinc sulfide fluorescent medium being worn out by the radiation from the radium. The appearance of an often thick layer of green or yellowish brown paint in devices from this period suggests a radioactive hazard. The radiation dose from an intact device is relatively low and usually not an acute risk; but the paint is dangerous if released and inhaled or ingested.

text: All other cerium isotopes are synthetic and radioactive. The most stable of them are Ce with a half-life of 284.9 days, Ce with a half-life of 137.6 days, Ce with a half-life of 33.04 days, and Ce with a half-life of 32.5 days. All other radioactive cerium isotopes have half-lives under four days, and most of them have half-lives under ten minutes. The isotopes between Ce and Ce inclusive occur as fission products of uranium. The primary decay mode of the isotopes lighter than Ce is inverse beta decay or electron capture to isotopes of lanthanum, while that of the heavier isotopes is beta decay to isotopes of praseodymium.

text: All other cerium isotopes are

text: synthetic

text: and

text: radioactive

text: . The most stable of them are Ce with a half-life of 284.9 days, Ce with a half-life of 137.6 days, Ce with a half-life of 33.04 days, and Ce with a half-life of 32.5 days. All other radioactive cerium isotopes have half-lives under four days, and most of them have half-lives under ten minutes. The isotopes between Ce and Ce inclusive occur as

text: fission product

text: s of

text: uranium

text: . The primary decay mode of the isotopes lighter than Ce is

text: inverse beta decay

text: or

text: electron capture

text: to

text: isotopes of lanthanum

text: , while that of the heavier isotopes is

text: beta decay

text: to

text: isotopes of praseodymium

text: Naturally occurring terbium is composed of its only stable isotope, terbium-159; the element is thus called mononuclidic and monoisotopic. Thirty six radioisotopes have been characterized, with the heaviest being terbium-171 (with atomic mass of 170.95330(86) u) and lightest being terbium-135 (exact mass unknown). The most stable synthetic radioisotopes of terbium are terbium-158, with a half-life of 180 years, and terbium-157, with a half-life of 71 years. All of the remaining radioactive isotopes have half-lives that are much less than a quarter of a year, and the majority of these have half-lives that are less than half a minute. The primary decay mode before the most abundant stable isotope, Tb, is electron capture, which results in production of gadolinium isotopes, and the primary mode after is beta minus decay, resulting in dysprosium isotopes.

text: Naturally occurring terbium is composed of its only stable

text: isotope

text: , terbium-159; the element is thus called

text: mononuclidic

text: and

text: monoisotopic

text: . Thirty six

text: radioisotope

text: s have been characterized, with the heaviest being terbium-171 (with

text: atomic mass

text: of 170.95330(86)

text: u

text: ) and lightest being terbium-135 (exact mass unknown). The most stable

text: synthetic radioisotope

text: s of terbium are terbium-158, with a

text: half-life

text: of 180 years, and terbium-157, with a half-life of 71 years. All of the remaining

text: radioactive

text: isotopes have half-lives that are much less than a quarter of a year, and the majority of these have half-lives that are less than half a minute. The primary

text: decay mode

text: before the most abundant stable isotope, Tb, is

text: electron capture

text: , which results in production of

text: gadolinium

text: isotopes, and the primary mode after is

text: beta minus decay

text: , resulting in

text: dysprosium

text: isotopes.

text: In the earlier period between 3.8 and 4.1 Ga, changes in the orbits of the giant planets may have caused a heavy bombardment by asteroids and comets that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have repeatedly sterilized the planet, had life appeared before that time. Geologically, the Hadean Earth would have been far more active than at any other time in its history. Studies of meteorites suggests that radioactive isotopes such as aluminium-26 with a half-life of 7.17×10 years, and potassium-40 with a half-life of 1.250×10 years, isotopes mainly produced in supernovae, were much more common. Coupled with internal heating as a result of gravitational sorting between the core and the mantle, there would have been a great deal of mantle convection, with the probable result of many smaller and much more active tectonic plates than now exist.

text: In the earlier period between 3.8 and 4.1 Ga, changes in the orbits of the

text: giant planet

text: s may have caused a

text: heavy bombardment

text: by asteroids and

text: comet

text: s that pockmarked the

text: Moon

text: and the other inner planets (

text: Mercury

text: Mars

text: , and presumably Earth and

text: Venus

text: ). This would likely have repeatedly sterilized the planet, had life appeared before that time. Geologically, the Hadean Earth would have been far more active than at any other time in its history. Studies of

text: meteorite

text: s suggests that

text: radioactive isotopes

text: such as

text: with a

text: half-life

text: of 7.17×10 years, and

text: with a half-life of 1.250×10 years, isotopes mainly produced in

text: supernova

text: e, were much more common. Coupled with internal heating as a result of

text: gravitational sorting

text: between the

text: core

text: and the

text: mantle

text: , there would have been a great deal of

text: mantle convection

text: , with the probable result of many smaller and much more active tectonic plates than now exist.

text: By this time, geologists recognised that the Earth was millions of years old. The exact chronology proposed by Darwin was disputed by other geologists, and the leading physicist William Thomson (later enobled as Lord Kelvin) produced an analyses of the heat energy and thermal histories of the Earth and Sun which yielded age estimates that were too short for gradual evolution. Thomson's colleague Fleeming Jenkin wrote an 1867 review of Darwin's On the Origin of species which opposed evolution on the basis of the shortened time available. Kelvin's age paradox was not resolved until it was discovered in the 20th century that the Earth is heated by radioactive decay, that its internal thermal gradient is affected by mantle convection, and that the Sun is heated by nuclear fusion.

text: By this time, geologists recognised that the Earth was millions of years old. The exact chronology proposed by Darwin was disputed by other geologists, and the leading physicist

text: William Thomson

text: (later enobled as Lord Kelvin) produced an analyses of the heat energy and thermal histories of

text: the Earth

text: and Sun which yielded age estimates that were too short for gradual evolution. Thomson's colleague

text: Fleeming Jenkin

text: wrote an 1867 review of Darwin's On the Origin of species which opposed evolution on the basis of the shortened time available. Kelvin's age paradox was not

text: resolved

text: until it was discovered in the 20th century that the Earth is heated by radioactive decay, that its internal thermal gradient is affected by mantle convection, and that the Sun is heated by nuclear fusion.

text: They started with developing expansions to Valve Software's Half-Life. Porting Half-Life to console platforms (each with new game content) followed, building the company's experience in console game-making, in addition to enhancing and building upon the successful Counter-Strike branch of the Half-Life franchise. Prior to Half-Life 2, they had developed or helped develop every Half-Life expansion game or port, including Opposing Force, Blue Shift, Counter-Strike: Condition Zero, Half-Life for the Sony PlayStation 2 (including Half-Life Decay), and Half-Life for the Sega Dreamcast (including Blue Shift). Branching out to other publishers, they pursued additional port work, each game being released with additional content, but this time from console to PC. These projects included their first non-first-person shooter, Tony Hawk's Pro Skater 3, and Halo: Combat Evolved, forging new publisher relationships with Activision and Microsoft Game Studios respectively. Additional new development, in the form of a PC game in the James Bond franchise (James Bond 007: Nightfire) for Electronic Arts, also occurred during the company's initial 5-year period.

text: They started with developing expansions to

text: Valve Software

text: 's

text: Half-Life

text: . Porting Half-Life to console platforms (each with new game content) followed, building the company's experience in console game-making, in addition to enhancing and building upon the successful

text: Counter-Strike

text: branch of the Half-Life franchise. Prior to

text: Half-Life 2

text: , they had developed or helped develop every Half-Life expansion game or port, including

text: Opposing Force

text: Blue Shift

text: Counter-Strike: Condition Zero

text: , Half-Life for the Sony

text: PlayStation 2

text: (including

text: Half-Life Decay

text: ), and Half-Life for the Sega

text: Dreamcast

text: (including Blue Shift). Branching out to other publishers, they pursued additional

text: port

text: work, each game being released with additional content, but this time from console to PC. These projects included their first non-first-person shooter,

text: Tony Hawk's Pro Skater 3

text: , and

text: Halo: Combat Evolved

text: , forging new publisher relationships with

text: Activision

text: and

text: Microsoft Game Studios

text: respectively. Additional new development, in the form of a PC game in the

text: James Bond

text: franchise (

text: James Bond 007: Nightfire

text: ) for

text: Electronic Arts

text: , also occurred during the company's initial 5-year period.

text: In hydraulic fracturing, radioactive tracer isotopes are injected with hydraulic fracturing fluid to determine the injection profile and location of created fractures.  Tracers with different half-lives are used for each stage of hydraulic fracturing. In the United States amounts per injection of radionuclide are listed in the US Nuclear Regulatory Commission (NRC) guidelines.  According to the NRC, some of the most commonly used tracers include antimony-124, bromine-82, iodine-125, iodine-131, iridium-192, and scandium-46. A 2003 publication by the International Atomic Energy Agency confirms the frequent use of most of the tracers above, and says that manganese-56, sodium-24, technetium-99m, silver-110m, argon-41, and xenon-133 are also used extensively because they are easily identified and measured.

text: In

text: hydraulic fracturing

text: , radioactive tracer isotopes are injected with hydraulic fracturing fluid to determine the injection profile and location of created fractures.  Tracers with different half-lives are used for each stage of hydraulic fracturing. In the United States amounts per injection of radionuclide are listed in the US

text: Nuclear Regulatory Commission

text: (NRC) guidelines.  According to the NRC, some of the most commonly used tracers include

text: , and

text: . A 2003 publication by the

text: International Atomic Energy Agency

text: confirms the frequent use of most of the tracers above, and says that

text: , and

text: are also used extensively because they are easily identified and measured.

text: It is important to note that a vast number of processes are able to release radioactivity into the environment, for example the action of cosmic rays on the air is responsible for the formation of radioisotopes (such as C and P), the decay of Ra forms Rn which is a gas which can diffuse through rocks before entering buildings and dissolve in water and thus enter drinking water in addition human activities such as bomb tests, accidents, and normal releases from industry have resulted in the release of radioactivity.

text: It is important to note that a vast number of processes are able to release radioactivity into the environment, for example the action of

text: cosmic ray

text: s on the air is responsible for the formation of radioisotopes (such as C and P), the decay of Ra forms Rn which is a gas which can diffuse through rocks before entering buildings and dissolve in water and thus enter

text: drinking water

text: in addition human activities such as

text: bomb test

text: s, accidents, and normal releases from industry have resulted in the release of radioactivity.

text: Some isotopes are radioactive, and are therefore referred to as radioisotopes or radionuclides, whereas others have never been observed to decay radioactively and are referred to as stable isotopes or stable nuclides. For example,  is a radioactive form of carbon, whereas  and  are stable isotopes. There are about 339 naturally occurring nuclides on Earth, of which 286 are primordial nuclides, meaning that they have existed since the Solar System's formation.

text: Some isotopes are

text: radioactive

text: , and are therefore referred to as radioisotopes or

text: radionuclide

text: s, whereas others have never been observed to decay radioactively and are referred to as stable isotopes or

text: stable nuclide

text: s. For example,  is a radioactive form of carbon, whereas  and  are stable isotopes. There are about 339 naturally occurring nuclides on Earth, of which 286 are

text: primordial nuclide

text: s, meaning that they have existed since the

text: Solar System

text: 's formation.

text: In nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations. They are also known as "radioactive cascades". Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.

text: In

text: nuclear science

text: , the decay chain refers to the

text: radioactive decay

text: of different discrete radioactive

text: decay product

text: s as a chained series of transformations. They are also known as "radioactive cascades". Most

text: radioisotopes

text: do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable

text: isotope

text: is reached.

text: The half-life of a reaction describes the time needed for half of the reactant to be depleted (same as the half-life involved in nuclear decay, which is a first order reaction). For a zero order reaction the half-life is given by

text: The half-life of a reaction describes the time needed for half of the reactant to be depleted (same as the

text: half-life

text: involved in

text: nuclear decay

text: , which is a first order reaction). For a zero order reaction the half-life is given by

text: Barium found in the Earth's crust is a mixture of seven primordial nuclides, barium-130, 132, and 134 through 138. Barium-130 undergoes very slow radioactive decay to xenon-130 by double beta plus decay, and barium-132 theoretically decays like xenon-132, with half-lives a thousand times greater than the age of the Universe. The abundance is ~0.1% that of natural barium. The radioactivity of these isotopes is so weak that they pose no danger to life.

text: Barium found in the Earth's crust is a mixture of seven primordial nuclides, barium-130, 132, and 134 through 138. Barium-130 undergoes very slow

text: radioactive decay

text: to

text: xenon

text: -130 by double

text: beta plus decay

text: , and barium-132 theoretically decays like xenon-132, with half-lives a thousand times greater than the

text: age of the Universe

text: . The abundance is ~0.1% that of natural barium. The radioactivity of these isotopes is so weak that they pose no danger to life.

text: Of the 94 naturally occurring elements, 84 are considered primordial and either stable or weakly radioactive. The remaining 10 naturally occurring elements possess half lives too short for them to have been present at the beginning of the Solar System, and are therefore considered transient elements. (Plutonium is usually also considered a transient element because primordial plutonium has by now decayed to almost undetectable traces.) Of these 10 transient elements, 5 (polonium, radon, radium, actinium, and protactinium) are relatively common decay products of thorium, uranium, and plutonium. The remaining 6 transient elements (technetium, promethium, astatine, francium, neptunium, and plutonium) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

text: Of the 94 naturally occurring elements, 84 are considered

text: primordial

text: and either

text: stable

text: or weakly radioactive. The remaining 10 naturally occurring elements possess

text: half lives

text: too short for them to have been present at the beginning of the

text: Solar System

text: , and are therefore considered transient elements. (Plutonium is usually also considered a transient element because primordial plutonium has by now decayed to almost undetectable traces.) Of these 10 transient elements, 5 (

text: polonium

text: radon

text: radium

text: actinium

text: , and

text: protactinium

text: ) are relatively common

text: decay products

text: of

text: thorium

text: uranium

text: , and

text: plutonium

text: . The remaining 6 transient elements (

text: technetium

text: promethium

text: astatine

text: francium

text: neptunium

text: , and

text: plutonium

text: ) occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.

text: Depend on properties of Sm-Nd radioactive isotope system can provide age estimation of sedimentary source rocks. It has been used in provenance studies. Nd is produced by α decay of Sm and has a half life of 1.06×10 years. Variation of Nd/Nd is caused by decay of Sm. Now Sm/Nd rato of the mantle is higher than that of the crust and Nd/Nd ratio is also higher than in the mantle than in the crust. Nd/Nd ratio is expressed in εNd notation (DePaolo and Wasserbur 1976).

text: Depend on properties of

text: Sm-Nd

text: radioactive isotope system can provide age estimation of sedimentary source rocks. It has been used in provenance studies. Nd is produced by α decay of Sm and has a half life of 1.06×10 years. Variation of Nd/Nd is caused by decay of Sm. Now Sm/Nd rato of the mantle is higher than that of the crust and Nd/Nd ratio is also higher than in the mantle than in the crust. Nd/Nd ratio is expressed in εNd notation (DePaolo and Wasserbur 1976).

text: Natural radioactivity is the spontaneous decay of the atoms of certain isotopes into other isotopes. If the resultant isotope is not stable, it undergoes further decay until a stable isotope forms. The decay process is usually accompanied by emissions of alpha, beta, and gamma radiation.  Natural gamma ray radiation is one form of spontaneous radiation emitted by unstable nuclei.  Gamma (γ) radiation may be considered either as an electromagnetic wave similar to visible light or X-rays, or as a particle of photon. Gamma rays are electromagnetic radiations emitted from an atomic nucleus during radioactive decay, with the wavelength in the range of 10 to 10cm

text: Natural radioactivity is the spontaneous decay of the atoms of certain isotopes into other isotopes. If the resultant isotope is not stable, it undergoes further decay until a stable isotope forms. The decay process is usually accompanied by emissions of alpha,

text: beta

text: , and gamma radiation.  Natural gamma ray radiation is one form of spontaneous radiation emitted by unstable nuclei.  Gamma (γ) radiation may be considered either as an electromagnetic wave similar to visible light or X-rays, or as a particle of photon. Gamma rays are electromagnetic radiations emitted from an atomic nucleus during radioactive decay, with the wavelength in the range of 10 to 10cm

text: Disintegrations per minute (dpm) and disintegrations per second (dps) are measures of the activity of the source of radioactivity. The SI unit of radioactivity, the becquerel (Bq), is equivalent to one disintegration per second. This unit should not be confused with cps, which is the number of counts received by an instrument from the source. One dps (one dpm) is the number of atoms that have decayed in one second (one minute), not the number of atoms that have been measured as decayed.

text: Disintegrations per minute (dpm) and disintegrations per second (dps) are measures of the activity of the source of radioactivity. The SI unit of radioactivity, the

text: becquerel

text: (Bq), is equivalent to one disintegration per second. This unit should not be confused with cps, which is the number of counts received by an instrument from the source. One dps (one dpm) is the number of atoms that have decayed in one second (one minute), not the number of atoms that have been measured as decayed.

text: At the same time, Daneel and Giskard are engaged in a struggle of wits with Fastolfe's rivals: the roboticists Kelden Amadiro and Vasilia Aliena, Fastolfe's estranged daughter. Frustrated by his series of failures, Amadiro accepts an ambitious and unscrupulous apprentice, Levular Mandamus, who plans to destroy the population of the Earth by a newly developed weapon, the "nuclear intensifier", with which to accelerate the natural radioactive decay in the upper crust of the Earth, thereby making the surface of the Earth radioactive. R. Daneel and R. Giskard discover the roboticists' plan, and attempt to stop Amadiro; but are hampered by the First Law of Robotics,

text: At the same time, Daneel and Giskard are engaged in a struggle of wits with Fastolfe's rivals: the

text: roboticist

text: s Kelden Amadiro and

text: Vasilia Aliena

text: , Fastolfe's estranged daughter. Frustrated by his series of failures, Amadiro accepts an ambitious and unscrupulous apprentice, Levular Mandamus, who plans to destroy the population of the Earth by a newly developed weapon, the "nuclear intensifier", with which to accelerate the natural

text: radioactive decay

text: in the upper crust of the Earth, thereby making the surface of the Earth radioactive. R. Daneel and R. Giskard discover the roboticists' plan, and attempt to stop Amadiro; but are hampered by the

text: First Law of Robotics

text: On 18 April 1959, the Navy disposed of the radioactive S2G plant by sealing it in a 30-foot high stainless steel containment vessel, towing it out to sea on a barge, and then sinking the barge at a point about 120 miles due east of Maryland in 9,100 feet of water. Twenty-one years later, the Navy was unable to relocate the container, but said that the radioactive materials inside should decay before the containment vessel deteriorated.

text: While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one element at each position on the periodic table. The term isotope was coined by Margaret Todd as a suitable name for these elements.

text: radioactive decay

text: , in 1913

text: radiochemist

text: Frederick Soddy

text: discovered that there appeared to be more than one element at each position on the

text: periodic table

text: . The term

text: isotope

text: was coined by

text: Margaret Todd

text: as a suitable name for these elements.

text: Note that there are two breaks where the unstable elements technetium (atomic number: 43) and promethium (atomic number: 61) would be. These are both extremely rare, since on Earth they are only produced through the spontaneous fission of very heavy radioactive elements (for example, uranium, thorium, or the trace amounts of plutonium that exist in uranium ores), or by the interaction of certain other elements with cosmic rays. Both of the first two of these elements have been identified spectroscopically in the atmospheres of stars, where they are produced by ongoing nucleosynthetic processes. There are also breaks where the six noble gases would be, since they are not chemically bound in the Earth's crust, and they are only generated by decay chains from radioactive elements and are therefore extremely rare there. The eight naturally occurring very rare, highly radioactive elements (polonium, astatine, francium, radium, actinium, protactinium, neptunium, and plutonium) are not included, since any of these elements that were present at the formation of the Earth have decayed away eons ago, and their quantity today is negligible and is only produced from the radioactive decay of uranium and thorium.

text: Note that there are two breaks where the unstable elements

text: technetium

text: (atomic number: 43) and

text: promethium

text: (atomic number: 61) would be. These are both extremely rare, since on Earth they are only produced through the

text: spontaneous fission

text: of very heavy

text: radioactive

text: elements (for example,

text: uranium

text: thorium

text: , or the trace amounts of

text: plutonium

text: that exist in uranium ores), or by the interaction of certain other elements with

text: cosmic ray

text: s. Both of the first two of these elements have been identified spectroscopically in the atmospheres of stars, where they are produced by ongoing nucleosynthetic processes. There are also breaks where the six

text: noble gas

text: es would be, since they are not chemically bound in the Earth's crust, and they are only generated by decay chains from radioactive elements and are therefore extremely rare there. The eight naturally occurring very rare, highly radioactive elements (

text: polonium

text: astatine

text: francium

text: radium

text: actinium

text: protactinium

text: neptunium

text: , and

text: plutonium

text: ) are not included, since any of these elements that were present at the formation of the Earth have decayed away eons ago, and their quantity today is negligible and is only produced from the

text: radioactive decay

text: of uranium and thorium.

text: Consistent with the theory of confirmation holism, some scholars assert "fact" to be necessarily "theory-laden" to some degree. Thomas Kuhn points out that knowing what facts to measure, and how to measure them, requires the use of other theories. For example, the age of fossils is based on radiometric dating, which is justified by reasoning that radioactive decay follows a Poisson process rather than a Bernoulli process. Similarly, Percy Williams Bridgman is credited with the methodological position known as operationalism, which asserts that all observations are not only influenced, but necessarily defined by the means and assumptions used to measure them.

text: Consistent with the theory of

text: confirmation holism

text: , some scholars assert "fact" to be necessarily "theory-laden" to some degree.

text: Thomas Kuhn

text: points out that knowing what facts to measure, and how to measure them, requires the use of other theories. For example, the age of

text: fossils

text: is based on

text: radiometric dating

text: , which is justified by reasoning that radioactive decay follows a

text: Poisson process

text: rather than a

text: Bernoulli process

text: . Similarly,

text: Percy Williams Bridgman

text: is credited with the methodological position known as operationalism, which asserts that all observations are not only influenced, but necessarily defined by the means and assumptions used to measure them.

text: More generally, nuclides with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission, and they also have relatively much longer partial half-lives for alpha or beta decay. Examples of these isotopes are uranium-238 and thorium-232. On the other hand, nuclei with an odd number of protons and an odd number of neutrons  (odd Z, odd N) are usually short-lived (a notable exception is neptunium-236 with a half-life of 154,000 years) because they readily decay by beta-particle emission to their isobars with an even number of protons and an even number of neutrons (even Z, even N) becoming much more stable.  The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.

text: More generally, nuclides with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to

text: absorb the neutron

text: but without gaining enough energy from the process to deform the nucleus enough for it to fission. These

text: "even-even"

text: isotopes are also less likely to undergo

text: spontaneous fission

text: , and they also have relatively much longer

text: partial half-lives

text: for

text: alpha

text: or

text: beta

text: decay. Examples of these isotopes are uranium-238 and

text: . On the other hand, nuclei with an odd number of protons and an odd number of neutrons  (odd Z, odd N) are usually short-lived (a notable exception is

text: with a half-life of 154,000 years) because they readily

text: decay by beta-particle emission

text: to their

text: isobars

text: with an even number of protons and an even number of neutrons (even Z, even N) becoming much more stable.  The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton–proton and neutron–neutron pairing. The relatively short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.

text: In isotopic labeling, there are multiple ways to detect the presence of labeling isotopes; through their mass, vibrational mode, or radioactive decay. Mass spectrometry detects the difference in an isotope's mass, while infrared spectroscopy detects the difference in the isotope's vibrational modes. Nuclear magnetic resonance detects atoms with different gyromagnetic ratios. The radioactive decay can be detected through an ionization chamber or autoradiographs of gels.

text: In isotopic labeling, there are multiple ways to detect the presence of labeling isotopes; through their

text: mass

text: vibrational mode

text: , or

text: radioactive decay

text: Mass spectrometry

text: detects the difference in an isotope's mass, while

text: infrared spectroscopy

text: detects the difference in the isotope's vibrational modes.

text: Nuclear magnetic resonance

text: detects atoms with different gyromagnetic ratios. The radioactive decay can be detected through an

text: ionization chamber

text: or

text: autoradiographs

text: of gels.

text: C-14 undergoes Beta decay giving non-radioactive Nitrogen and high energy Beta particles .

text: C-14 undergoes

text: Beta decay

text: giving non-radioactive Nitrogen and high energy

text: Beta particles

text: St. Gallen is notable for reporting the highest maximum radioactivity measurements of any Swiss city, as published in the 2009 yearly report by the Federal Office of Public Health. While the daily average level of gamma-ray radioactivity in the city is unremarkable at 105 nSv/h, the maximum can reach 195 nSv/h, as high as the average for Jungfraujoch, the location with the highest reported level of radioactivity in Switzerland, due to its high elevation and therefore greater exposure to cosmic rays. The same report explains that the unusually high spikes of radioactivity measured in St. Gallen are due to radioactive products of radon gas being washed to the ground during heavy storms, but does not explain where the sufficient quantities of radon gas and its products to account for the anomaly would come from. The yearly report for 2009 on risks associated with radon published by the same governmental agency shows St. Gallen to lie in an area of the lowest level of radon exposure. In addition to the measured gamma-radiation, the city may be subject to radioactive tritium pollution in Teufen, a satellite town situated 4 km south of the city in the canton of Appenzell Outer Rhodes (this pollution is also covered in the report).

text: St. Gallen is notable for reporting the highest maximum radioactivity measurements of any Swiss city, as published in the 2009 yearly report by the

text: Federal Office of Public Health

text: . While the daily average level of gamma-ray radioactivity in the city is unremarkable at 105 nSv/h, the maximum can reach 195 nSv/h, as high as the average for

text: Jungfraujoch

text: , the location with the highest reported level of radioactivity in Switzerland, due to its high elevation and therefore greater exposure to cosmic rays. The same report explains that the unusually high spikes of radioactivity measured in St. Gallen are due to radioactive products of

text: radon

text: gas being washed to the ground during heavy storms, but does not explain where the sufficient quantities of radon gas and its products to account for the anomaly would come from. The yearly report for 2009 on risks associated with radon published by the same governmental agency shows St. Gallen to lie in an area of the lowest level of radon exposure. In addition to the measured gamma-radiation, the city may be subject to radioactive

text: tritium

text: pollution in

text: Teufen

text: , a satellite town situated 4 km south of the city in the canton of Appenzell Outer Rhodes (this pollution is also covered in the report).

text: The most common forms of radioactive decay are:

text: In nuclear physics, the following fundamental relationship governs radioactive decay: the number of radioactive atoms  in a sample of a radioactive isotope decays at a rate proportional to .  This leads to the first order linear differential equation

text: In

text: nuclear physics

text: , the following fundamental relationship governs

text: radioactive decay

text: : the number of radioactive atoms  in a sample of a radioactive

text: isotope

text: decays at a rate proportional to .  This leads to the first order linear differential equation

text: Since the fraction of a radioisotope's atoms decaying per unit of time is inversely proportional to its half-life, the relative radioactivity of a quantity of buried human radioactive waste would diminish over time compared to natural radioisotopes (such as the decay chain of 120 trillion tons of thorium and 40 trillion tons of uranium which are at relatively trace concentrations of parts per million each over the crust's 3 ton mass). For instance, over a timeframe of thousands of years, after the most active short half-life radioisotopes decayed, burying U.S. nuclear waste would increase the radioactivity in the top 2000 feet of rock and soil in the United States (10 million km) by ≈ 1 part in 10 million over the cumulative amount of natural radioisotopes in such a volume, although the vicinity of the site would have a far higher concentration of artificial radioisotopes underground than such an average.

text: Since the fraction of a radioisotope's atoms decaying per unit of time is inversely proportional to its half-life, the relative radioactivity of a quantity of buried human

text: radioactive waste

text: would diminish over time compared to natural radioisotopes (such as the decay chain of 120 trillion tons of thorium and 40 trillion tons of uranium which are

text: at relatively trace concentrations of parts per million each

text: over the crust's 3 ton mass). For instance, over a timeframe of thousands of years, after the most active short half-life radioisotopes decayed, burying U.S. nuclear waste would increase the radioactivity in the top 2000 feet of rock and soil in the United States (10 million km) by

text: 1 part in 10 million over the cumulative amount of

text: natural radioisotopes

text: in such a volume, although the vicinity of the site would have a far higher concentration of artificial radioisotopes underground than such an average.

text: A more commonly used parameter is the half-life. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. For the case of one-decay nuclear reactions:

text: A more commonly used parameter is the

text: half-life

text: . Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. For the case of one-decay nuclear reactions:

text: Radioactive tracer isotopes are sometimes injected with hydraulic fracturing fluid to determine the injection profile and location of created fractures. Sand containing gamma-emitting tracer isotopes is used to trace and measure fractures.  A 1995 study found that radioactive tracers were used in over 15% of stimulated oil and gas wells. In the United States, injection of radionuclides are licensed and regulated by the Nuclear Regulatory Commission (NRC).  According to the NRC, some of the most commonly used tracers include antimony-124, bromine-82, iodine-125, iodine-131, iridium-192, and scandium-46. A 2003 publication by the International Atomic Energy Agency confirms the frequent use of most of the tracers above, and says that manganese-56, sodium-24, technetium-99m, silver-110m, argon-41, and xenon-133 are also used extensively because they are easily identified and measured.  According to a 2013 meeting of researchers who examined low (never exceeding drinking water standards) but persistent detections of iodine-131 in a stream used for Philadelphia drinking water: “Workshop participants concluded that the likely source of 131-I in Philadelphia’s source waters is residual 131-I excreted from patients following medical treatments,” but suggested that other potential sources also be studied, including hydraulic fracturing.

text: Radioactive tracer isotopes are sometimes injected with hydraulic fracturing fluid to determine the injection profile and location of created fractures. Sand containing gamma-emitting tracer isotopes is used to trace and measure fractures.  A 1995 study found that radioactive tracers were used in over 15% of stimulated oil and gas wells. In the United States, injection of radionuclides are licensed and regulated by the

text: Nuclear Regulatory Commission

text: (NRC).  According to the NRC, some of the most commonly used tracers include

text: , and

text: . A 2003 publication by the

text: International Atomic Energy Agency

text: , and

text: are also used extensively because they are easily identified and measured.  According to a 2013 meeting of researchers who examined low (never exceeding drinking water standards) but persistent detections of iodine-131 in a stream used for Philadelphia drinking water: “Workshop participants concluded that the likely source of 131-I in Philadelphia’s source waters is residual 131-I excreted from patients following medical treatments,” but suggested that other potential sources also be studied, including hydraulic fracturing.

text: Less is known about her personal life, only that she had an unhappy childhood. Mărăcineanu received her degree in physical and chemical sciences in 1910, going on to teach at the Central School for Girls in Bucharest. She then travelled to Paris to do research at the Radium Institute with Marie Curie from around 1919 until 1926, where she also received her doctorate in 1924. At the Radium Institute, Mărăcineanu researched the half-life of polonium and devised methods of measuring alpha decay. This work led her to believe that radioactive isotopes could be formed from atoms as a result of exposure to polonium's alpha rays; an observation which would lead to the Joliot-Curies' 1935 Nobel Prize. She also investigated the possibility of sunlight inducing radioactivity; work which was contested by other researchers.

text: Less is known about her personal life, only that she had an unhappy childhood. Mărăcineanu received her degree in physical and chemical sciences in 1910, going on to teach at the Central School for Girls in Bucharest. She then travelled to Paris to do research at the

text: Radium Institute

text: with

text: Marie Curie

text: from around 1919 until 1926, where she also received her

text: doctorate

text: in 1924. At the Radium Institute, Mărăcineanu researched the

text: half-life

text: of

text: polonium

text: and devised methods of measuring

text: alpha decay

text: . This work led her to believe that

text: radioactive isotopes

text: could be formed from

text: atom

text: s as a result of exposure to polonium's alpha rays; an observation which would lead to the

text: Joliot-Curies

text: ' 1935 Nobel Prize. She also investigated the possibility of sunlight inducing radioactivity; work which was contested by other researchers.

text: Some radioactive materials have short half-lives; some have long half-lives. Uranium and thorium have long half-lives, and so persist in Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a stable element. Such "decay series", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity, and provided a basis for constructing techniques of radiometric dating.

text: Some radioactive materials have short half-lives; some have long half-lives.

text: Uranium

text: and

text: thorium

text: have long half-lives, and so persist in Earth's crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive ("stable") elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a

text: stable element

text: . Such "decay series", such as the uranium-radium and thorium series, were known within a few years of the discovery of radioactivity, and provided a basis for constructing techniques of radiometric dating.

text: Radiometric dating measures the steady decay of radioactive elements in an object to determine its age. It is used to calculate dates for the older part of the planet’s geological record. The theory is very complicated but, in essence, the radioactive elements within an object decay to form isotopes of each chemical element. Isotopes are atoms of the element that differ in mass but share the same general properties. Geologists are most interested in the decay of isotopes carbon-14 (into nitrogen-14) and potassium-40 (into argon-40). Carbon-14 aka radiocarbon dating works for organic materials that are less than about 50,000 years old. For older periods, the potassium-argon dating process is more accurate.

text: Radiometric dating

text: measures the steady decay of

text: radioactive element

text: s in an object to determine its age. It is used to calculate dates for the older part of the planet’s geological record. The theory is very complicated but, in essence, the radioactive elements within an object decay to form

text: isotope

text: s of each

text: chemical element

text: . Isotopes are

text: atom

text: s of the element that differ in

text: mass

text: but share the same general properties. Geologists are most interested in the decay of isotopes

text: (into

text: ) and

text: (into

text: ). Carbon-14 aka

text: radiocarbon dating

text: works for organic materials that are less than about 50,000 years old. For older periods, the

text: potassium-argon dating

text: process is more accurate.

text: By measuring the amount of radioactive decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the radiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope. Two or more radiometric methods can be used in concert to achieve more robust results. Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the Ar/Ar dating method can be extended into the time of early human life and into recorded history.

text: By measuring the amount of

text: radioactive decay

text: of a

text: radioactive isotope

text: with a known

text: half-life

text: , geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the

text: radiocarbon method

text: , most of these techniques are actually based on measuring an increase in the abundance of a

text: radiogenic

text: isotope, which is the decay-product of the radioactive parent isotope. Two or more radiometric methods can be used in concert to achieve more robust results. Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the Ar/Ar dating method can be extended into the time of early human life and into recorded history.

text: When the radioactive atoms decay, they emit fast charged particles (e.g., α-particles and β-particles). Because of their charge, the emitted particles interact in characteristic ways with the electrons and nuclei of the crystal atoms, giving rise to channeling and blocking directions for the particle escaping the crystal. The intensity (or yield) of the emitted particles is therefore dependent on the position of the detector relative to crystal planes and axes. This fact is used to infer the location of the radioactive species in the lattice by varying the emission angles and subsequent comparison to simulation results. For the simulations, the manybeam formalism can be employed, and resolutions below 1 Å are achievable.

text: When the

text: radioactive

text: atom

text: s decay, they emit fast charged particles (e.g.,

text: α-particles

text: and

text: β-particles

text: ). Because of their

text: charge

text: , the emitted particles interact in characteristic ways with the

text: electron

text: s and

text: nuclei

text: of the

text: crystal

text: atom

text: s, giving rise to channeling and blocking directions for the particle escaping the crystal. The intensity (or yield) of the emitted particles is therefore dependent on the position of the detector relative to crystal planes and axes. This fact is used to infer the location of the radioactive species in the

text: lattice

text: by varying the emission angles and subsequent comparison to simulation results. For the simulations, the manybeam formalism can be employed, and resolutions below 1

text: Å

text: are achievable.

text: There are many other kinds of radioactivity, e.g. cluster decay, proton decay, various beta-delayed decay modes (p, 2p, 3p, n, 2n, 3n, 4n, d, t, alpha, f), fission isomers, particle accompanied (ternary) fission, etc. The height of the potential barrier, mainly of Coulomb nature, for emission of the charged particles is much higher than the observed kinetic energy of the emitted particles. The spontaneous decay can only be explained by quantum tunneling in a similar way to the first application of the Quantum Mechanics to Nuclei given by G. Gamow for alpha decay.

text: There are many other kinds of radioactivity, e.g. cluster decay,

text: proton decay

text: , various beta-delayed decay modes (p, 2p, 3p, n, 2n, 3n, 4n, d, t, alpha, f),

text: fission isomer

text: s, particle accompanied (ternary) fission, etc. The height of the potential barrier, mainly of Coulomb nature, for emission of the charged particles is much higher than the observed kinetic energy of the emitted particles. The spontaneous decay can only be explained by

text: quantum tunneling

text: in a similar way to the first application of the Quantum Mechanics to Nuclei given by G. Gamow for alpha decay.

text: The daughter nuclide of a decay event may also be unstable (radioactive). In this case, it too will decay, producing radiation. The resulting second daughter nuclide may also be radioactive. This can lead to a sequence of several decay events called a decay chain (see this article for specific details of important natural decay chains).Eventually, a stable nuclide is produced.

text: The daughter nuclide of a decay event may also be unstable (radioactive). In this case, it too will decay, producing radiation. The resulting second daughter nuclide may also be radioactive. This can lead to a sequence of several decay events called a

text: decay chain

text: (see this article for specific details of important natural decay chains).Eventually, a stable nuclide is produced.

text: The extended length of the observed light curve found a remarkably uniform 0.01 magnitudes per day decline starting about 60 days after discovery.  Translated into other units, this is almost exactly a 77-day half-life, which is the half-life of Co.  In the standard model for Type Ia supernovae, approximately a solar mass of Ni is formed and ejected from a white dwarf which accretes mass from a binary companion and is raised over the Chandrasekhar limit and explodes.  This Ni decays with a half-life of about 6 days to Co, and the decay of the cobalt provides the energy radiated away by the supernova remnant.  The model also produces an estimate for the luminosity of such a supernova.  The observations of SN1972e, both peak brightness and fade rate, were in general agreement with these predictions, and led to rapid acceptance of this degenerate-explosion model.

text: The extended length of the observed light curve found a remarkably uniform 0.01

text: magnitudes

text: per day decline starting about 60 days after discovery.  Translated into other units, this is almost exactly a 77-day

text: half-life

text: , which is the half-life of

text: Co

text: .  In the

text: standard model for Type Ia supernovae

text: , approximately a

text: solar mass

text: of

text: Ni

text: is formed and ejected from a

text: white dwarf

text: which accretes mass from a binary companion and is raised over the

text: Chandrasekhar limit

text: and explodes.  This Ni decays with a half-life of about 6 days to Co, and the decay of the cobalt provides the energy radiated away by the supernova remnant.  The model also produces an estimate for the luminosity of such a supernova.  The observations of SN1972e, both peak brightness and fade rate, were in general agreement with these predictions, and led to rapid acceptance of this degenerate-explosion model.

text: After securing a license to the Quake engine through the help of friend Michael Abrash of id Software in late 1996, Newell and Harrington began working on Half-Life. Originally planned for release in late 1997, Half-Life launched on November 19, 1998. Valve acquired TF Software Pty. Ltd., the makers of the Team Fortress mod for Quake, in May 1998 with the intent to create a standalone Team Fortress game. The Team Fortress Classic mod, essentially a port of the original Team Fortress mod for Quake, was released for Half-Life in 1999. Gearbox contributed much after the release of Half-Life. Gearbox Software is responsible for the Half-Life expansion packs, Half-Life: Opposing Force and Half-Life: Blue Shift, along with the home console versions of Half-Life for the Sega Dreamcast and Sony PlayStation 2 which included a third expansion pack called Half-Life: Decay, which enabled two-player split-screen co-op.

text: After securing a license to the

text: Quake engine

text: through the help of friend

text: Michael Abrash

text: of

text: id Software

text: in late 1996, Newell and Harrington began working on

text: Half-Life

text: . Originally planned for release in late 1997, Half-Life launched on November 19, 1998. Valve acquired TF Software Pty. Ltd., the makers of the

text: Team Fortress

text: mod

text: for

text: Quake

text: , in May 1998 with the intent to create a standalone Team Fortress game. The

text: Team Fortress Classic

text: mod

text: , essentially a port of the original Team Fortress mod for Quake, was released for Half-Life in 1999. Gearbox contributed much after the release of Half-Life.

text: Gearbox Software

text: is responsible for the Half-Life expansion packs,

text: Half-Life: Opposing Force

text: and

text: Half-Life: Blue Shift

text: , along with the home console versions of Half-Life for the

text: Sega

text: Dreamcast

text: and

text: Sony

text: PlayStation 2

text: which included a third expansion pack called

text: Half-Life: Decay

text: , which enabled two-player split-screen

text: co-op

text: A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive (a radionuclide) or stable (a stable nuclide).

text: A radiogenic nuclide is a

text: nuclide

text: that is produced by a process of

text: radioactive decay

text: . It may itself be radioactive (a

text: radionuclide

text: ) or stable (a

text: stable nuclide

text: Headcrabs are one of the first enemies introduced in Half-Life. They are frequently encountered by players throughout Half-Life and its three expansions, Opposing Force, Blue Shift and Decay. They also see appearances in the official remake of Half-Life, titled Half-Life: Source, as well as the third-party fan modification remake titled Black Mesa. Because the games are remakes, headcrabs play essentially the same role as they do in the original Half-Life.

text: Headcrabs are one of the first enemies introduced in Half-Life. They are frequently encountered by players throughout Half-Life and its three expansions,

text: Opposing Force

text: Blue Shift

text: and

text: Decay

text: . They also see appearances in the official remake of Half-Life, titled

text: Half-Life: Source

text: , as well as the third-party fan modification remake titled

text: Black Mesa

text: . Because the games are remakes, headcrabs play essentially the same role as they do in the original Half-Life.

text: The mathematical expression that relates radioactive decay to geologic time is

text: Ra is the most stable isotope of radium and is the last isotope in the (4n + 2) decay chain of uranium-238 with a half-life of over a millennium: it makes up almost all of natural radium. Its immediate decay product is the dense radioactive noble gas radon, which is responsible for much of the danger of environmental radium. It is 2.7 million times more radioactive than the same molar amount of natural uranium (mostly uranium-238), due to its proportionally shorter half-life.

text: Ra is the most stable isotope of radium and is the last isotope in the (4n + 2) decay chain of uranium-238 with a half-life of over a millennium: it makes up almost all of natural radium. Its immediate decay product is the dense radioactive

text: noble gas

text: radon

text: , which is responsible for much of the danger of environmental radium. It is 2.7 million times more radioactive than the same

text: molar amount

text: of natural

text: uranium

text: (mostly uranium-238), due to its proportionally shorter half-life.

text: Half-life (T) is defined as the length of time for half of a given quantity of radioactive atoms to undergo radioactive decay:

text: When particles are subject to radioactive decay, Decay correction may be used to compute the decay rate or number of particles at a point in time relative to when they were measured. This is necessary when comparing measurements obtained at different points in time. For example, the decay rate of a parent radionuclide is proportional to the number of atoms; as time passes some of atoms decay into daughter nuclides reducing the number of parent atoms.

text: When particles are subject to

text: radioactive decay

text: , Decay correction may be used to compute the decay rate or number of particles at a point in time relative to when they were measured. This is necessary when comparing measurements obtained at different points in time. For example, the decay rate of a parent

text: radionuclide

text: is proportional to the number of atoms; as time passes some of atoms decay into daughter

text: nuclides

text: reducing the number of parent atoms.

text: Although every element up to bismuth (element 83) has an isotope that is practically stable for all purposes ("classically stable"), with the exceptions of technetium and promethium (elements 43 and 61), all elements from polonium (element 84) onward are noticeably radioactive. Of these, thorium (element 90) is the most stable, closely followed by uranium (element 92): the isotope Th has a half-life of 14.05 billion years, about three times the age of the earth, and even slightly longer than the generally accepted age of the universe (about 13.8 billion years). As such, Th still occurs naturally today: four-fifths of the thorium present at Earth's formation has survived to the present. Thorium and uranium are thus the most well-studied of all the radioactive elements.

text: Although every element up to

text: bismuth

text: (element 83) has an isotope that is practically stable for all purposes ("classically stable"), with the exceptions of

text: technetium

text: and

text: promethium

text: (elements 43 and 61), all elements from

text: polonium

text: (element 84) onward are noticeably radioactive. Of these, thorium (element 90) is the most stable, closely followed by uranium (element 92): the isotope Th has a half-life of 14.05 billion years, about three times the

text: age of the earth

text: , and even slightly longer than the generally accepted

text: age of the universe

text: (about 13.8 billion years). As such, Th still occurs naturally today: four-fifths of the thorium present at Earth's formation has survived to the present. Thorium and uranium are thus the most well-studied of all the radioactive elements.

text: A series of different experimental methods exist, these have been designed to enable the measurement of a range of different elements in different matrices. To reduce the effect of the matrix it is common to use the chemical extraction of the wanted element and/or to allow the radioactivity due to the matrix elements to decay before the measurement of the radioactivity. Since the matrix effect can be corrected for by observing the decay spectrum, little or no sample preparation is required for some samples, making neutron activation analysis less susceptible to contamination.

text: A series of different experimental methods exist, these have been designed to enable the measurement of a range of different elements in different matrices. To reduce the effect of the

text: matrix

text: it is common to use the chemical extraction of the wanted element and/or to allow the radioactivity due to the matrix elements to decay before the measurement of the radioactivity. Since the matrix effect can be corrected for by observing the decay spectrum, little or no sample preparation is required for some samples, making neutron activation analysis less susceptible to contamination.

text: There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb. Pb is the only non-radiogenic lead isotope, therefore is not one of the daughter isotopes. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U andTh respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This shifts the ratio of radiogenic Pb versus non-radiogenic Pb (Pb/Pb or Pb/Pb) in favor of radiogenic Pb or Pb. This can be expressed by the following decay equations:

text: There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb. Pb is the only non-

text: radiogenic

text: lead isotope, therefore is not one of the daughter isotopes. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U andTh respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This shifts the ratio of radiogenic Pb versus non-radiogenic Pb (Pb/Pb or Pb/Pb) in favor of radiogenic Pb or Pb. This can be expressed by the following decay equations:

text: Other types of radioactive decay were found to emit previously-seen particles, but via different mechanisms. An example is internal conversion, which results in an initial electron emission, and then often further characteristic X-rays and Auger electrons emissions, although the internal conversion process involves neither beta nor gamma decay. A neutrino is not emitted, and none of the electron(s) and photon(s) emitted originate in the nucleus, even though the energy to emit all of them does originate there. Internal conversion decay, like isomeric transition gamma decay and neutron emission, involves the release of energy by an excited nuclide, without the transmutation of one element into another.

text: Other types of radioactive decay were found to emit previously-seen particles, but via different mechanisms. An example is

text: internal conversion

text: , which results in an initial electron emission, and then often further

text: characteristic X-ray

text: s and

text: Auger electron

text: s emissions, although the internal conversion process involves neither beta nor gamma decay. A neutrino is not emitted, and none of the electron(s) and photon(s) emitted originate in the nucleus, even though the energy to emit all of them does originate there. Internal conversion decay, like

text: isomeric transition

text: gamma decay and neutron emission, involves the release of energy by an excited nuclide, without the transmutation of one element into another.

text: Naturally occurring cadmium is composed of 8 isotopes. Two of them are radioactive, and three are expected to decay but have not done so under laboratory conditions. The two natural radioactive isotopes are Cd (beta decay, half-life is 7.7 × 10 years) and Cd (two-neutrino double beta decay, half-life is 2.9 × 10 years). The other three are Cd, Cd (both double electron capture), and Cd (double beta decay); only lower limits on these half-lives have been determined. At least three isotopes – Cd, Cd, and Cd – are stable. Among the isotopes that do not occur naturally, the most long-lived are Cd with a half-life of 462.6 days, and Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives of less than 2.5 hours, and the majority have half-lives of less than 5 minutes. Cadmium has 8 known meta states, with the most stable being Cd (t = 14.1 years), Cd (t = 44.6 days), and Cd (t = 3.36 hours).

text: Naturally occurring cadmium is composed of 8

text: isotope

text: s. Two of them are

text: radioactive

text: , and three are expected to

text: decay

text: but have not done so under laboratory conditions. The two natural radioactive isotopes are Cd (

text: beta decay

text: , half-life is 7.7 × 10 years) and Cd (two-neutrino

text: double beta decay

text: , half-life is 2.9 × 10 years). The other three are Cd, Cd (both

text: double electron capture

text: ), and Cd (double beta decay); only lower limits on these half-lives have been determined. At least three isotopes – Cd, Cd, and Cd – are stable. Among the isotopes that do not occur naturally, the most long-lived are Cd with a half-life of 462.6 days, and Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives of less than 2.5 hours, and the majority have half-lives of less than 5 minutes. Cadmium has 8 known

text: meta state

text: s, with the most stable being Cd (t = 14.1 years), Cd (t = 44.6 days), and Cd (t = 3.36 hours).

text: If the magnitude of v were to describe a measurable physical quantity, that would mean that the laws of physics would not appear the same if the universe was viewed in a mirror. In fact, this is exactly what happens in the weak interaction: Certain radioactive decays treat "left" and "right" differently, a phenomenon which can be traced to the summation of a polar vector with a pseudovector in the underlying theory. (See parity violation.)

text: If the magnitude of v were to describe a measurable physical quantity, that would mean that the laws of physics would not appear the same if the universe was viewed in a mirror. In fact, this is exactly what happens in the

text: weak interaction

text: : Certain radioactive decays treat "left" and "right" differently, a phenomenon which can be traced to the summation of a polar vector with a pseudovector in the underlying theory. (See

text: parity violation

text: Elements with atomic numbers 1 through 40 are all stable, while those with atomic numbers 41 through 82 (except technetium and promethium) are metastable. The half-lives of these metastable "theoretical radionuclides" are so long (at least 100 million times longer than the estimated age of the universe) that their radioactive decay has yet to be detected by experiment. Elements with atomic numbers 83 through 94 are unstable to the point that their radioactive decay can be detected. Three of these elements, bismuth (element 83), thorium (element 90), and uranium (element 92) have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy elements before the formation of our solar system. For example, at over 1.9 years, over a billion times longer than the current estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any naturally occurring element. The very heaviest 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half-lives and cannot be produced as daughters of longer-lived elements, and thus they do not occur in nature at all.

text: Elements with atomic numbers 1 through 40 are all stable, while those with atomic numbers 41 through 82 (except technetium and promethium) are metastable. The half-lives of these metastable "

entity_section: Nuclides with no experimentally observed decays

text: theoretical radionuclides

text: " are so long (at least 100 million times longer than the estimated

text: age of the universe

text: ) that their radioactive decay has yet to be detected by experiment. Elements with atomic numbers 83 through 94 are unstable to the point that their radioactive decay can be detected. Three of these elements, bismuth (element 83), thorium (element 90), and uranium (element 92) have one or more isotopes with half-lives long enough to survive as remnants of the explosive

text: stellar nucleosynthesis

text: that produced the heavy elements before the formation of our solar system. For example, at over 1.9 years, over a billion times longer than the current estimated age of the universe,

text: has the longest known

text: alpha decay

text: half-life of any naturally occurring element. The very heaviest 24 elements (those beyond plutonium, element 94) undergo radioactive decay with short half-lives and cannot be produced as daughters of longer-lived elements, and thus they do not occur in nature at all.

text: According to former Immanuel Velikovsky assistant turned prolific critic, C. Leroy Ellenberger, "[Sitchin states that] from an equal start, the Nephilim evolved on Nibiru 45 million years ahead of comparable development on Earth with its decidedly more favorable environment. Such an outcome is unlikely, to say the least, since Nibiru would spend over 99% of its time beyond Pluto. Sitchin's explanation that heat from radioactive decay and a thick atmosphere keep Nibiru warm is absurd and does not address the problem of darkness in deep space. Also unexplained is how the Nephilim, who evolved long after Nibiru arrived, knew what happened when Nibiru first entered the solar system."

text: Radiocarbon dating has been used since 1946 to determine the age of organic material as old as 50,000 years. As the organism dies, the exchange of C with the environment ceases and the incorporated C decays. Given the decay of this isotope (the half-life of C is about 5,730 years), the amount of C left in the dead organism permits to calculate how long ago it died. Bomb pulse dating should be considered a special form of carbon dating. As discussed above and in the Radiolab episode, Elements (section 'Carbon'),  in bomb pulse dating the slow absorption of atmospheric C by the biosphere, can be considered as a chronometer. Starting from the pulse around the years 1963 (see figure), atmospheric radiocarbon decreased with 1% a year. So in bomb pulse dating it is the amount of C in the atmosphere that is decreasing and not the amount of C in a dead organisms, as is the case in classical radiocarbon dating. This decrease in atmospheric C can be measured in cells and tissues and has permitted scientists to determine the age of individual cells and of deceased people. These applications are very similar to the experiments conducted with pulse-chase analysis in which cellular processes are examined over time by exposing the cells to a labeled compound (pulse) and then to the same compound in an unlabeled form (chase). Radioactivity is a commonly used label in these experiments. An important difference between pulse-chase analysis and bomb pulse dating is the absence of the chase in the latter.

text: Radiocarbon dating

text: has been used since 1946 to determine the age of organic material as old as 50,000 years. As the organism dies, the exchange of C with the environment ceases and the incorporated C decays. Given the decay of this isotope (the

text: half-life

text: of C is about 5,730 years), the amount of C left in the dead organism permits to calculate how long ago it died. Bomb pulse dating should be considered a special form of carbon dating. As discussed above and in the

text: Radiolab

text: episode, Elements (section 'Carbon'),  in bomb pulse dating the slow absorption of atmospheric C by the biosphere, can be considered as a chronometer. Starting from the pulse around the years 1963 (see figure), atmospheric radiocarbon decreased with 1% a year. So in bomb pulse dating it is the amount of C in the atmosphere that is decreasing and not the amount of C in a dead organisms, as is the case in classical radiocarbon dating. This decrease in atmospheric C can be measured in cells and tissues and has permitted scientists to determine the age of individual cells and of deceased people. These applications are very similar to the experiments conducted with

text: pulse-chase analysis

text: in which cellular processes are examined over time by exposing the cells to a labeled compound (pulse) and then to the same compound in an unlabeled form (chase).

text: Radioactivity

text: is a commonly used label in these experiments. An important difference between pulse-chase analysis and bomb pulse dating is the absence of the chase in the latter.

text: Argon–argon (or Ar/Ar) dating is a radiometric dating method invented to supersede potassium-argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. Ar/Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (K) into the radioactive Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the K/Ar* ratio, and thus to calculate the age of the unknown sample. Ar* refers to the radiogenic Ar, i.e. the Ar produced from radioactive decay of K. Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the Ar (which is assumed to be of atmospheric origin) and assuming that Ar is found in a constant ratio to Ar in atmospheric gases.

text: Argon–argon (or Ar/Ar) dating is a

text: radiometric dating

text: method invented to supersede

text: potassium-argon (K/Ar) dating

text: in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. Ar/Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (

text: K

text: ) into the radioactive

text: Ar

text: . As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the K/Ar* ratio, and thus to calculate the age of the unknown sample. Ar* refers to the radiogenic Ar, i.e. the Ar produced from radioactive decay of K. Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the Ar (which is assumed to be of atmospheric origin) and assuming that Ar is found in a constant ratio to Ar in atmospheric gases.

text: The other iodine radioisotopes have much shorter half-lives, no longer than days. Some of them have medical applications involving the thyroid gland, where the iodine that enters the body is stored and concentrated. Iodine-123 has a half-life of thirteen hours and decays by electron capture to tellurium-123, emitting gamma radiation; it is used in nuclear medicine imaging, including single photon emission computed tomography (SPECT) and X-ray computed tomography (X-Ray CT) scans. Iodine-125 has a half-life of fifty-nine days, decaying by electron capture to tellurium-125 and emitting low-energy gamma radiation; the second-longest-lived iodine radioisotope, it has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat a number of conditions, including prostate cancer, uveal melanomas, and brain tumours. Finally, iodine-131, with a half-life of eight days, beta decays to an excited state of stable xenon-131 that then converts to the ground state by emitting gamma radiation. It is a common fission product and thus is present in high levels in radioactive fallout. It may then be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of iodine-131 is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis. The usual means of protection against the negative effects of iodine-131 is by saturating the thyroid gland with stable iodine-127 in the form of potassium iodide tablets, taken daily for optimal prophylaxis. However, iodine-131 may also be used for medicinal purposes in radiation therapy for this very reason, when tissue destruction is desired after iodine uptake by the tissue. Iodine-131 is also used as a radioactive tracer.

text: The other iodine radioisotopes have much shorter half-lives, no longer than days. Some of them have medical applications involving the

text: thyroid gland

text: , where the iodine that enters the body is stored and concentrated.

text: has a half-life of thirteen hours and decays by

text: electron capture

text: to

text: , emitting

text: gamma radiation

text: ; it is used in

text: nuclear medicine

text: imaging, including

text: (SPECT) and

text: X-ray computed tomography

text: (X-Ray CT) scans.

text: has a half-life of fifty-nine days, decaying by electron capture to

text: and emitting low-energy gamma radiation; the second-longest-lived iodine radioisotope, it has uses in

text: biological assay

text: s,

text: nuclear medicine

text: imaging and in

text: radiation therapy

text: as

text: brachytherapy

text: to treat a number of conditions, including

text: prostate cancer

text: uveal melanoma

text: s, and

text: brain tumour

text: s. Finally,

text: , with a half-life of eight days, beta decays to an excited state of stable

text: that then converts to the ground state by emitting gamma radiation. It is a common

text: fission product

text: and thus is present in high levels in radioactive

text: fallout

text: . It may then be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of iodine-131 is the chance occurrence of

text: radiogenic

text: thyroid cancer

text: in later life. Other risks include the possibility of non-cancerous growths and

text: thyroiditis

text: . The usual means of protection against the negative effects of iodine-131 is by saturating the thyroid gland with stable iodine-127 in the form of

text: potassium iodide

text: tablets, taken daily for optimal prophylaxis. However, iodine-131 may also be used for medicinal purposes in

text: radiation therapy

text: for this very reason, when tissue destruction is desired after iodine uptake by the tissue. Iodine-131 is also used as a

text: radioactive tracer

text: Michael Shapiro is most famous for voicing Barney Calhoun and the G-Man in the computer games Half-Life, Half-Life: Opposing Force, Half-Life: Blue Shift, Half-Life: Decay, Half-Life 2, Half-Life 2: Episode One, and Half-Life 2: Episode Two.  Games in which he is credited as “Mike Shapiro” are Freddi Fish 4: The Case of the Hogfish Rustlers of Briny Gulch, Blood II: The Chosen, Grand Theft Auto: Liberty City Stories and Torin's Passage. He has also voiced many radio and TV commercials. Michael is a director of television shows and theater productions, with some of his shows being shown on Strike.TV. In 2007, his TV pilot Family Values, starring Rob Riggle, was screened at the New York Television Festival.

text: Michael Shapiro is most famous for voicing

text: Barney Calhoun

text: and the

text: G-Man

text: in the computer games

text: Half-Life

text: Half-Life: Opposing Force

text: Half-Life: Blue Shift

text: Half-Life: Decay

text: Half-Life 2

text: Half-Life 2: Episode One

text: , and

text: Half-Life 2: Episode Two

text: .  Games in which he is credited as “Mike Shapiro” are

text: Freddi Fish 4: The Case of the Hogfish Rustlers of Briny Gulch

text: Blood II: The Chosen

text: Grand Theft Auto: Liberty City Stories

text: and

text: Torin's Passage

text: . He has also voiced many

text: radio

text: and TV commercials. Michael is a director of

text: television

text: shows and

text: theater

text: productions, with some of his shows being shown on

text: Strike.TV

text: . In 2007, his TV pilot Family Values, starring

text: Rob Riggle

text: , was screened at the

text: New York Television Festival

text: There are also five primordial long-lived radioactive odd-even isotopes, , , , , and . The last two were only recently found to decay, with half-lives greater than 10 years.

text: While a radioactive isotope decays almost perfectly according to so-called "first order kinetics" where the rate constant is a fixed number, the elimination of a substance from a living organism usually follows more complex chemical kinetics.

text: Naturally occurring actinium is composed of two radioactive isotopes;  (from the radioactive family of ) and  (a granddaughter of ).  decays mainly as a beta emitter with a very small energy, but in 1.38% of cases it emits an alpha particle, so it can readily be identified through alpha spectrometry. Thirty-six radioisotopes have been identified, the most stable being  with a half-life of 21.772 years,  with a half-life of 10.0 days and  with a half-life of 29.37 hours. All remaining radioactive isotopes have half-lives that are less than 10 hours and the majority of them have half-lives shorter than one minute. The shortest-lived known isotope of actinium is  (half-life of 69 nanoseconds) which decays through alpha decay and electron capture. Actinium also has two known meta states. The most significant isotopes for chemistry are Ac, Ac, and Ac.

text: Naturally occurring actinium is composed of two radioactive

text: isotope

text: s;  (from the radioactive family of ) and  (a granddaughter of ).  decays mainly as a

text: beta emitter

text: with a very small energy, but in 1.38% of cases it emits an

text: alpha particle

text: , so it can readily be identified through

text: alpha spectrometry

text: . Thirty-six

text: radioisotope

text: s have been identified, the most stable being  with a

text: half-life

text: of 21.772 years,  with a half-life of 10.0 days and  with a half-life of 29.37 hours. All remaining

text: radioactive

text: isotopes have half-lives that are less than 10 hours and the majority of them have half-lives shorter than one minute. The shortest-lived known isotope of actinium is  (half-life of 69 nanoseconds) which decays through

text: alpha decay

text: and

text: electron capture

text: . Actinium also has two known

text: meta state

text: s. The most significant isotopes for chemistry are Ac, Ac, and Ac.

text: The term stable isotope has a similar meaning to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundance of such stable isotopes can be measured experimentally (isotope analysis), yielding an isotope ratio that can be used as a research tool. Theoretically, such stable isotopes could include the radiogenic daughter products of radioactive decay, used in radiometric dating. However, the expression stable isotope ratio is preferably used to refer to isotopes whose relative abundances are affected by isotope fractionation in nature. This field is termed stable isotope geochemistry.

text: The term stable isotope has a similar meaning to

text: stable nuclide

text: , but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundance of such stable isotopes can be measured experimentally (

text: isotope analysis

text: ), yielding an isotope ratio that can be used as a research tool. Theoretically, such stable isotopes could include the radiogenic daughter products of radioactive decay, used in

text: radiometric dating

text: . However, the expression stable isotope ratio is preferably used to refer to isotopes whose relative abundances are affected by

text: isotope fractionation

text: in nature. This field is termed

text: stable isotope geochemistry

text: Radioactive decay often proceeds via a sequence of steps known as a decay chain.  For example, U decays to Th which decays to Pa and so on, eventually reaching Pb:

text: Radioactive decay often proceeds via a sequence of steps known as a decay chain.  For example,

text: U

text: decays to Th which decays to Pa and so on, eventually reaching

text: Pb

text: It is a two-dimensional graphical representation in the Segrè-arrangement with the neutron number N on the abscissa and the proton number Z on the ordinate. Each nuclide is represented at the intersection of its respective neutron and proton number by a small square box with the chemical symbol and the nucleon number A. By columnar subdivision of such a field, in addition to ground states also nuclear isomers can be shown. The coloring of a field (segmented if necessary) shows in addition to the existing text entries the observed types of radioactive decay of the nuclide and a rough classification of their relative shares: stable, nonradioactive nuclides completely black, primordial radionuclides partially black, proton emission orange, alpha decay yellow, beta plus decay/electron capture red, isomeric transition (gamma decay, internal conversion) white, beta minus decay blue, spontaneous fission green, cluster emission violet, neutron emission light blue. For each radionuclide its field includes (if known) information about its half-life and essential energies of the emitted radiation, for stable nuclides and primordial radionuclides there are data on mole fraction abundances in the natural isotope mixture of the corresponding chemical element. Furthermore, for many nuclides cross sections for nuclear reactions with thermal neutrons are quoted, usually for the (n, γ)-reaction (neutron capture), partly fission cross sections for the induced nuclear fission and cross sections for the (n, α)-reaction or (n, p)-reaction. For the chemical elements cross sections and relative atomic masses (both averaged over natural isotopic composition) are specified (the relative atomic masses partially as an interval to reflect the variability of the composition of the element's natural isotope mixture). For the nuclear fission of U and Pu with thermal neutrons, percentage isobaric chain yields of fission products are listed.

text: It is a two-dimensional graphical representation in the

text: Segrè

text: -arrangement with the

text: neutron number

text: N on the

text: abscissa

text: and the

text: proton number

text: Z on the

text: ordinate

text: . Each

text: nuclide

text: is represented at the intersection of its respective neutron and proton number by a small square box with the

text: chemical symbol

text: and the

text: nucleon number

text: A. By columnar subdivision of such a field, in addition to

text: ground state

text: s also

text: nuclear isomer

text: s can be shown. The coloring of a field (segmented if necessary) shows in addition to the existing text entries the observed types of

text: radioactive decay

text: of the nuclide and a rough classification of their relative shares:

text: stable, nonradioactive nuclides

text: completely black,

text: primordial radionuclides

text: partially black,

text: proton emission

text: orange,

text: alpha decay

text: yellow,

text: beta plus decay

text: electron capture

text: red,

text: isomeric transition

text: gamma decay

text: internal conversion

text: ) white,

text: beta minus decay

text: blue,

text: spontaneous fission

text: green,

text: cluster emission

text: violet,

text: neutron emission

text: light blue. For each

text: radionuclide

text: its field includes (if known) information about its

text: half-life

text: and essential energies of the emitted radiation, for stable nuclides and primordial radionuclides there are data on

text: mole fraction

text: abundances in the

text: natural isotope mixture

text: of the corresponding

text: chemical element

text: . Furthermore, for many nuclides

text: cross sections

text: for

text: nuclear reaction

text: s with

text: thermal neutrons

text: are quoted, usually for the (n, γ)-reaction (

text: neutron capture

text: ), partly fission cross sections for the induced

text: nuclear fission

text: and cross sections for the (n, α)-reaction or (n, p)-reaction. For the chemical elements cross sections and

text: relative atomic mass

text: es (both averaged over natural isotopic composition) are specified (the relative atomic masses partially as an interval to reflect the variability of the composition of the element's natural isotope mixture). For the nuclear fission of

text: U

text: and

text: Pu

text: with thermal neutrons, percentage

text: isobaric chain yields

text: of

text: fission product

text: s are listed.

text: The equation for the radioactive decay of  is:

text: A thorium atom has 90 protons and therefore 90 electrons, of which four are valence electrons. Thorium metal is silvery and tarnishes black when exposed to air, forming the dioxide. Thorium is weakly radioactive: all its known isotopes are unstable. Thorium-232 (Th), which has 142 neutrons, is the most stable isotope of thorium and accounts for nearly all natural thorium, with six other natural isotopes occurring only as trace radioisotopes. Thorium has the longest half-life of all the significantly radioactive elements, 14.05 billion years, or about the age of the universe; it decays very slowly through alpha decay to radium-228 (Ra), starting a decay chain named the thorium series that ends at stable lead-208 (Pb). Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare earth metals.

text: A thorium atom has 90

text: proton

text: s and therefore 90

text: electron

text: s, of which four are

text: valence electron

text: s. Thorium metal is silvery and

text: tarnish

text: es black when exposed to air, forming the dioxide. Thorium is weakly radioactive: all its known

text: isotope

text: s are unstable. Thorium-232 (Th), which has 142

text: neutron

text: s, is the most stable isotope of thorium and accounts for nearly all natural thorium, with six other natural isotopes occurring only as

text: trace radioisotope

text: s. Thorium has the longest half-life of all the significantly radioactive elements, 14.05 billion years, or about the

text: age of the universe

text: ; it decays very slowly through

text: alpha decay

text: to

text: (Ra), starting a

text: decay chain

text: named the

text: thorium series

text: that ends at stable

text: lead

text: -208 (Pb). Thorium is estimated to be about three to four times more

text: abundant

text: than uranium in the Earth's crust, and is chiefly refined from

text: monazite

text: sands as a by-product of extracting

text: rare earth metal

text: s.

text: A neutron by itself is an unstable particle: It undergoes  decay (a type of radioactive decay) by turning into a proton, electron, and electron antineutrino, with a half-life around ten minutes. (See the Neutron article for further discussion of neutron decay.) A proton by itself is thought to be stable, or at least its lifetime is too long to measure. (This is an important issue in particle physics, see Proton decay.)

text: A neutron by itself is an unstable particle: It undergoes

text: decay

text: (a type of

text: radioactive decay

text: ) by turning into a proton, electron, and

text: electron antineutrino

text: , with a half-life around ten minutes. (See the

text: Neutron

text: article for further discussion of neutron decay.) A proton by itself is thought to be stable, or at least its lifetime is too long to measure. (This is an important issue in particle physics, see

text: Proton decay

text: The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists. Libby received the Nobel Prize for his work in 1960. The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire  by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of  it contains begins to decrease as the  undergoes radioactive decay. Measuring the amount of  in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less  there is to be detected, and because the half-life of  (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by radiocarbon dating are around 50,000 years ago, although special preparation methods occasionally permit dating of older samples.

text: The method was developed by

text: Willard Libby

text: in the late 1940s and soon became a standard tool for archaeologists. Libby received the

text: Nobel Prize

text: for his work in 1960. The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of

text: cosmic ray

text: s with atmospheric

text: nitrogen

text: . The resulting radiocarbon combines with atmospheric

text: oxygen

text: to form radioactive

text: carbon dioxide

text: , which is incorporated into plants by

text: photosynthesis

text: ; animals then acquire  by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of  it contains begins to decrease as the  undergoes

text: radioactive decay

text: . Measuring the amount of  in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less  there is to be detected, and because the

text: half-life

text: of  (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by radiocarbon dating are around 50,000 years ago, although special preparation methods occasionally permit dating of older samples.

text: The palladium isotope Pd decays by beta emission to Ag with a half-life of 6.5 million years. Iron meteorites are the only objects with a high enough palladium/silver ratio to yield measurable variations in Ag abundance. Radiogenic Ag was first discovered in the Santa Clara meteorite in 1978.

text: The palladium

text: isotope

text: Pd decays by beta emission to Ag with a half-life of 6.5 million years.

text: Iron

text: meteorite

text: s are the only objects with a high enough palladium/silver ratio to yield measurable variations in Ag abundance.

text: Radiogenic

text: Ag was first discovered in the

text: Santa Clara

text: meteorite in 1978.

text: The potassium-argon method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive isotope of potassium (potassium 40) to argon 40, an isotope of argon, an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work. The age of Pilot Knob volcanism dated through the potassium-argon method is 79.5 +/- 3 million years, and agrees with the age derived by correlation with fossils to other radiometrically dated deposits.

text: The

text: potassium-argon

text: method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive

text: isotope

text: of

text: potassium

text: (potassium 40) to argon 40, an isotope of

text: argon

text: , an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work. The age of Pilot Knob volcanism dated through the potassium-argon method is 79.5 +/- 3 million years, and agrees with the age derived by correlation with fossils to other radiometrically dated deposits.

text: The pharmacokinetics of NMT have been studied in rabbits and mice using drug that had been radiolabeled with tritium at C-3 and C-5 on the benzene ring. Plasma concentrations were measured in the rabbits, whereas distribution, metabolism and excretion were determined in the mice. After i.v. administration to rabbits, the α-phase T was found to be 0.3 minutes, and the β-phase T was 5.6 minutes. These figures were indicative of a rapid distribution from blood to tissue and a very short plasma half-life. Within 2 minutes of injection, significant levels of radioactivity were detected in all tissues examined, with the highest amounts being in kidney and liver. No detectable radioactivity was left in the plasma after 30 minutes. Some NMT was found in the brains of mice treated with the drug, indicating that a small amount did cross the blood-brain barrier. ~ 80% of the administered dose was recovered from  the urine of mice within 1 hour.

text: The pharmacokinetics of NMT have been studied in rabbits and mice using drug that had been

text: radiolabeled

text: with

text: tritium

text: at C-3 and C-5 on the benzene ring. Plasma concentrations were measured in the rabbits, whereas distribution, metabolism and excretion were determined in the mice. After i.v. administration to rabbits, the α-phase T was found to be 0.3 minutes, and the β-phase T was 5.6 minutes. These figures were indicative of a rapid distribution from blood to tissue and a very short plasma half-life. Within 2 minutes of injection, significant levels of

text: radioactivity

text: were detected in all tissues examined, with the highest amounts being in kidney and liver. No detectable radioactivity was left in the plasma after 30 minutes. Some NMT was found in the brains of mice treated with the drug, indicating that a small amount did cross the

text: blood-brain barrier

text: . ~ 80% of the administered dose was recovered from  the urine of mice within 1 hour.

text: The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with the exception of dubnium-268. Nevertheless, because of reasons not very well understood yet, there is a slight increased nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted.

text: The stability of nuclei decreases greatly with the increase in atomic number after

text: plutonium

text: , the heaviest

text: primordial element

text: , so that all isotopes with an atomic number above

text: decay radioactively

text: with a

text: half-life

text: under a day, with the exception of

text: dubnium

text: -268. Nevertheless, because of

text: reasons

text: not very well understood yet, there is a slight increased nuclear stability around atomic numbers

text: , which leads to the appearance of what is known in nuclear physics as the "

text: island of stability

text: ". This concept, proposed by

text: University of California

text: professor

text: Glenn Seaborg

text: , explains why

text: superheavy element

text: s last longer than predicted.

text: Decay energy therefore remains associated with a certain measure of mass of the decay system, called invariant mass, which does not change during the decay, even though the energy of decay is distributed among decay particles. The energy of photons, the kinetic energy of emitted particles, and, later, the thermal energy of the surrounding matter, all contribute to the invariant mass of the system. Thus, while the sum of the rest masses of the particles is not conserved in radioactive decay, the system mass and system invariant mass (and also the system total energy) is conserved throughout any decay process. This is a restatement of the equivalent laws of conservation of energy and conservation of mass.

text: Decay energy therefore remains associated with a certain measure of mass of the decay system, called

text: invariant mass

text: , which does not change during the decay, even though the energy of decay is distributed among decay particles. The energy of photons, the kinetic energy of emitted particles, and, later, the thermal energy of the surrounding matter, all contribute to the

text: invariant mass

text: of the system. Thus, while the sum of the rest masses of the particles is not conserved in radioactive decay, the system mass and system

text: invariant mass

text: (and also the system total energy) is conserved throughout any decay process. This is a restatement of the equivalent laws of

text: conservation of energy

text: and

text: conservation of mass

text: Isotopes of a chemical element differ only in the mass number. For example, the isotopes of hydrogen can be written as H, H and H, with the mass number at top left. When the atomic nucleus of an isotope is unstable, compounds containing this isotope are radioactive. Tritium is an example of a radioactive isotope.

text: Isotope

text: s of a

text: chemical element

text: differ only in the mass number. For example, the isotopes of

text: hydrogen

text: can be written as

text: H

text: H

text: and

text: H

text: , with the mass number at top left. When the

text: atomic nucleus

text: of an isotope is unstable, compounds containing this isotope are

text: radioactive

text: Tritium

text: is an example of a radioactive isotope.

text: Radioactive isotopes can also be used as a light source, to provide an illuminated reticle for low-light condition aiming. In sights like the SUSAT or Elcan C79 Optical Sight tritium-illuminated reticles are used for low-light condition aiming. Trijicon Corporation uses tritium in their combat and hunting-grade firearm optics, including the ACOG. The (radioactive) tritium light source has to be replaced every 8–12 years, since it gradually loses its brightness due to radioactive decay.

text: Radioactive isotopes

text: can also be used as a light source, to provide an illuminated reticle for low-light condition aiming. In sights like the

text: SUSAT

text: or

text: Elcan

text: C79 Optical Sight

text: tritium

text: -illuminated reticles are used for low-light condition aiming.

text: Trijicon

text: Corporation uses tritium in their combat and hunting-grade firearm optics, including the

text: ACOG

text: . The (radioactive) tritium light source has to be replaced every 8–12 years, since it gradually loses its brightness due to

text: radioactive decay

text: While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., Samarium-147).

text: While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays

text: exponentially

text: at a rate described by a parameter known as the

text: half-life

text: , usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or

text: decay product

text: . In many cases, the daughter nuclide itself is radioactive, resulting in a

text: decay chain

text: , eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g.,

text: tritium

text: ) to over 100 billion years (e.g.,

text: In Seibersdorf, Paul had a magnetic intermediate-image beta ray spectrometer at his disposal.  With it, he measured the shapes of beta ray spectra. Above all, he investigated the spectrometer itself, in order to show that the measured shapes are not due to distortions produced by the apparatus.  An essential part of his work was devoted to the measurement of the electron-neutrino angular correlation in neutron decay, a difficult project that went on for years and was finally concluded when Paul was no longer at Seibersdorf.In Brookhaven, Paul attempted to find a possible parity admixture in an excited state of a radioactive hafnium isotope, which would manifest itself by a small circular polarization of the emitted gamma radiation. The result was negative: there was no circular polarization. While at Brookhaven, Paul also published a summarizing article on the shapes of beta spectra which he had already begun at Seibersdorf.In 1970, Paul received a call to the young  University of Social Sciences, Economics and Business in Linz (from 1975: Johannes Kepler University of Linz) for the newly established chair of Experimental Physics. Paul had the chance to cooperate in the planning of his professorship even prior to beginning his office. Since no chair for experimental physics had existed before, everything had to be built up from scratch (teaching, laboratories, electronics, mechanical workshop).

text: In Seibersdorf, Paul had a magnetic intermediate-image beta ray spectrometer at his disposal.  With it, he measured the shapes of beta ray spectra. Above all, he investigated the spectrometer itself, in order to show that the measured shapes are not due to distortions produced by the apparatus.  An essential part of his work was devoted to the measurement of the electron-neutrino angular correlation in

text: neutron

text: decay, a difficult project that went on for years and was finally concluded when Paul was no longer at Seibersdorf.In Brookhaven, Paul attempted to find a possible

text: parity

text: admixture in an excited state of a radioactive

text: hafnium

text: isotope, which would manifest itself by a small

text: circular polarization

text: of the emitted

text: gamma radiation

text: . The result was negative: there was no circular polarization. While at Brookhaven, Paul also published a summarizing article on the shapes of beta spectra which he had already begun at Seibersdorf.In 1970, Paul received a call to the young

text: University of Social Sciences, Economics and Business

text: in

text: Linz

text: (from 1975: Johannes Kepler University of Linz) for the newly established chair of Experimental Physics. Paul had the chance to cooperate in the planning of his professorship even prior to beginning his office. Since no chair for experimental physics had existed before, everything had to be built up from scratch (teaching, laboratories, electronics, mechanical workshop).

text: In the early 20th century, measurements of the atomic weight of lead showed significant variations depending on the origin of the sample. These differences were considered to be an exception attributed to lead isotopes being products of the natural radioactive decay chains of uranium. In 1930s, however, Malcolm Dole reported that the atomic weight of oxygen in air was slightly different from that in water. Soon thereafter, Alfred Nier reported natural variation in the isotopic composition of carbon. It was becoming clear that atomic weights are not constants of nature. At the Commission’s meeting in 1951, it was recognized that the isotopic-abundance variation of sulfur had a significant effect on the internationally accepted value of an atomic weight. In order to indicate the span of atomic-weight values that may apply to sulfur from different natural sources, the value ± 0.003 was attached to the atomic weight of sulfur. By the 1969, the Commission had assigned uncertainties to all atomic-weight values.

text: In the early 20th century, measurements of the atomic weight of lead showed significant variations depending on the origin of the sample. These differences were considered to be an exception attributed to lead isotopes being products of the natural radioactive decay chains of uranium. In 1930s, however,

text: Malcolm Dole

text: reported that the atomic weight of oxygen in air was slightly different from that in water. Soon thereafter,

text: Alfred Nier

text: reported natural variation in the isotopic composition of carbon. It was becoming clear that atomic weights are not constants of nature. At the Commission’s meeting in 1951, it was recognized that the isotopic-abundance variation of sulfur had a significant effect on the internationally accepted value of an atomic weight. In order to indicate the span of atomic-weight values that may apply to sulfur from different natural sources, the value ± 0.003 was attached to the atomic weight of sulfur. By the 1969, the Commission had assigned uncertainties to all atomic-weight values.

text: Naturally occurring xenon (Xe) is made of eight stable isotopes and one very long-lived isotope. (Xe, Xe, and Xe are predicted to undergo double beta decay, but this has never been observed in these isotopes, so they are considered to be stable.) Xenon has the second highest number of stable isotopes. Only tin, with 10 stable isotopes, has more. Beyond these stable forms, there are over 30 unstable isotopes and isomers that have been studied, the longest-lived of which is Xe, which undergoes double beta decay with a half-life of  with the next longest lived being Xe with a half-life of 36.345 days. Of known isomers, the longest-lived is Xe with a half-life of 11.934 days. Xe is produced by beta decay of I (half-life: 16 million years); Xe, Xe, Xe, and Xe are some of the fission products of both U and Pu, and therefore used as indicators of nuclear explosions.

text: Naturally occurring

text: xenon

text: (Xe) is made of eight

text: stable

text: isotope

text: s and one very long-lived isotope. (Xe, Xe, and Xe are predicted to undergo

text: double beta decay

text: , but this has never been observed in these isotopes, so they are considered to be stable.) Xenon has the second highest

text: number of stable isotopes

text: . Only

text: tin

text: , with 10 stable isotopes, has more. Beyond these stable forms, there are over 30

text: unstable isotope

text: s and isomers that have been studied, the longest-lived of which is Xe, which undergoes

text: double beta decay

text: with a

text: half-life

text: of  with the next longest lived being Xe with a

text: half-life

text: of 36.345 days. Of known isomers, the longest-lived is Xe with a half-life of 11.934 days. Xe is produced by

text: beta decay

text: of

text: I

text: half-life

text: : 16 million years); Xe, Xe, Xe, and Xe are some of the

text: fission product

text: s of both

text: U

text: and

text: Pu

text: , and therefore used as indicators of

text: nuclear explosion

text: s.

text: The law describes which chemical element and isotope is created during the particular type of radioactive decay:

text: The law describes which

text: chemical element

text: and

text: isotope

text: is created during the particular type of radioactive decay:

text: Once formed, proteins only exist for a certain period of time and are then degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan is measured in terms of its half-life and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable.

text: Once formed, proteins only exist for a certain period of time and are then

text: degraded

text: and recycled by the cell's machinery through the process of

text: protein turnover

text: . A protein's lifespan is measured in terms of its

text: half-life

text: and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable.

text: Fenton’s post-doctoral work at the University of Tasmania resulted in the development of a technique to age fish based on the radioactive decay of radium-226 to lead-210. This revolutionized the understanding of the age of the important fishery species, orange roughy. Her work revealed that this species is extremely slow-growing, not maturing until between 20 and 25 years of age, with significant implications for sustainable management of catch limits for the species.

text: Half-life (abbreviated t) is the time required for a quantity to reduce to half its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay. The term is also used more generally to characterize any type of exponential or non-exponential decay. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the body. The converse of half-life is doubling time.

text: Half-life (abbreviated t) is the time required for a quantity to reduce to half its initial value. The term is commonly used in

text: nuclear physics

text: to describe how quickly unstable atoms undergo, or how long stable atoms survive,

text: radioactive decay

text: . The term is also used more generally to characterize any type of

text: exponential

text: or

text: non-exponential

text: decay. For example, the medical sciences refer to the

text: biological half-life

text: of drugs and other chemicals in the body. The converse of half-life is

text: doubling time

text: This article compares the radioactivity release and decay from the Chernobyl disaster with various other events which involved a release of uncontrolled radioactivity.

text: This article compares the radioactivity release and decay from the

text: Chernobyl disaster

text: with various other events which involved a release of uncontrolled radioactivity.

text: See Large, John H: Radioactive Decay Characteristics of Irradiated Nuclear Fuels, January 2006.

text: Terrestrial radiation, for the purpose of the table above, only includes sources that remain external to the body. The major radionuclides of concern are potassium, uranium and thorium and their decay products, some of which, like radium and radon are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, due to radioactive decay since the formation of the Earth, because there is no significant amount currently transported to the Earth. Thus, the present activity on earth from uranium-238 is only half as much as it originally was because of its 4.5 billion year half-life, and potassium-40 (half-life 1.25 billion years) is only at about 8% of original activity. The effects on humans of the actual diminishment (due to decay) of these isotopes is, however, minimal. This is because humans evolved too recently for the difference in activity over a fraction of a half-life to be significant. Put another way, human history is so short in comparison to a half-life of a billion years, that the activity of these long-lived isotopes has been effectively constant throughout our time on this planet.

text: Terrestrial radiation, for the purpose of the table above, only includes sources that remain external to the body. The major

text: radionuclide

text: s of concern are

text: potassium

text: uranium

text: and

text: thorium

text: and their decay products, some of which, like

text: radium

text: and

text: radon

text: are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, due to

text: radioactive decay

text: since the formation of the Earth, because there is no significant amount currently transported to the Earth. Thus, the present activity on earth from

text: is only half as much as it originally was because of its 4.5

text: billion

text: year half-life, and

text: (half-life 1.25 billion years) is only at about 8% of original activity. The effects on humans of the actual diminishment (due to decay) of these isotopes is, however, minimal. This is because humans evolved too recently for the difference in activity over a fraction of a half-life to be significant. Put another way, human history is so short in comparison to a half-life of a billion years, that the activity of these long-lived isotopes has been effectively constant throughout our time on this planet.

text: The equation governing the decay of a radioactive isotope is:

text: A trace radioisotope is a radioisotope that occurs naturally in trace amounts (i.e., extremely small). Generally speaking, trace radioisotopes have half-lives that are short in comparison to the age of the Earth, since primordial nuclides tend to occur in larger than trace amounts. Trace radioisotopes are therefore present only because they are continually produced on Earth by natural processes. Natural processes which produce trace radioisotopes include cosmic ray bombardment of stable nuclides, ordinary alpha and beta decay of the long-lived heavy nuclides, thorium-232, uranium-238, and uranium-235, spontaneous fission of uranium-238, and nuclear transmutation reactions induced by natural radioactivity, such as the production of plutonium-239 and uranium-236 from neutron capture by natural uranium.

text: A trace radioisotope is a

text: radioisotope

text: that occurs naturally in trace amounts (i.e., extremely small). Generally speaking, trace radioisotopes have

text: half-lives

text: that are short in comparison to the

text: age of the Earth

text: , since

text: primordial nuclide

text: s tend to occur in larger than trace amounts. Trace radioisotopes are therefore present only because they are continually produced on Earth by natural processes. Natural processes which produce trace radioisotopes include

text: cosmic ray

text: bombardment of stable nuclides, ordinary

text: alpha

text: and

text: beta decay

text: of the long-lived heavy nuclides,

text: , and

text: spontaneous fission

text: of

text: , and

text: nuclear transmutation

text: reactions induced by

text: natural radioactivity

text: , such as the production of

text: and

text: from

text: neutron capture

text: by

text: natural uranium

text: Dr. Colette Green is a Black Mesa scientist and one half of the protagonist team in Half-Life: Decay.

text: The intermediate stages each emit the same amount of radioactivity as the original radioisotope (though not the same energy). When equilibrium is achieved, a granddaughter isotope is present in direct proportion to its half-life; but since its activity is inversely proportional to its half-life, each nuclide in the decay chain finally contributes as much radioactivity as the head of the chain, though not the same energy. For example, uranium-238 is weakly radioactive, but pitchblende, a uranium ore, is 13 times more radioactive than the pure uranium metal of the same amount because of the radium and other daughter isotopes it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain they also generate radon, a heavy, inert, naturally occurring radioactive gas. Rock containing thorium and/or uranium (such as some granites) emits radon gas that can accumulate in enclosed places such as basements or underground mines. Radon exposure is considered the leading cause of lung cancer in non-smokers.

text: The intermediate stages each emit the same amount of radioactivity as the original radioisotope (though not the same energy). When equilibrium is achieved, a granddaughter isotope is present in direct proportion to its half-life; but since its activity is inversely proportional to its half-life, each nuclide in the decay chain finally contributes as much radioactivity as the head of the chain, though not the same energy. For example, uranium-238 is weakly radioactive, but

text: pitchblende

text: , a uranium ore, is 13 times more radioactive than the pure uranium metal of the same amount because of the

text: radium

text: and other daughter isotopes it contains. Not only are unstable radium isotopes significant radioactivity emitters, but as the next stage in the decay chain they also generate

text: radon

text: , a heavy, inert, naturally occurring radioactive gas. Rock containing thorium and/or uranium (such as some granites) emits radon gas that can accumulate in enclosed places such as basements or underground mines. Radon exposure is considered the leading cause of

text: lung cancer

text: in non-smokers.

text: Radioactive beams of specific isotopes are widely used in the fields of experimental physics, biology and materials science. The production and formation of these radioactive atoms into an ionic beam for study is an entire field of research carried out at many laboratories throughout the world. The first isotope separator was developed at the Copenhagen Cyclotron by Bohr and co-workers using the principle of electromagnetic separation. Today, there are many laboratories around the world which supply beams of radioactive ions for use. Arguably the principal Isotope Separator On-Line (ISOL) is ISOLDE at CERN, which is a joint European facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses mainly proton spallation of uranium carbide targets to produce a wide range of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), a uranium carbide target is heated to several thousand degrees so that radioactive atoms produced in the nuclear reaction are released. Once out of the target, the vapour of radioactive atoms travels to an ionizer cavity. This ionizer cavity is a thin tube made of a refractory metal with a high work function allowing for collisions with the walls to liberate a single electron from a free atom (surface ionization effect). Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approximately equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.

text: work function

text: allowing for collisions with the walls to liberate a single electron from a free atom (

text: surface ionization

text: effect). Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approximately equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.

text: L.W. Alvarez and Robert Cornog of the United States first used an accelerator as a mass spectrometer in 1939 when they employed a cyclotron to demonstrate that He was stable; from this observation they immediately and correctly concluded that the other mass-3 isotope tritium was radioactive. In 1977, inspired by this early work, Richard A. Muller at the Lawrence Berkeley Laboratory recognized that modern accelerators could accelerate radioactive particles to an energy where the background interferences could be separated using particle identification techniques.  He published the seminal paper in Science showing how accelerators (cyclotrons and linear) could be used for detection of tritium, radiocarbon (C), and several other isotopes of scientific interest including Be; he also reported the first successful radioisotope date experimentally obtained using tritium (H).  His paper was the direct inspiration for other groups using cyclotrons (G. Raisbeck and F. Yiou, in France) and tandem linear accelerators (D. Nelson, R. Korteling, W. Stott at McMaster).  K. Purser and colleagues also published the successful detection of radiocarbon using their tandem at Rochester.  Soon afterwards the Berkeley and French teams reported the successful detection of Be, an isotope widely used in geology. Soon the accelerator technique, since it was more sensitive by a factor of about 1,000, virtually supplanted the older “decay counting” methods for these and other radioisotopes.

text: L.W. Alvarez

text: and

text: Robert Cornog

text: of the United States first used an accelerator as a mass spectrometer in 1939 when they employed a

text: cyclotron

text: to demonstrate that He was stable; from this observation they immediately and correctly concluded that the other mass-3 isotope

text: tritium

text: was radioactive. In 1977, inspired by this early work,

text: Richard A. Muller

text: at the

text: Lawrence Berkeley Laboratory

text: recognized that modern accelerators could accelerate radioactive particles to an energy where the background interferences could be separated using particle identification techniques.  He published the seminal paper in

text: Science

text: showing how accelerators (cyclotrons and linear) could be used for detection of tritium, radiocarbon (C), and several other isotopes of scientific interest including Be; he also reported the first successful radioisotope date experimentally obtained using tritium (H).  His paper was the direct inspiration for other groups using cyclotrons (G. Raisbeck and F. Yiou, in France) and tandem linear accelerators (D. Nelson, R. Korteling, W. Stott at McMaster).  K. Purser and colleagues also published the successful detection of radiocarbon using their tandem at Rochester.  Soon afterwards the Berkeley and French teams reported the successful detection of Be, an isotope widely used in geology. Soon the accelerator technique, since it was more sensitive by a factor of about 1,000, virtually supplanted the older “decay counting” methods for these and other radioisotopes.

text: Inheritance: Genome-wide analyses showed that the bulk of Helitrons tend to be quite recent. The young age of Helitron families is of course biased by the genomes that have been examined carefully, which are predominately plant and insect where the unconstrained DNA half-life (the average amount of time when half of DNA not conserved for function is lost) is quite rapid. In contrast to other DNA transposons, Helitrons from some species have been reported to exhibit long-term activity probably due to the mechanism of transposition or inability of the host to recognize Helitrons because of either sequence heterogeneity or host gene capture. In contrast, to the relatively faster unconstrained DNA half-life (2.5–14 my) of the plant and insect genomes, the mammalian DNA half-life is estimated to be much slower (884 my) which along with the minimal requirements of Helitron transposition and the slow rate of decay in mammals have caused this pattern of vertical persistence.

text: The becquerel (symbol Bq) (pronounced:  ) is the  SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The becquerel is therefore equivalent to an inverse second, s. The becquerel is named after Henri Becquerel, who shared a Nobel Prize with Pierre Curie and Marie Curie in 1903 for their work in discovering radioactivity.

text: The becquerel (symbol Bq) (pronounced:  ) is the

text: SI derived unit

text: of

text: radioactivity

text: . One becquerel is defined as the

text: activity

text: of a quantity of radioactive material in which one

text: nucleus

text: decays per

text: second

text: . The becquerel is therefore equivalent to an inverse second,

text: s

text: . The becquerel is named after

text: Henri Becquerel

text: , who shared a

text: Nobel Prize

text: with

text: Pierre Curie

text: and

text: Marie Curie

text: in 1903 for their work in discovering radioactivity.

text: Not all neutrons are emitted as a direct product of fission;  some are instead due to the radioactive decay of some of the fission fragments.  The neutrons that occur directly from fission are called "prompt neutrons," and the ones that are a result of radioactive decay of fission fragments are called "delayed neutrons." The fraction of neutrons that are delayed is called β, and this fraction is typically less than 1% of all the neutrons in the chain reaction.

text: Not all neutrons are emitted as a direct product of fission;  some are instead due to the

text: radioactive decay

text: of some of the fission fragments.  The neutrons that occur directly from fission are called "

text: prompt neutron

text: s," and the ones that are a result of radioactive decay of fission fragments are called "delayed neutrons." The fraction of neutrons that are delayed is called β, and this fraction is typically less than 1% of all the neutrons in the chain reaction.

text: The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass (or 12:1 by number of atoms), along with tiny traces of the next two elements, lithium and beryllium. Almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are naturally produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are also naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, and other rarer modes of decay.

text: The lightest chemical elements are hydrogen and

text: helium

text: , both created by

text: Big Bang nucleosynthesis

text: during the

text: first 20 minutes of the universe

text: in a

text: ratio

text: of around 3:1 by mass (or 12:1 by number of atoms), along with tiny traces of the next two elements,

text: lithium

text: and

text: beryllium

text: . Almost all other elements found in nature were made by various natural methods of

text: nucleosynthesis

text: . On Earth, small amounts of new atoms are naturally produced in

text: nucleogenic

text: reactions, or in

text: cosmogenic

text: processes, such as

text: cosmic ray spallation

text: . New atoms are also naturally produced on Earth as

text: radiogenic

text: daughter isotope

text: s of ongoing

text: radioactive decay

text: processes such as

text: alpha decay

text: beta decay

text: spontaneous fission

text: cluster decay

text: , and other rarer modes of decay.

text: Most fission products have too many neutrons to be stable so they are radioactive by beta decay, converting neutrons into protons by throwing off beta particles (electrons) and gamma rays. Their half lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability. In reactors, the radioactive products are the nuclear waste in spent fuel. In bombs, they become radioactive fallout, both local and global.

text: Most fission products have too many neutrons to be stable so they are radioactive by

text: beta decay

text: , converting neutrons into protons by throwing off beta particles (electrons) and gamma rays. Their half lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability. In reactors, the radioactive products are the nuclear waste in spent fuel. In bombs, they become radioactive fallout, both local and global.

text: National LEB figures reported by statistical national agencies and international organizations are indeed estimates of period LEB. In the Bronze Age and the Iron Age, LEB was 26 years; the 2010 world LEB was 67.2 years. For recent years, in Swaziland LEB is about 49, and in Japan, it is about 83. The combination of high infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers LEB. But for those who survive early hazards, a life expectancy of 60 or 70 would not be uncommon. For example, a society with a LEB of 40 may have few people dying at precisely 40: most will die before 30 or very few after 55. In populations with high infant mortality rates, LEB is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, LEB can be subjected to gross misinterpretation, leading one to believe that a population with a low LEB will necessarily have a small proportion of older people. For example, in a hypothetical stationary population in which half the population dies before the age of five but everybody else dies at exactly 70 years old, LEB will be about 36, but about 25% of the population will be between the ages of 50 and 70. Another measure, such as life expectancy at age 5 (e), can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood; in the hypothetical population above, life expectancy at 5 would be another 65. Aggregate population measures, such as the proportion of the population in various age groups, should also be used along individual-based measures like formal life expectancy when analyzing population structure and dynamics.

text: National LEB figures reported by statistical national agencies and international organizations are indeed estimates of period LEB. In the

text: Bronze Age

text: and the

text: Iron Age

text: , LEB was 26 years; the 2010 world LEB was 67.2 years. For recent years, in

text: Swaziland

text: LEB is about 49, and in

text: Japan

text: , it is about 83. The combination of high

text: infant mortality

text: Due to the abundance of radioactive elements that can be incorporated into zircons, they are also susceptible to damage from radioactive decay through the process of metamictization. As radioactive elements within the crystal lattice decay, they bombard the interior of the crystal with radioactive particles. This weakens the crystal and leave it fractured or destroyed. This increases the chance of isotopes leaking out of the crystal and affecting titanium, or other elements, measurements.

text: Due to the abundance of radioactive elements that can be incorporated into zircons, they are also susceptible to damage from radioactive decay through the process of

text: metamictization

text: . As radioactive elements within the crystal lattice decay, they bombard the interior of the crystal with radioactive particles. This weakens the crystal and leave it fractured or destroyed. This increases the chance of isotopes leaking out of the crystal and affecting titanium, or other elements, measurements.

text: In 1895, Perry published a paper challenging Kelvin's assumption of low thermal conductivity inside the Earth, and thus disputing Kelvin's estimate that the Earth was only 20–400 million years old, but this had little impact. It was not until the discovery in 1903 that radioactive decay releases heat and the development a few years later of radiometric dating of rocks that it was accepted that the age of the earth was many times older, as Perry had argued. Perry's reasoning held that if the interior of the Earth was fluid, or partly fluid, it would transfer heat much more effectively than the conductivity which Kelvin assumed, and he stated that "much internal fluidity would practically mean infinite conductivity for our purpose."

text: In 1895, Perry published a paper challenging Kelvin's assumption of low

text: thermal conductivity

text: inside the Earth, and thus disputing Kelvin's estimate that the Earth was only 20–400 million years old, but this had little impact. It was not until the discovery in 1903 that

text: radioactive decay

text: releases heat and the development a few years later of

text: radiometric dating

text: of rocks that it was accepted that the age of the earth was many times older, as Perry had argued. Perry's reasoning held that if the interior of the Earth was fluid, or partly fluid, it would transfer heat much more effectively than the conductivity which Kelvin assumed, and he stated that "much internal fluidity would practically mean infinite conductivity for our purpose."

text: Isochron dating is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks. Isochron dating can be further separated into mineral isochron dating and whole rock isochron dating; both techniques are applied frequently to date terrestrial and also extraterrestrial rocks (meteorites). The advantage of isochron dating as compared to simple radiometric dating techniques is that no assumptions are needed about the initial amount of the daughter nuclide in the radioactive decay sequence. Indeed, the initial amount of the daughter product can be determined using isochron dating. This technique can be applied if the daughter element has at least one stable isotope other than the daughter isotope into which the parent nuclide decays.

text: Isochron dating is a common technique of

text: radiometric dating

text: and is applied to date certain events, such as

text: crystal

text: lization,

text: metamorphism

text: , shock events, and differentiation of precursor melts, in the history of

text: rocks

text: . Isochron dating can be further separated into mineral isochron dating and whole rock isochron dating; both techniques are applied frequently to date terrestrial and also extraterrestrial rocks (

text: meteorite

text: s). The advantage of isochron dating as compared to simple radiometric dating techniques is that no assumptions are needed about the initial amount of the daughter

text: nuclide

text: in the

text: radioactive decay

text: sequence. Indeed, the initial amount of the daughter product can be determined using isochron dating. This technique can be applied if the daughter element has at least one stable

text: isotope

text: other than the daughter isotope into which the parent nuclide decays.

text: Radioactive isotopes of phosphorus include:

text: Radioactive

text: isotope

text: s of phosphorus include:

text: Nucleation is usually a stochastic process, so even in two identical systems nucleation will occur at different times. This behaviour is similar to radioactive decay. A common mechanism is illustrated in the animation to the right. This shows nucleation of a new phase (shown in red) in an existing phase (white). In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of the new red phase is so large it is more favourable for it to grow than to shrink back to nothing. This nucleus of the red phase then grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called classical nucleation theory.

text: Nucleation is usually a

text: stochastic

text: process, so even in two identical systems nucleation will occur at different times. This behaviour is similar to

text: radioactive decay

text: . A common mechanism is illustrated in the animation to the right. This shows nucleation of a new phase (shown in red) in an existing phase (white). In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of the new red phase is so large it is more favourable for it to grow than to shrink back to nothing. This nucleus of the red phase then grows and converts the system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called

text: classical nucleation theory

text: WITCH (standing for "Weak Interaction Trap for Charged particles"), or experiment IS433, is a double Penning trap experiment to measure the recoil energy of decaying nuclei. A spectrometer in combination with a position-sensitive microchannel plate detector is used to count ions (with the MCP) while scanning their energy (using the spectrometer). The experiment is located at the ISOLDE Radioactive Ion Beam Facility in CERN. The beam from ISOLDE is bunched by REXTRAP after which it is transferred to the WITCH set-up.

text: WITCH (standing for "Weak Interaction Trap for Charged particles"), or experiment IS433, is a double

text: Penning trap

text: experiment to measure the recoil energy of decaying

text: nuclei

text: . A spectrometer in combination with a position-sensitive

text: microchannel plate detector

text: is used to count ions (with the MCP) while scanning their energy (using the spectrometer). The experiment is located at the

text: ISOLDE Radioactive Ion Beam Facility

text: in

text: CERN

text: . The beam from ISOLDE is bunched by

text: REXTRAP

text: after which it is transferred to the WITCH set-up.

text: Gradually it was realized that the radiation produced by radioactive decay was ionizing radiation, and that even quantities too small to burn could pose a severe long-term hazard. Many of the scientists working on radioactivity died of cancer as a result of their exposure. Radioactive patent medicines mostly disappeared, but other applications of radioactive materials persisted, such as the use of radium salts to produce glowing dials on meters.

text: Gradually it was realized that the radiation produced by radioactive decay was

text: ionizing radiation

text: , and that even quantities too small to burn could pose a

text: severe long-term hazard

text: . Many of the scientists working on radioactivity died of

text: cancer

text: as a result of their exposure. Radioactive patent medicines mostly disappeared, but other applications of radioactive materials persisted, such as the use of radium salts to produce

text: glowing dials on meters

text: There are naturally occurring radioactive material (e.g., radium and radon) in shale deposits. Hydraulic fracturing can dislodge naturally occurring heavy metals and radioactive materials from shale deposits, and these substances return to the surface with flowback, also referred to as wastewater.  These naturally occurring radionuclides are of more concern than some man-made radionuclides used in fracture monitoring because of their long half lives. Radium (Ra) is a product of Uranium-238 decay, and is the longest-lived isotope of radium with a half-life of 1601 years; next longest is Radium-228, a product of Thorium-232 breakdown, with a half-life of 5.75 years. Radon (Rn) is a naturally occurring product of the decay of uranium or thorium. Its most stable isotope, Radon-222, has a half-life of 3.8 days. Strontium is also naturally occurring and may be dislodged by the process.

text: There are naturally occurring radioactive material (e.g.,

text: radium

text: and

text: radon

text: ) in shale deposits. Hydraulic fracturing can dislodge naturally occurring heavy metals and radioactive materials from shale deposits, and these substances return to the surface with flowback, also referred to as wastewater.  These naturally occurring radionuclides are of more concern than some man-made radionuclides used in fracture monitoring because of their long half lives. Radium (Ra) is a product of

text: decay, and is the longest-lived isotope of radium with a

text: half-life

text: of 1601 years; next longest is

text: , a product of

text: breakdown, with a half-life of 5.75 years. Radon (Rn) is a naturally occurring product of the decay of

text: uranium

text: or

text: thorium

text: . Its most stable

text: isotope

text: , has a

text: half-life

text: of 3.8 days.

text: Strontium

text: is also naturally occurring and may be dislodged by the process.

text: The only primordial isotope of bismuth, bismuth-209, was traditionally regarded as the heaviest stable isotope, but it had long been suspected to be unstable on theoretical grounds. This was finally demonstrated in 2003, when researchers at the Institut d'Astrophysique Spatiale in Orsay, France, measured the alpha emission half-life of  to be , over a billion times longer than the current estimated age of the universe. Owing to its extraordinarily long half-life, for all presently known medical and industrial applications, bismuth can be treated as if it is stable and nonradioactive. The radioactivity is of academic interest because bismuth is one of few elements whose radioactivity was suspected and theoretically predicted before being detected in the laboratory. Bismuth has the longest known alpha decay half-life, although tellurium-128 has a double beta decay half-life of over .

text: The only primordial

text: isotope

text: of bismuth,

text: , was traditionally regarded as the heaviest stable isotope, but it had long been suspected to be unstable on theoretical grounds. This was finally demonstrated in 2003, when researchers at the Institut d'Astrophysique Spatiale in

text: Orsay

text: , France, measured the

text: alpha emission

text: half-life

text: of  to be , over a

text: billion

text: times longer than the current estimated

text: age of the universe

text: . Owing to its extraordinarily long half-life, for all presently known medical and industrial applications, bismuth can be treated as if it is stable and nonradioactive. The radioactivity is of academic interest because bismuth is one of few elements whose radioactivity was suspected and theoretically predicted before being detected in the laboratory. Bismuth has the longest known alpha decay half-life, although

text: has a

text: double beta decay

text: half-life of over .

text: Some naturally occurring isotopes are entirely radiogenic, but all these are isotopes that are radioactive, with half-lives too short to occur primordially. Thus, they are only present as radiogenic daughters of either ongoing decay processes, or else cosmogenic (cosmic ray induced) processes that produce them in nature freshly. A few others are naturally produced by nucleogenic processes (natural nuclear reactions of other types, such as neutron absorption).

text: Some naturally occurring isotopes are entirely radiogenic, but all these are isotopes that are radioactive, with half-lives too short to occur primordially. Thus, they are only present as radiogenic daughters of either ongoing decay processes, or else cosmogenic (cosmic ray induced) processes that produce them in nature freshly. A few others are naturally produced by

text: nucleogenic

text: processes (natural nuclear reactions of other types, such as neutron absorption).

text: In January 1934, Irène Joliot-Curie and Frédéric Joliot announced that they had bombarded elements with alpha particles and induced radioactivity in them. By March, Fermi's assistant Gian-Carlo Wick had provided a theoretical explanation using Fermi's theory of beta decay. Fermi decided to switch to experimental physics, using the neutron, which James Chadwick had discovered in 1932. In March 1934, Fermi wanted to see if he could induce radioactivity with Rasetti's polonium-beryllium neutron source. Neutrons had no electric charge, and so would not be deflected by the positively charged nucleus. This meant that they needed much less energy to penetrate the nucleus than charged particles, and so would not require a particle accelerator, which the Via Panisperna boys did not have.

text: In January 1934,

text: Irène Joliot-Curie

text: and

text: Frédéric Joliot

text: announced that they had bombarded elements with

text: alpha particle

text: s and induced

text: radioactivity

text: in them. By March, Fermi's assistant

text: Gian-Carlo Wick

text: had provided a theoretical explanation using Fermi's theory of beta decay. Fermi decided to switch to experimental physics, using the

text: neutron

text: , which

text: James Chadwick

text: had discovered in 1932. In March 1934, Fermi wanted to see if he could induce radioactivity with Rasetti's

text: polonium

text: beryllium

text: neutron source

text: . Neutrons had no electric charge, and so would not be deflected by the positively charged nucleus. This meant that they needed much less energy to penetrate the nucleus than charged particles, and so would not require a

text: particle accelerator

text: , which the Via Panisperna boys did not have.

text: Americium-241 (Am) is an isotope of americium. Like all isotopes of americium, it is radioactive. Am is the most common isotope of americium. It is the most prevalent isotope of americium in nuclear waste. Americium-241 has a half-life of 432.2 years. It is commonly found in ionization type smoke detectors. It is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β from Pu, EC from Cm and α from Bk. Am is fissile and the critical mass of a bare sphere is 57.6-75.6 kilograms and a sphere diameter of 19–21 centimeters. Americium-241 has a specific activity of 3.43 Ci/g (Curies per gram or 117.29 Gigabequerels (GBq) per gram). It is commonly found in the form of americium-241 dioxide (AmO). This isotope also has one meta state; Am, with an exitation energy of 2.2 MeV, and a half-life of 1.23 μs. Its presence in plutonium is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of Am. A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of plutonium pits) may be required in some cases.

text: Americium-241 (Am) is an

text: isotope of americium

text: . Like all isotopes of americium, it is

text: radioactive

text: . Am is the most common

text: isotope

text: of

text: americium

text: . It is the most prevalent isotope of americium in

text: nuclear waste

text: . Americium-241 has a

text: half-life

text: of 432.2 years. It is commonly found in ionization type smoke detectors. It is a potential fuel for long-lifetime

text: radioisotope thermoelectric generators

text: (RTGs). Its common parent nuclides are

text: β

text: from Pu,

text: EC

text: from Cm and

text: α

text: from Bk. Am is

text: fissile

text: and the

text: critical mass

text: of a bare sphere is 57.6-75.6 kilograms and a sphere diameter of 19–21 centimeters. Americium-241 has a specific activity of 3.43 Ci/g (Curies per gram or 117.29 Gigabequerels (GBq) per gram). It is commonly found in the form of

text: americium-241 dioxide

text: (AmO). This isotope also has one

text: meta state

text: ; Am, with an exitation energy of 2.2

text: MeV

text: , and a

text: half-life

text: of 1.23 μs. Its presence in

text: plutonium

text: is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of Am. A chemical removal of americium-241 from reworked plutonium (e.g. during reworking of

text: plutonium pit

text: s) may be required in some cases.

text: The half-life of a population is the time taken for the population to decline to half its size. We can calculate the half-life of a geometric population using the equation:

text: The

text: half-life

text: of a population is the time taken for the population to decline to half its size. We can calculate the half-life of a geometric population using the equation:

text: Einsteinium is a soft, silvery, paramagnetic metal. Its chemistry is typical of the late actinides, with a preponderance of the +3 oxidation state; the +2 oxidation state is also accessible, especially in solids. The high radioactivity of einsteinium-253 produces a visible glow and rapidly damages its crystalline metal lattice, with released heat of about 1000 watts per gram. Difficulty in studying its properties is due to einsteinium-253's decay to berkelium-249 and then californium-249 at a rate of about 3% per day. The isotope of einsteinium with the longest half life, einsteinium-252 (half life 471.7 days) would be more suitable for investigation of physical properties, but it has proven far more difficult to produce and is available only in minute quantities, and not in bulk. Einsteinium is the element with the highest atomic number which has been observed in macroscopic quantities in its pure form, and this was the common short-lived isotope einsteinium-253.

text: Einsteinium is a soft, silvery,

text: paramagnetic

text: metal

text: . Its chemistry is typical of the late actinides, with a preponderance of the +3

text: oxidation state

text: ; the +2 oxidation state is also accessible, especially in solids. The high radioactivity of einsteinium-253 produces a visible glow and rapidly damages its crystalline metal lattice, with released heat of about 1000

text: watt

text: s per gram. Difficulty in studying its properties is due to einsteinium-253's decay to

text: berkelium

text: -249 and then

text: californium

text: -249 at a rate of about 3% per day. The isotope of einsteinium with the longest half life, einsteinium-252 (half life 471.7 days) would be more suitable for investigation of physical properties, but it has proven far more difficult to produce and is available only in minute quantities, and not in bulk. Einsteinium is the element with the highest atomic number which has been observed in macroscopic quantities in its pure form, and this was the common short-lived isotope einsteinium-253.

text: The principle behind the use of radioactive tracers is that an atom in a chemical compound is replaced by another atom, of the same chemical element. The substituting atom, however, is a radioactive isotope. This process is often called radioactive labeling. The power of the technique is due to the fact that radioactive decay is much more energetic than chemical reactions. Therefore, the radioactive isotope can be present in low concentration and its presence detected by sensitive radiation detectors such as Geiger counters and scintillation counters. George de Hevesy won the 1943 Nobel Prize for Chemistry "for his work on the use of isotopes as tracers in the study of chemical processes".

text: The principle behind the use of radioactive tracers is that an

text: atom

text: in a

text: chemical compound

text: is replaced by another atom, of the same chemical element. The substituting atom, however, is a radioactive isotope. This process is often called radioactive labeling. The power of the technique is due to the fact that radioactive decay is much more energetic than chemical reactions. Therefore, the radioactive isotope can be present in low concentration and its presence detected by sensitive

text: radiation detector

text: s such as

text: Geiger counter

text: s and

text: scintillation counter

text: s.

text: George de Hevesy

text: won the 1943

text: Nobel Prize for Chemistry

text: "for his work on the use of isotopes as tracers in the study of chemical processes".

text: The genome of the flu virus includes about 13,000 base pairs, which had decayed into pieces as small as 100 base pairs. In order to make PCR work, primers have to be constructed, i.e. a short bit of DNA with mirror sequences of the sequence at the two end points of the fragment. They bind to the fragment, and with the help of a polymerase bases are added between the primers to make a copy. The millions of copies of the gene segment are labelled with a radioactive probe as they are being made. They can then be separated on a thin gel by running an electric current across the gel. The radioactive labels create a black mark on an X-ray film which is put over the gel.

text: The genome of the flu virus includes about 13,000 base pairs, which had decayed into pieces as small as 100 base pairs. In order to make PCR work,

text: primers

text: have to be constructed, i.e. a short bit of DNA with mirror sequences of the sequence at the two end points of the fragment. They bind to the fragment, and with the help of a

text: polymerase

text: bases are added between the primers to make a copy. The millions of copies of the gene segment are labelled with a radioactive probe as they are being made. They can then be separated on a thin gel by running an electric current across the gel. The radioactive labels create a black mark on an X-ray film which is put over the gel.

text: The Berkeley team, consisting of Albert Ghiorso, Glenn T. Seaborg, John R. Walton and Torbjørn Sikkeland, then claimed the synthesis of element 102 in 1958. The team used the new heavy-ion linear accelerator (HILAC) to bombard a curium target (95% Cm and 5% Cm) with C and C ions. They were unable to confirm the 8.5 MeV activity claimed by the Swedes but were instead able to detect decays from fermium-250, supposedly the daughter of 102 (produced from the curium-246), which had an apparent half-life of ~3 s. Later 1963 Dubna work confirmed that 102 could be produced in this reaction, but that its half-life was actually 50 ± 10 s. In 1967, the Berkeley team attempted to defend their work, stating that the isotope found was indeed Fm but the isotope that the half-life measurements actually related to was californium-244, granddaughter of 102, produced from the more abundant curium-244. Energy differences were then attributed to "resolution and drift problems", although these had not been previously reported and should also have influenced other results. 1977 experiments showed that 102 indeed had a 2.3-second half-life. However, 1973 work also showed that the Fm recoil could have also easily been produced from the isomeric transition of Fm (half-life 1.8 s) which could also have been formed in the reaction at the energy used. Given this, it is probable that no nobelium was actually produced in this experiment.

text: The Berkeley team, consisting of

text: Albert Ghiorso

text: Glenn T. Seaborg

text: John R. Walton

text: and

text: Torbjørn Sikkeland

text: , then claimed the synthesis of element 102 in 1958. The team used the new heavy-

text: ion

text: linear accelerator

text: (HILAC) to bombard a

text: curium

text: target (95%

text: Cm

text: and 5% Cm) with

text: C

text: and C ions. They were unable to confirm the 8.5 MeV activity claimed by the Swedes but were instead able to detect decays from fermium-250, supposedly the daughter of 102 (produced from the curium-246), which had an apparent

text: half-life

text: of ~3 s. Later 1963 Dubna work confirmed that 102 could be produced in this reaction, but that its half-life was actually 50 ± 10 s. In 1967, the Berkeley team attempted to defend their work, stating that the isotope found was indeed Fm but the isotope that the half-life measurements actually related to was californium-244,

text: granddaughter

text: of 102, produced from the more abundant curium-244. Energy differences were then attributed to "resolution and drift problems", although these had not been previously reported and should also have influenced other results. 1977 experiments showed that 102 indeed had a 2.3-second half-life. However, 1973 work also showed that the Fm recoil could have also easily been produced from the

text: isomeric transition

text: of Fm (half-life 1.8 s) which could also have been formed in the reaction at the energy used. Given this, it is probable that no nobelium was actually produced in this experiment.

text: The factor of  in the above relations results from the fact that concept of "half-life" is merely a way of selecting a different base other than the natural base  for the lifetime expression. The time constant  is the -life, the time until only 1/e remains, about 36.8%, rather than the 50% in the half-life of a radionuclide. Thus,  is longer than . The following equation can be shown to be valid:

text: It is likely that the fissile material of an old bomb which is due for refitting will contain decay products of the plutonium isotopes used in it, these are likely to include U-236 from Pu-240 impurities, plus some U-235 from decay of the Pu-239; due to the relatively long half-life of these Pu isotopes, these wastes from radioactive decay of bomb core material would be very small, and in any case, far less dangerous (even in terms of simple radioactivity) than the Pu-239 itself.

text: It is likely that the fissile material of an old bomb which is due for refitting will contain decay products of the plutonium isotopes used in it, these are likely to include

text: from Pu-240 impurities, plus some U-235 from decay of the Pu-239; due to the relatively long half-life of these Pu isotopes, these wastes from radioactive decay of bomb core material would be very small, and in any case, far less dangerous (even in terms of simple radioactivity) than the Pu-239 itself.

text: Much of the light curve, or graph of luminosity as a function of time, after the explosion of a Type II Supernova such as SN 1987A is provided its energy by radioactive decay. Although the luminous emission consists of optical photons, it is the radioactive power absorbed that keeps the remnant hot enough to radiate light. Without radioactive heat it would quickly dim. The radioactive decay of Ni through its daughters Co to Fe produces gamma-ray photons that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of Ni to Co (half life 6 days) while energy for the later light curve in particular fit very closely with the 77.3 day half-life of Co decaying to Fe. Later measurements by space gamma-ray telescopes of the small fraction of the Co  and Co gamma rays that escaped the SN1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power source.

text: Much of the

text: light curve

text: , or graph of luminosity as a function of time, after the explosion of a

text: Type II Supernova

text: such as SN 1987A is provided its energy by

text: radioactive decay

text: . Although the luminous emission consists of optical photons, it is the radioactive power absorbed that keeps the remnant hot enough to radiate light. Without radioactive heat it would quickly dim. The

text: radioactive decay

text: of

text: Ni

text: through its daughters

text: Co

text: to

text: Fe

text: produces gamma-ray

text: photon

text: s that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of

text: Ni

text: to Co (half life 6 days) while energy for the later light curve in particular fit very closely with the 77.3 day half-life of

text: Co

text: decaying to Fe. Later measurements by space gamma-ray telescopes of the small fraction of the Co  and Co gamma rays that escaped the SN1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power source.

text: In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed.  So, for example, digoxin has a half-life (or t) of 24–36 h; this means that a change in the dose will take the best part of a week to take full effect. For this reason, drugs with a long half-life (e.g., amiodarone, elimination t of about 58 days) are usually started with a loading dose to achieve their desired clinical effect more quickly.

text: In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed.  So, for example, digoxin has a half-life (or t) of 24–36 h; this means that a change in the dose will take the best part of a week to take full effect. For this reason, drugs with a long half-life (e.g.,

text: amiodarone

text: , elimination t of about 58 days) are usually started with a

text: loading dose

text: to achieve their desired clinical effect more quickly.

text: The energy and the type of the ionizing radiation emitted by a radioactive substance are also important factors in determining its threat to humans. The chemical properties of the radioactive element will determine how mobile the substance is and how likely it is to spread into the environment and contaminate humans. This is further complicated by the fact that many radioisotopes do not decay immediately to a stable state but rather to radioactive decay products within a decay chain before ultimately reaching a stable state.

text: The energy and the type of the

text: ionizing radiation

text: emitted by a radioactive substance are also important factors in determining its threat to humans. The chemical properties of the radioactive

text: element

text: will determine how mobile the substance is and how likely it is to spread into the environment and

text: contaminate

text: humans. This is further complicated by the fact that many radioisotopes do not decay immediately to a stable state but rather to radioactive

text: decay product

text: s within a

text: decay chain

text: before ultimately reaching a stable state.

text: The interplay of these forces produces a number of different phenomena in which energy may be released by rearrangement of particles in the nucleus, or else the change of one type of particle into others. These rearrangements and transformations may be hindered energetically, so that they do not occur immediately. In certain cases, random quantum vacuum fluctuations are theorized to promote relaxation to a lower energy state (the "decay") in a phenomenon known as quantum tunneling. Radioactive decay half-life of nuclides has been measured over timescales of 55 orders of magnitude, from 2.3 x 10 seconds (for hydrogen-7) to 6.9 x 10 seconds (for tellurium-128). The limits of these timescales are set by the sensitivity of instrumentation only, and there are no known natural limits to how brief or long a decay half life for radioactive decay of a radionuclide may be.

text: The interplay of these forces produces a number of different phenomena in which energy may be released by rearrangement of particles in the nucleus, or else the change of one type of particle into others. These rearrangements and transformations may be hindered energetically, so that they do not occur immediately. In certain cases, random

text: quantum vacuum fluctuation

text: s are theorized to promote relaxation to a lower energy state (the "decay") in a phenomenon known as

text: quantum tunneling

text: . Radioactive decay

text: half-life

text: of nuclides has been measured over timescales of 55 orders of magnitude, from 2.3 x 10 seconds (for

text: ) to 6.9 x 10 seconds (for

text: ). The limits of these timescales are set by the sensitivity of instrumentation only, and there are no known natural limits to how brief or long a decay

text: half life

text: for radioactive decay of a

text: radionuclide

text: may be.

text: Indium (In) consists of two primordial nuclides, with the most common (~ 95.7%) nuclide (In) being measurably though weakly radioactive. Its spin-forbidden decay has a half life of 441 trillion years.

text: Indium

text: (In) consists of two

text: primordial nuclide

text: s, with the most common (~ 95.7%) nuclide (In) being measurably though weakly radioactive. Its spin-forbidden decay has a half life of 441 trillion years.

text: In October 2005, work was begun by a group of Ukrainian developers to port Decay over to Windows, as a modification for both the old World Opponent Network and current Steam versions of Half-Life. The port relied on the discovery of a method that allowed Decays PS2 game files to be deciphered, thereby allowing the files to be ported to Windows. The port went into the beta development stage in December 2007 and was released publicly on September 23, 2008. The port was received well by the journalists in the industry; GameSpy site Planet Half-Life noted that it was a shame that an official PC version of Decay never emerged, while British journalist Alec Meer stated that it was "fantastic to have this short but sweet lost Half-Life episode on PC at last, and it even has something the PS2 version didn't—online play".

text: In October 2005, work was begun by a group of Ukrainian developers to

text: port

text: Decay over to

text: Windows

text: , as a

text: modification

text: for both the old

text: World Opponent Network

text: and current

text: Steam

text: versions of Half-Life. The port relied on the discovery of a method that allowed Decays PS2 game files to be deciphered, thereby allowing the files to be ported to Windows. The port went into the

text: beta development stage

text: in December 2007 and was released publicly on September 23, 2008. The port was received well by the journalists in the industry;

text: GameSpy

text: site

text: Planet Half-Life

text: noted that it was a shame that an official PC version of Decay never emerged, while British journalist Alec Meer stated that it was "fantastic to have this short but sweet lost Half-Life episode on PC at last, and it even has something the PS2 version didn't—online play".

text: An additional ~3000 radioactive isotopes not found in nature have been created in nuclear reactors and in particle accelerators. Many short-lived isotopes not found naturally on Earth have also been observed by spectroscopic analysis, being naturally created in stars or supernovae. An example is aluminium-26, which is not naturally found on Earth, but is found in abundance on an astronomical scale.

text: An additional ~3000 radioactive isotopes not found in nature have been created in nuclear reactors and in particle accelerators. Many short-lived isotopes not found naturally on Earth have also been observed by spectroscopic analysis, being naturally created in stars or supernovae. An example is

text: , which is not naturally found on Earth, but is found in abundance on an astronomical scale.

text: It is now known by direct observation that much of the light curve (the graph of luminosity as a function of time) after the explosion of a Type II Supernova such as SN 1987A is provided its energy by those predicted radioactive decays. Although the luminous emission consists of optical photons, it is the radioactive power absorbed by the ejected gases that keeps the remnant hot enough to radiate light. The radioactive decay of Ni through its daughters Co to Fe produces gamma-ray photons , primarily of 847keV and 1238keV, that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of Ni to Co (half life 6 days) while energy for the later light curve in particular fit very closely with the 77.3 day half-life of Co decaying to Fe. Later measurements by space gamma-ray telescopes of the small fraction of the Co  and Co gamma rays that escaped the SN 1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources.

text: It is now known by direct observation that much of the

text: light curve

text: (the graph of luminosity as a function of time) after the explosion of a

text: Type II Supernova

text: such as SN 1987A is provided its energy by those predicted

text: radioactive decay

text: s. Although the luminous emission consists of optical photons, it is the radioactive power absorbed by the ejected gases that keeps the remnant hot enough to radiate light. The

text: radioactive decay

text: of

text: Ni

text: through its daughters

text: Co

text: to

text: Fe

text: produces gamma-ray

text: photon

text: s , primarily of 847keV and 1238keV, that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of

text: Ni

text: Co

text: decaying to Fe. Later measurements by space gamma-ray telescopes of the small fraction of the Co  and Co gamma rays that escaped the

text: SN 1987A

text: remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources.

text: Although it cannot be predicted whether any given atom of a radioactive substance will decay at any given time, the decay products of a radioactive substance are extremely predictable. Because of this, decay products are important to scientists in many fields who need to know the quantity or type of the parent product. Such studies are done to measure pollution levels (in and around nuclear facilities) and for other matters.

text: Different types of radioactive decay are characterized by their changes in mass number as well as atomic number, according to the radioactive displacement law of Fajans and Soddy. For example, uranium-238 usually decays by alpha decay, where the nucleus loses two neutrons and two protons in the form of an alpha particle.  Thus the atomic number and the number of neutrons each decrease by 2 (Z: 92→90, N: 146→144), so that the mass number decreases by 4 (A = 238→234); the result is an atom of thorium-234 and an alpha particle ():

text: Different types of

text: radioactive decay

text: are characterized by their changes in mass number as well as

text: atomic number

text: , according to the

text: . For example,

text: usually decays by

text: alpha decay

text: , where the nucleus loses two neutrons and two protons in the form of an

text: alpha particle

text: .  Thus the atomic number and the number of neutrons each decrease by 2 (Z: 92→90, N: 146→144), so that the mass number decreases by 4 (A = 238→234); the result is an atom of

text: and an alpha particle ():

text: Thorium is radioactive and produces a radioactive gas, radon-220, as one of its decay products.  Moreover, when heated to incandescence the thorium volatilizes its ingrowth radio-daughters, particularly radium-224.  Despite its very short half-life, radium quickly replenishes from its radio-parent (thorium-228), and every new heating of the mantle to incandescence releases a fresh flush of radium-224 into the air. This byproduct can be inhaled if the mantle is being used indoors, and is an internal alpha-emitter radio-toxicity concern. Secondary decay products of thorium include radium and actinium. Because of this, there are concerns about the safety of thorium mantles. Some nuclear safety agencies make recommendations about their use.

text: Thorium

text: is radioactive and produces a radioactive gas,

text: radon

text: -220, as one of its

text: decay products

text: .  Moreover, when heated to incandescence the thorium volatilizes its ingrowth

text: radio-daughters

text: , particularly

text: radium

text: -224.  Despite its very short half-life, radium quickly replenishes from its radio-parent (thorium-228), and every new heating of the mantle to incandescence releases a fresh flush of radium-224 into the air. This byproduct can be inhaled if the mantle is being used indoors, and is an internal

text: alpha-emitter

text: radio-toxicity concern. Secondary decay products of thorium include

text: radium

text: and

text: actinium

text: . Because of this, there are concerns about the safety of thorium mantles. Some

text: nuclear safety

text: agencies make recommendations about their use.

text: The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with an exception of dubnium-268. No elements with atomic numbers above 82 (after lead) have stable isotopes. Nevertheless, because of reasons not very well understood yet, there is a slightly increased nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, hypothesizes why superheavy elements last longer than predicted. Oganesson is radioactive and has a half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values, thus giving further support to the idea of this "island of stability".

text: plutonium

text: , the heaviest

text: primordial element

text: decay radioactively

text: with a

text: half-life

text: under a day, with an exception of

text: dubnium

text: -268. No elements with

text: atomic number

text: s above 82 (after

text: lead

text: ) have stable isotopes. Nevertheless, because of

text: reasons

text: not very well understood yet, there is a slightly increased nuclear stability around atomic numbers

text: island of stability

text: ". This concept, proposed by

text: University of California

text: professor

text: Glenn Seaborg

text: , hypothesizes why

text: superheavy element

text: s last longer than predicted. Oganesson is

text: radioactive

text: and has a

text: half-life

text: that appears to be less than a

text: millisecond

text: . Nonetheless, this is still longer than some predicted values, thus giving further support to the idea of this "island of stability".

text: 254 nuclides are considered stable. Many of these in theory could decay through spontaneous fission, alpha decay, double beta decay, etc. with a very long half-life, but no radioactive decay has yet been observed. Thus the number of stable nuclides is subject to change if some of these 254 are determined to be very long-lived radioactive nuclides in the future.  If a decay has been predicted theoretically but never observed experimentally, it is given in parentheses. In theory, spontaneous fission is possible for all elements with atomic numbers >40, but has not been observed for most elements up to lead (82). The list lists nuclides unstable only to this fission mechanism, before nuclides theoretically unstable to additional mechanisms. Those that have been checked for radioactivity are indicated with "> number" and show the lower limit for the half-life based on experimental observation. Such nuclides are considered to be "stable" until a decay has been observed in some fashion. For example, tellurium-123 was reported to be radioactive, but the same experimental group later retracted this report, and it presently remains observationally stable.

text: 254 nuclides are considered

text: stable

text: . Many of these in theory could decay through spontaneous fission, alpha decay, double beta decay, etc. with a very long

text: half-life

text: , but no

text: radioactive decay

text: has yet been observed. Thus the number of stable nuclides is subject to change if some of these 254 are determined to be very long-lived radioactive nuclides in the future.  If a decay has been predicted theoretically but never observed experimentally, it is given in parentheses. In theory, spontaneous fission is possible for all elements with atomic numbers >40, but has not been observed for most elements up to lead (82). The list lists nuclides unstable only to this fission mechanism, before nuclides theoretically unstable to additional mechanisms. Those that have been checked for radioactivity are indicated with "> number" and show the lower limit for the half-life based on experimental observation. Such nuclides are considered to be "stable" until a decay has been observed in some fashion. For example,

text: was reported to be radioactive, but the same experimental group later retracted this report, and it presently remains observationally stable.

text: All radioactivity of F-FDG, both the 20% which is rapidly excreted in the first several hours of urine which is made after the exam, and the 80% which remains in the patient, decays with a half-life of 110 minutes (just under 2 hours). Thus, within 24 hours (13 half-lives after the injection), the radioactivity in the patient and in any initially voided urine which may have contaminated bedding or objects after the PET exam, will have decayed to 2 = 1/8192 of the initial radioactivity of the dose. In practice, patients who have been injected with F-FDG are told to avoid the close vicinity of especially radiation-sensitive persons such as infants, children and pregnant women, for at least 12 hours (7 half-lives, or decay to 1/128th the initial radioactive dose).

text: Where decay constant λ is related to specific radioactivity a by the following equation:

text: Due to its extraordinarily long half-life, for nearly all applications Bi can still be treated as if it were non-radioactive. Although Bi holds the half-life record for alpha decay, bismuth does not have the longest half-life of any radionuclide to be found experimentally—this distinction belongs to Tellurium-128 (Te) with a  half-life estimated at 7.7 x 10 years by double β-decay (beta decay).

text: Due to its extraordinarily long half-life, for nearly all applications Bi can still be treated as if it were non-radioactive. Although Bi holds the half-life record for alpha decay, bismuth does not have the longest half-life of any

text: radionuclide

text: to be found experimentally—this distinction belongs to Tellurium-128 (

text: Te

text: ) with a  half-life estimated at 7.7 x 10 years by

text: double β-decay

text: (beta decay).

text: Lanthanum-140 was chosen for having the right emitted gamma ray energy (1.60 megaelectronvolts (MeV), with fraction of 0.49 MeV) and a sufficient radiation intensity to provide satisfying signal from the ionization chambers, combined with a rapid radioactive decay into stable Cerium-140 (reducing the radiation hazard for the operators after a period of several half lives). It was also potentially available in larger quantities because its parent nuclide barium-140 is an abundant fission product of uranium. As a consequence, Lanthanum-140 samples contained small amounts of barium-140, caesium-140, and especially strontium-90, which still presents a radioactive contamination problem in the area of the tests. Lanthanum-140 has a specific activity of 5.57×10 Ci/g (20.6 PBq/g); a  La-140 source therefore equals about 1.8 mg of lanthanum.

text: Lanthanum-140 was chosen for having the right emitted gamma ray energy (1.60

text: megaelectronvolts

text: (MeV), with fraction of 0.49 MeV) and a sufficient

text: radiation intensity

text: to provide satisfying signal from the ionization chambers, combined with a rapid

text: radioactive decay

text: into stable

text: (reducing the radiation hazard for the operators after a period of several

text: half lives

text: ). It was also potentially available in larger quantities because its

text: parent nuclide

text: barium-140 is an abundant

text: fission product

text: of uranium. As a consequence, Lanthanum-140 samples contained small amounts of

text: , and especially

text: , which still presents a

text: radioactive contamination

text: problem in the area of the tests. Lanthanum-140 has a

text: specific activity

text: of 5.57×10 Ci/g (20.6 PBq/g); a  La-140 source therefore equals about 1.8 mg of lanthanum.

text: Twenty isotopes of sodium are known, but only Na is stable. Na is created in the carbon-burning process in stars by fusing two carbon atoms together; this requires temperatures above 600 megakelvins and a star of at least three solar masses. Two radioactive, cosmogenic isotopes are the byproduct of cosmic ray spallation: Na has a half-life of 2.6 years and Na, a half-life of 15 hours; all other isotopes have a half-life of less than one minute. Two nuclear isomers have been discovered, the longer-lived one being Na with a half-life of around 20.2 milliseconds. Acute neutron radiation, as from a nuclear criticality accident, converts some of the stable Na in human blood to Na; the neutron radiation dosage of a victim can be calculated by measuring the concentration of Na relative to Na.

text: Twenty isotopes of sodium are known, but only Na is stable. Na is created in the

text: carbon-burning process

text: in stars by fusing two

text: carbon

text: atoms together; this requires temperatures above 600 megakelvins and a star of at least three solar masses. Two

text: radioactive

text: cosmogenic

text: isotopes are the byproduct of

text: cosmic ray spallation

text: : Na has a

text: half-life

text: of 2.6 years and Na, a half-life of 15 hours; all other isotopes have a half-life of less than one minute. Two

text: nuclear isomer

text: s have been discovered, the longer-lived one being Na with a half-life of around 20.2 milliseconds. Acute neutron radiation, as from a nuclear

text: criticality accident

text: , converts some of the stable Na in human blood to Na; the neutron radiation dosage of a victim can be calculated by measuring the concentration of Na relative to Na.

text: The electron that is captured is one of the atom's own electrons, and not a new, incoming electron, as might be suggested by the way the above reactions are written. Radioactive isotopes that decay by pure electron capture can be inhibited from radioactive decay if they are fully ionized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the r-process in exploding supernovae, are ejected fully ionized and so do not undergo radioactive decay as long as they do not encounter electrons in outer space. Anomalies in elemental distributions are thought to be partly a result of this effect on electron capture. Inverse decays can also be induced by full ionisation; for instance, Ho decays into Dy by electron capture; however, a fully ionised Dy decays into a bound state of Ho by the process of bound-state β decay.

text: The electron that is captured is one of the atom's own electrons, and not a new, incoming electron, as might be suggested by the way the above reactions are written. Radioactive isotopes that decay by pure electron capture can be inhibited from radioactive decay if they are fully

text: ion

text: ized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the

text: r-process

text: in exploding

text: supernova

text: e, are ejected fully ionized and so do not undergo radioactive decay as long as they do not encounter electrons in outer space. Anomalies in elemental distributions are thought to be partly a result of this effect on electron capture. Inverse decays can also be induced by full ionisation; for instance,

text: Ho

text: decays into

text: Dy

text: by electron capture; however, a fully ionised Dy decays into a bound state of Ho by the process of

text: bound-state β decay

text: The half-life of this alpha decay was first measured as 150 days and then corrected to 162.8 days.

text: The

text: half-life

text: of this

text: alpha decay

text: was first measured as 150 days and then corrected to 162.8 days.

text: Calcium (Ca) has 24 isotopes, from Ca to Ca. There are five stable isotopes (Ca, Ca, Ca, Ca and Ca), plus one isotope (Ca) with such a long half-life that for all practical purposes it can be considered stable. The most abundant isotope, Ca, as well as the rare Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a cosmogenic isotope, radioactive Ca, which has a half-life of 102,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, Ca is produced by neutron activation of Ca. Most of its production is in the upper metre or so of the soil column, where the cosmogenic neutron flux is still sufficiently strong. Ca has received much attention in stellar studies because it decays to K, a critical indicator of solar-system anomalies. The most stable artificial radioisotope is Ca, with a half-life of 163 days.

text: Calcium

text: (Ca) has 24 isotopes, from Ca to Ca. There are five

text: stable isotope

text: s (Ca, Ca, Ca, Ca and Ca), plus one isotope (

text: Ca

text: ) with such a long

text: half-life

text: that for all practical purposes it can be considered stable. The most abundant isotope, Ca, as well as the rare Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a

text: cosmogenic isotope

text: radioactive

text: Ca, which has a

text: half-life

text: of 102,000 years. Unlike

text: cosmogenic isotope

text: s that are produced in the

text: atmosphere

text: , Ca is produced by

text: neutron activation

text: of Ca. Most of its production is in the upper metre or so of the soil column, where the cosmogenic neutron flux is still sufficiently strong. Ca has received much attention in stellar studies because it decays to K, a critical indicator of solar-system anomalies. The most stable artificial radioisotope is Ca, with a half-life of 163 days.

text: Radon and its first decay products being very short-lived, the seed is left in place. After 12 half-lives (43 days), radon radioactivity is at 1/2000 of its original level. At this stage, the predominant residual activity originates from the radon decay product Pb, whose half-life (22.3 years) is 2000 times that of radon (and whose activity is thus 1/2000 of radon's), and its descendants Bi and Po.

text: Out of the six known chalcogens, one (oxygen) has an atomic number equal to a nuclear magic number, which means that their atomic nuclei tend to have increased stability towards radioactive decay. Oxygen has three stable isotopes, and 14 unstable ones. Sulfur has four stable isotopes, 20 radioactive ones, and one isomer. Selenium has six observationally stable or nearly stable isotopes, 26 radioactive isotopes, and 9 isomers. Tellurium has eight stable or nearly stable isotopes, 31 unstable ones, and 17 isomers. Polonium has 42 isotopes, none of which are stable. It has an additional 28 isomers. In addition to the stable isotopes, some radioactive chalcogen isotopes occur in nature, either because they are decay products, such as Po, because they are primordial, such as Se, because of cosmic ray spallation, or via nuclear fission of uranium. Livermorium isotopes 290 through 293 have been discovered. The most stable livermorium isotope is Lv, which has a half-life of 0.061 seconds.

text: Out of the six known chalcogens, one (oxygen) has an atomic number equal to a nuclear

text: magic number

text: , which means that their

text: atomic nuclei

text: tend to have increased stability towards radioactive decay. Oxygen has three stable isotopes, and 14 unstable ones. Sulfur has four stable isotopes, 20 radioactive ones, and one

text: isomer

text: . Selenium has six

text: observationally stable

text: or nearly stable isotopes, 26 radioactive isotopes, and 9 isomers. Tellurium has eight stable or nearly stable isotopes, 31 unstable ones, and 17 isomers. Polonium has 42 isotopes, none of which are stable. It has an additional 28 isomers. In addition to the stable isotopes, some radioactive chalcogen isotopes occur in nature, either because they are decay products, such as

text: Po

text: , because they are

text: primordial

text: , such as Se, because of

text: cosmic ray

text: spallation

text: , or via

text: nuclear fission

text: of uranium. Livermorium isotopes 290 through 293 have been discovered. The most stable livermorium isotope is Lv, which has a half-life of 0.061 seconds.

text: Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average". In other words, the probability of a radioactive atom decaying within its half-life is 50%.

text: Instead, the half-life is defined in terms of

text: probability

text: : "Half-life is the time required for exactly half of the entities to decay

text: on average

text: ". In other words, the probability of a radioactive atom decaying within its half-life is 50%.

text: The decay of a mixture of two or more materials which each decay exponentially, but with different half-lives, is not exponential. Mathematically, the sum of two exponential functions is not a single exponential function. A common example of such a situation is the waste of nuclear power stations, which is a mix of substances with vastly different half-lives. Consider a mixture of a rapidly decaying element A, with a half-life of 1 second, and a slowly decaying element B, with a half-life of 1 year. In a couple of minutes, almost all atoms of element A will have decayed after repeated halving of the initial number of atoms, but very few of the atoms of element B will have done so as only a tiny fraction of its half-life has elapsed. Thus, the mixture taken as a whole will not decay by halves.

text: There may have been observations of . In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps a  nucleus. The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce , which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton'—a pair of protons bound together as a  nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time—so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.

text: There may have been observations of . In 2000, physicists first observed a new type of radioactive decay in which a nucleus

text: emits two protons

text: at once—perhaps a  nucleus. The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the

text: creation of elements inside stars

text: . Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce , which then decayed into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton'—a pair of protons bound together as a  nucleus—which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time—so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.

text: Researchers use radioactive atoms to determine the age of materials that were once part of a living organism. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain in a process called radiocarbon dating. Similarly, using other radioactive elements, the age of rocks and other geological features (even some man-made objects) can be determined; this is called Radiometric dating. Environmental scientists use radioactive atoms, known as tracer atoms, to identify the pathways taken by pollutants through the environment.

text: Researchers use radioactive atoms to determine the age of materials that were once part of a living organism. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain in a process called

text: radiocarbon dating

text: . Similarly, using other radioactive elements, the age of rocks and other geological features (even some man-made objects) can be determined; this is called

text: Radiometric dating

text: . Environmental scientists use radioactive atoms, known as

text: tracer atoms

text: , to identify the pathways taken by pollutants through the environment.

text: Many of these nuclides have a very short half-life, and therefore are very radioactive. For instance, strontium-90, strontium-89 and strontium-94 are all fission products, they are produced in similar quantities, and each nucleus decays by shooting off one beta particle (electron).  But Sr-90 has a 30-year half-life, Sr-89 a 50.5-day half-life, and Sr-94 a 75-second half-life.  The same number of atoms of Sr-89 will decay 10,600 times faster than Sr-90, and Sr-94 will do so 915 million times faster. It is these short-half-life nuclides that make spent fuel so dangerous, in addition to generating much heat, immediately after the reactor itself has been shut down. The most radioactive also decay the fastest; after 50 days, Sr-94 has had 58,000 half-lives and is  effectively gone; Sr-89 is at half its original quantity, but 99.68% of the Sr-90 remains.  As there are hundreds of different nuclides created, the initial radioactivity level fades quickly, but never fades out completely.

text: Many of these

text: nuclides

text: have a very short

text: half-life

text: , and therefore are very radioactive. For instance, strontium-90, strontium-89 and strontium-94 are all fission products, they are produced in similar quantities, and each nucleus decays by shooting off one

text: beta particle

text: electron

text: ).  But Sr-90 has a 30-year half-life, Sr-89 a 50.5-day half-life, and Sr-94 a 75-second half-life.  The same number of atoms of Sr-89 will decay 10,600 times faster than Sr-90, and Sr-94 will do so 915 million times faster. It is these short-half-life nuclides that make spent fuel so dangerous, in addition to generating much heat, immediately after the reactor itself has been shut down. The most radioactive also decay the fastest; after 50 days, Sr-94 has had 58,000 half-lives and is  effectively gone; Sr-89 is at half its original quantity, but 99.68% of the Sr-90 remains.  As there are hundreds of different nuclides created, the initial radioactivity level fades quickly, but never fades out completely.

text: For decades after Libby performed the first radiocarbon dating experiments, the only way to measure the  in a sample was to detect the radioactive decay of individual carbon atoms. In this approach, what is measured is the activity, in number of decay events per unit mass per time period, of the sample. This method is also known as "beta counting", because it is the beta particles emitted by the decaying  atoms that are detected. In the late 1970s an alternative approach became available: directly counting the number of  and  atoms in a given sample, via accelerator mass spectrometry, usually referred to as AMS. AMS counts the / ratio directly, instead of the activity of the sample, but measurements of activity and / ratio can be converted into each other exactly. For some time, beta counting methods were more accurate than AMS, but as of 2014 AMS is more accurate and has become the method of choice for radiocarbon measurements.  In addition to improved accuracy, AMS has two further significant advantages over beta counting: it can perform accurate testing on samples much too small for beta counting; and it is much faster – an accuracy of 1% can be achieved in minutes with AMS, which is far quicker than would be achievable with the older technology.

text: The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of 5,730 years. After an organism has been dead for 60,000 years, so little carbon-14 is left that accurate dating can not be established. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.

text: For example, the isotope copper-64, commonly used in medical research, has a half-life of 12.7 hours.  If you inject a large group of animals at "time zero", but measure the radioactivity in their organs at two later times, the later groups must be "decay corrected" to adjust for the decay that has occurred between the two time points.

text: For example, the

text: isotope

text: copper-64, commonly used in medical research, has a half-life of 12.7 hours.  If you inject a large group of animals at "time zero", but measure the radioactivity in their organs at two later times, the later groups must be "decay corrected" to adjust for the decay that has occurred between the two time points.

text: Quantum tunnelling was developed from the study of radioactivity, which was discovered in 1896 by Henri Becquerel. Radioactivity was examined further by Marie Curie and Pierre Curie, for which they earned the Nobel Prize in Physics in 1903. Ernest Rutherford and Egon Schweidler studied its nature, which was later verified empirically by Friedrich Kohlrausch. The idea of the half-life and the impossibility of predicting decay was created from their work.

text: Quantum tunnelling was developed from the study of

text: radioactivity

text: , which was discovered in 1896 by

text: Henri Becquerel

text: . Radioactivity was examined further by

text: Marie Curie

text: and

text: Pierre Curie

text: , for which they earned the

text: Nobel Prize in Physics

text: in 1903.

text: Ernest Rutherford

text: and

text: Egon Schweidler

text: studied its nature, which was later verified empirically by

text: Friedrich Kohlrausch

text: . The idea of the

text: half-life

text: and the impossibility of predicting decay was created from their work.

text: Radioactive decay results in a reduction of summed rest mass, once the released energy (the disintegration energy) has escaped in some way. Although decay energy is sometimes defined as associated with the difference between the mass of the parent nuclide products and the mass of the decay products, this is true only of rest mass measurements, where some energy has been removed from the product system. This is true because the decay energy must always carry mass with it, wherever it appears (see mass in special relativity) according to the formula E = mc. The decay energy is initially released as the energy of emitted photons plus the kinetic energy of massive emitted particles (that is, particles that have rest mass). If these particles come to thermal equilibrium with their surroundings and photons are absorbed, then the decay energy is transformed to thermal energy, which retains its mass.

text: Radioactive decay results in a reduction of summed rest

text: mass

text: , once the released energy (the disintegration energy) has escaped in some way. Although decay energy is sometimes defined as associated with the difference between the mass of the parent nuclide products and the mass of the decay products, this is true only of rest mass measurements, where some energy has been removed from the product system. This is true because the decay energy must always carry mass with it, wherever it appears (see

text: mass in special relativity

text: ) according to the formula E = mc. The decay energy is initially released as the energy of emitted photons plus the kinetic energy of massive emitted particles (that is, particles that have rest mass). If these particles come to

text: thermal equilibrium

text: with their surroundings and photons are absorbed, then the decay energy is transformed to thermal energy, which retains its mass.

text: George Tilton was born in Danville Illinois on the 3rd of June, 1923. He enrolled at Blackburn College in Illinois, but his education was interrupted in 1943 when was called to serve in the US Army. He participated in the campaign to liberate France, where he was wounded in November 1944. After the war, in 1947, Tilton completed his B.S. in Chemistry from the University of Illinois, including a term paper on "reactions of slow and fast neutrons" and a senior thesis on the use of radioactive tracers to study chemical reactions. He earned his Ph.D. in Chemistry at the University of Chicago in 1951, working on clean lab chemistry, isotope dilution and mass spectrometry techniques to measure the uranium content of meteorites. Fellow graduate student Clair Patterson and he achieved two momentous results: the first accurate age of Earth and the solar system based on Pb isotope ratios, and the ages for terrestrial rocks using U-Pb isotopic dates. He was a researcher at the Carnegie Institution of Washington from 1951-1965, working on U-Pb dating and the use of isotope tracers. In 1965 George accepted the offer of a Professorship at the University of California, Santa Barbara. There he mentored many graduate students and researchers until his retirement in 1991. He remained an active Emeritus Professor there until a few years before his death.

text: George Tilton was born in Danville Illinois on the 3rd of June, 1923. He enrolled at Blackburn College in Illinois, but his education was interrupted in 1943 when was called to serve in the US Army. He participated in the campaign to liberate France, where he was wounded in November 1944. After the war, in 1947, Tilton completed his B.S. in Chemistry from the

entity_name: University of Illinois at Urbana–Champaign

text: University of Illinois

text: , including a term paper on "reactions of slow and fast neutrons" and a senior thesis on the use of radioactive tracers to study chemical reactions. He earned his Ph.D. in Chemistry at the

text: University of Chicago

text: in 1951, working on clean lab chemistry, isotope dilution and mass spectrometry techniques to measure the uranium content of meteorites. Fellow graduate student

text: Clair Patterson

text: and he achieved two momentous results: the first accurate

text: age of Earth

text: and the solar system based on Pb isotope ratios, and the ages for terrestrial rocks using U-Pb isotopic dates. He was a researcher at the

text: Carnegie Institution of Washington

text: from 1951-1965, working on U-Pb dating and the use of isotope tracers. In 1965 George accepted the offer of a Professorship at the

text: University of California, Santa Barbara

text: . There he mentored many graduate students and researchers until his retirement in 1991. He remained an active Emeritus Professor there until a few years before his death.

text: Decay products are important in understanding radioactive decay and the management of radioactive waste.

text: Decay products are important in understanding radioactive decay and the management of

text: radioactive waste

text: The radioactive decay impact of theU andU on its semiconducting properties was not measured . Due to the slow decay rate of these isotopes, it should not meaningfully influence the properties of uranium dioxide solar cells and thermoelectric devices, but it may become an important factor for VLSI chips. Use of depleted uranium oxide is necessary for this reason. The capture of alpha particles emitted during radioactive decay as helium atoms in the crystal lattice may also cause gradual long-term changes in its properties.

text: The

text: radioactive decay

text: impact of theU andU on its semiconducting properties was not measured . Due to the slow decay rate of these isotopes, it should not meaningfully influence the properties of uranium dioxide solar cells and thermoelectric devices, but it may become an important factor for

text: VLSI

text: chips. Use of

text: depleted uranium

text: oxide is necessary for this reason. The capture of alpha particles emitted during radioactive decay as helium atoms in the crystal lattice may also cause gradual long-term changes in its properties.

text: Other more rare types of radioactive decay include ejection of neutrons or protons or clusters of nucleons from a nucleus, or more than one beta particle. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is internal conversion— a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called spontaneous nuclear fission.

text: Other more rare types of

text: radioactive decay

text: include ejection of neutrons or protons or clusters of

text: nucleon

text: s from a nucleus, or more than one

text: beta particle

text: . An analog of gamma emission which allows excited nuclei to lose energy in a different way, is

text: internal conversion

text: — a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays. A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called spontaneous

text: nuclear fission

text: The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule. In such cases, the half-life is defined the same way as before: as the time elapsed before half of the original quantity has decayed. However, unlike in an exponential decay, the half-life depends on the initial quantity, and the prospective half-life will change over time as the quantity decays.

text: In a land or water surface burst, heat vaporizes large amounts of earth or water, which is drawn up into the radioactive cloud. This material becomes radioactive when it condenses with fission products and other radiocontaminants that have become neutron-activated. The table below summarizes the abilities of common isotopes to form fallout. Some radiation taints large amounts of land and drinking water causing formal mutations throughout animal and human life.

text: In a land or water

text: surface burst

text: , heat

text: vapor

text: izes large amounts of earth or water, which is drawn up into the

text: radioactive cloud

text: . This material becomes radioactive when it condenses with

text: fission products

text: and other radiocontaminants that have become

text: neutron-activated

text: . The table below summarizes the abilities of common isotopes to form fallout. Some radiation taints large amounts of land and

text: drinking water

text: causing formal

text: mutation

text: s throughout animal and human life.

text: The heat causing the melting of a reactor may originate from the nuclear chain reaction, but more commonly decay heat of the fission products contained in the fuel rods is the primary heat source. The heat production from radioactive decay drops quickly, as the short half-life isotopes provide most of the heat and radioactive decay, with the curve of decay heat being a sum of the decay curves of numerous isotopes of elements decaying at different exponential half-life rates. A significant additional heat source can be the chemical reaction of hot metals with oxygen or steam.

text: The heat causing the

entity_name: Behavior of nuclear fuel during a reactor accident

text: melting of a reactor

text: may originate from the

text: nuclear chain reaction

text: , but more commonly

text: decay heat

text: of the

text: fission products

text: contained in the fuel rods is the primary heat source. The heat production from radioactive decay drops quickly, as the short

text: half-life

text: isotopes provide most of the heat and radioactive decay, with the curve of decay heat being a sum of the decay curves of numerous

text: isotope

text: s of elements decaying at different exponential half-life rates. A significant additional heat source can be the

text: chemical reaction

text: of hot metals with oxygen or

text: steam

text: All uranium compounds are radioactive. However, uranium is usually in depleted form, except in the context of the nuclear industry. Depleted uranium consists mainly of U which decays by alpha decay with a half-life of 4.468(3) × 10 years. Even if the uranium contained U which decays with a similar half-life of about 7.038 × 10 years, both of them would still be regarded as weak alpha emitters and their radioactivity is only hazardous with direct contact or ingestion.

text: All uranium compounds are

text: radioactive

text: . However, uranium is usually in depleted form, except in the context of the nuclear industry. Depleted uranium consists mainly of

text: U

text: which decays by

text: alpha decay

text: with a half-life of 4.468(3) × 10 years. Even if the uranium contained

text: U

text: which decays with a similar half-life of about 7.038 × 10 years, both of them would still be regarded as weak alpha emitters and their radioactivity is only hazardous with direct contact or ingestion.

text: There are two stable and naturally occurring isotopes of bromine, bromine-79 and bromine-81. A total of 32 isotopes of bromine have been discovered, with atomic masses ranging 67 to 98. There is one stable and naturally occurring isotope of iodine, iodine-127. However, there are trace amounts in nature of the radioactive isotope iodine-129, which occurs via spallation and from the radioactive decay of uranium in ores. Several other radioactive isotopes of iodine have also been created naturally via the decay of uranium. A total of 38 isotopes of iodine have been discovered, with atomic masses ranging from 108 to 145.

text: There are two stable and naturally occurring

text: isotopes of bromine

text: , bromine-79 and bromine-81. A total of 32 isotopes of bromine have been discovered, with atomic masses ranging 67 to 98. There is one stable and naturally occurring

text: isotope of iodine

text: . However, there are trace amounts in nature of the radioactive isotope

text: , which occurs via spallation and from the radioactive decay of uranium in ores. Several other radioactive isotopes of iodine have also been created naturally via the decay of uranium. A total of 38 isotopes of iodine have been discovered, with atomic masses ranging from 108 to 145.

text: Because nucleogenic isotopes have been produced later than the birth of the solar system (and the nucleosynthetic events that preceded it), nucleogenic isotopes, by definition, are not primordial nuclides. However, nucleogenic isotopes should not be confused with much more common radiogenic nuclides that are also younger than primordial nuclides, but which arise as simple daughter isotopes from radioactive decay. Nucleogenic isotopes, as noted, are the result of a more complicated nuclear reaction, although such reactions may begin with a radioactive decay event.

text: Because nucleogenic isotopes have been produced later than the birth of the solar system (and the nucleosynthetic events that preceded it), nucleogenic isotopes, by definition, are not

text: primordial nuclide

text: s. However, nucleogenic isotopes should not be confused with much more common

text: radiogenic

text: nuclides

text: that are also younger than primordial nuclides, but which arise as simple

text: daughter isotope

text: s from

text: radioactive decay

text: . Nucleogenic isotopes, as noted, are the result of a more complicated

text: nuclear reaction

text: , although such reactions may begin with a radioactive decay event.

text: This is a list of radioactive isotopes ordered by half-life from shortest to longest.

text: This is a list of radioactive

text: isotope

text: s ordered by

text: half-life

text: from shortest to longest.

text: A scintillating bolometer (or luminescent bolometer) is a scientific instrument used particle physics in the search for events with low energy deposition. These events could include dark matter, low energy solar neutrinos, double beta decay or rare radioactive decay. It works by simultaneously measuring both the light pulse and heat pulse generated by a particle interaction within its internal scintillator crystal. The device was originally proposed by L. Gonzalez-Mestres and D. Perret-Gallix (LAPP, IN2P3/CNRS)

text: A scintillating bolometer (or luminescent bolometer) is a

text: scientific instrument

text: used

text: particle physics

text: in the search for events with low energy deposition. These events could include

text: dark matter

text: , low energy

text: solar neutrinos

text: double beta decay

text: or rare

text: radioactive decay

text: . It works by simultaneously measuring both the light pulse and heat pulse generated by a particle interaction within its internal

text: scintillator

text: crystal. The device was originally proposed by L. Gonzalez-Mestres and D. Perret-Gallix (LAPP, IN2P3/CNRS)

text: A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom that has excess nuclear energy, making it unstable. This excess energy can be either emitted from the nucleus as gamma radiation, or create and emit from the nucleus a new particle (alpha particle or beta particle), or transfer this excess energy to one of its electrons, causing that electron to be ejected as a conversion electron. During those processes, the radionuclide is said to undergo radioactive decay. These emissions constitute ionizing radiation. The unstable nucleus is more stable following the emission, but will sometimes undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, and thus the half-life (t) for that collection can be calculated from their measured decay constants. The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.

text: A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an

text: atom

text: that has excess nuclear energy, making it unstable. This excess energy can be either emitted from the nucleus as

text: gamma radiation

text: , or create and emit from the nucleus a new

text: particle

text: alpha particle

text: or

text: beta particle

text: ), or transfer this excess energy to one of its

text: electron

text: s, causing that electron to be ejected as a

text: conversion electron

text: . During those processes, the radionuclide is said to undergo

text: radioactive decay

text: . These emissions constitute

text: ionizing radiation

text: . The unstable nucleus is more stable following the emission, but will sometimes undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, and thus the

text: half-life

text: (t) for that collection can be calculated from their measured

text: decay constant

text: s. The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.

text: Cornano maintained that longevity was desirable and "God wills it". He rejected ascetics who believed man must suffer in this life to attain salvation in the next, arguing that there was no reason one could not enjoy both his earthly existence and his heavenly one. In addition, he rejected conventional wisdom that old age was a period of misery and decay, writing "And now some sensual and unreasonable individuals pretend that the existence of a man after he passes the age of 65 cannot be termed a living one, but a dead one. I will plainly demonstrate that they are mistaken, for I have a desire that all men should attain my age, which is the most beautiful period of life." He also commented that at the age of 83, his health was good, he could perform most functions unassisted, and he had a wide circle of younger friends and correspondents. Cornano also firmly condemned those with a live-fast-and-die-young mentality, stating that "They don't stop and consider the virtue of ten more years of active life, at a point where we've reached a high point of experience and wisdom, two things that can only be honed with time."

text: Some of the essential elements that make up the human body, mainly potassium and carbon, have radioactive isotopes that add significantly to our background radiation dose. An average human contains about 30 milligrams of potassium-40 (K) and about 10 nanograms (10 g) of carbon-14 (C),  which has a decay half-life of 5,730 years. Excluding internal contamination by external radioactive material, the largest component of internal radiation exposure from biologically functional components of the human body is from potassium-40. The decay of about 4,000 nuclei of K per second makes potassium the largest source of radiation in terms of number of decaying atoms. The energy of beta particles produced by K is also about 10 times that from the beta particles from C decay.C is present in the human body at a level of 3700 Bq with a biological half-life of 40 days. There are about 1,200 beta particles per second produced by the decay of C. However, a C atom is in the genetic information of about half the cells, while potassium is not a component of DNA. The decay of a C atom inside DNA in one person happens about 50 times per second, changing a carbon atom to one of nitrogen. The global average internal dose from radionuclides other than radon and its decay products is 0.29 mSv/a, of which 0.17 mSv/a comes from K, 0.12 mSv/a comes from the uranium and thorium series, and 12 μSv/a comes from C.

text: Some of the essential elements that make up the human body, mainly potassium and carbon, have radioactive isotopes that add significantly to our background radiation dose. An average human contains about 30 milligrams of

text: (K) and about 10 nanograms (10 g) of

text: (C),  which has a decay half-life of 5,730 years. Excluding internal contamination by external radioactive material, the largest component of internal radiation exposure from biologically functional components of the human body is from potassium-40. The decay of about 4,000 nuclei of K per second makes potassium the largest source of radiation in terms of number of decaying atoms. The energy of

text: beta particle

text: s produced by K is also about 10 times that from the beta particles from C decay.C is present in the human body at a level of 3700 Bq with a

text: biological half-life

text: of 40 days. There are about 1,200 beta particles per second produced by the decay of C. However, a C atom is in the genetic information of about half the cells, while potassium is not a component of

text: DNA

text: . The decay of a C atom inside DNA in one person happens about 50 times per second, changing a carbon atom to one of

text: nitrogen

text: . The global average internal dose from radionuclides other than radon and its decay products is 0.29 mSv/a, of which 0.17 mSv/a comes from K, 0.12 mSv/a comes from the uranium and thorium series, and 12 μSv/a comes from C.

text: This section deals with characters that appear in Half-Life, Opposing Force, Blue Shift and Half-Life: Decay

text: This section deals with characters that appear in Half-Life,

text: Opposing Force

text: Blue Shift

text: and

text: Half-Life: Decay

text: This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 10 years, such as for the very nearly stable nuclide Bi, to 10 seconds for highly unstable ones.

text: This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than

text: 10 years

text: , such as for the very nearly stable nuclide Bi, to 10 seconds for highly unstable ones.

text: By emitting an electron and an electron antineutrino, one of the neutrons in the carbon-14 atom decays to a proton and the carbon-14 (half-life of 5700 ± 30 years) decays into the stable (non-radioactive) isotope nitrogen-14.

text: By emitting an

text: electron

text: and an

text: electron antineutrino

text: , one of the neutrons in the carbon-14 atom decays to a proton and the carbon-14 (

text: half-life

text: of 5700 ± 30 years) decays into the stable (non-radioactive) isotope

text: Gearbox has developed a total of six games in the Half-Life series: the expansion packs Opposing Force and Blue Shift; ports of Half-Life for Dreamcast (which included Blue Shift) and Half-Life for PlayStation 2 (which included Half-Life: Decay); they also did a large amount of work on both the retail release of Counter-Strike and the main portion of Counter-Strike: Condition Zero.

text: Gearbox has developed a total of six games in the Half-Life series: the expansion packs

text: Opposing Force

text: and

text: Blue Shift

text: ; ports of Half-Life for Dreamcast (which included Blue Shift) and Half-Life for PlayStation 2 (which included

text: Half-Life: Decay

text: ); they also did a large amount of work on both the retail release of

text: Counter-Strike

text: and the main portion of

text: Counter-Strike: Condition Zero

text: During his investigations of radioactivity, Ernest Rutherford coined the terms alpha rays, beta rays and gamma rays for the three types of emissions that occur during radioactive decay.

text: During his investigations of radioactivity,

text: Ernest Rutherford

text: coined the terms

text: alpha rays

text: beta rays

text: and

text: gamma ray

text: s for the three types of emissions that occur during

text: radioactive decay

text: Radioactive decay (also known as nuclear decay or radioactivity) is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays, and conversion electrons. A material that spontaneously emits such radiation is considered radioactive.

text: Radioactive decay (also known as nuclear decay or radioactivity) is the process by which the

text: nucleus

text: of an unstable

text: atom

text: loses energy by emitting radiation, including

text: alpha particle

text: s,

text: beta particle

text: s,

text: gamma ray

text: s, and

text: conversion electrons

text: . A material that spontaneously emits such radiation is considered radioactive.

text: Aenea, Raul and de Soya cast to Pacem and de Soya leads them through unused underground train lines and catacombs to St Peter's Basilica where Hoyt (as pope) is celebrating High Mass on Holy Thursday. She sends de Soya away then interrupts the mass and she and Raul are arrested. Raul is sentenced to death but not wishing to sully their hands by murder he is placed in his far away Schrodinger cell with cyanide that will be triggered by radioactive decay. In this sealed cell he spends what is to him an indeterminate amount of time and he confesses as narrator of the story to having lied about not knowing her fate (as announced at the start of the previous book).

text: Cobalt-60, , is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope . Measurable quantities are also produced as a by-product of typical nuclear power plant operation and may be detected externally when leaks occur. In the latter case (in the absence of added cobalt) the incidentally produced  is largely the result of multiple stages of neutron activation of iron isotopes in the reactor's steel structures<ref>US EPA Radiation Protection: Cobalt</ref> via the creation of {{SimpleNuclide2|cobalt|59}} precursor. The simplest case of the latter would result from the activation of {{SimpleNuclide2|iron|58|link=yes}}. {{SimpleNuclide2|cobalt|60}} decays by [[beta decay] to the stable isotope nickel-60 (). The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is

text: Cobalt-60, , is a

text: synthetic

text: radioactive

text: isotope of cobalt

text: with a

text: half-life

text: of 5.2714 years. It is produced artificially in

text: nuclear reactor

text: s. Deliberate industrial production depends on

text: neutron activation

text: of bulk samples of the

text: monoisotopic

text: and

text: mononuclidic

text: cobalt

text: isotope

text: . Measurable quantities are also produced as a by-product of typical nuclear power plant operation and may be detected externally when leaks occur. In the latter case (in the absence of added cobalt) the incidentally produced  is largely the result of multiple stages of neutron activation of

text: iron isotopes

text: in the reactor's steel structures<ref>US

entity_name: United States Environmental Protection Agency

text: EPA

text: Radiation Protection: Cobalt</ref> via the creation of {{SimpleNuclide2|cobalt|59}} precursor. The simplest case of the latter would result from the activation of {{SimpleNuclide2|iron|58|link=yes}}. {{SimpleNuclide2|cobalt|60}} decays by [[beta decay] to the stable isotope

text: (). The activated nickel nucleus emits two

text: gamma ray

text: s with energies of 1.17 and 1.33

text: MeV

text: , hence the overall nuclear equation of the reaction is

text: Clayton created new tools for calculating the interstellar abundances of radioactive nuclei in the Galaxy. In 1964 he discovered a new method for measuring the age of interstellar nuclei based on the larger than expected observed abundances of stable daughters of radioactive nuclei. The decays of rhenium-187 to osmium-187 and of uranium and thorium to three differing isotopes of lead (Pb) defined the cosmoradiogenic chronologies. Merging his method with an earlier method based only on the abundances of uranium and thorium themselves still did not yield a precise galactic age, however. Clayton wrote that the discord arose from inadequate treatments of both the history of star formation in the Galaxy and of the rate of infall of pristine metal-free gas onto the young Milky Way, compounded by a prevailing but erroneous technique for computation of the radioactive abundances within interstellar gas. Clayton reasoned that interstellar gas contains higher concentrations of shorter-lived radioactive nuclei than do the stars. With that insight he invented in 1985 new mathematical solutions for the simplified differential equation of galactic abundance evolution that for the first time rendered these relationships transparent, ending decades of confusion. Clayton calculated an age of 13-15 billion years for the oldest galactic nuclei, which would necessarily be approximately equal to the age of our galaxy. More recently radioactive  cosmochronology has diminished in importance because more accurate techniques for determining the age of the Milky Way have been discovered in the microwave background; but agreement confirmed correctness of his treatment of radioactivity in astronomy. His analytic mathematical models demonstrated that the concentration of short-lived radioactive nuclei in interstellar gas had routinely been underestimated by the factor 1/(k+1), where k is an integer near 2 or 3 that measures the steepness of the rate of decline of the infall of pristine gas onto our galaxy. In a parallel scientific development the identities and initial abundances of  shorter-lived radioactive nuclei that remained alive at variable levels within the interstellar gas cloud that formed the early solar system, but which are now extinct, has grown in importance with more experimental discoveries of such nuclei within the meteorites. These are called extinct radioactivities because none remain on earth today. Simultaneous solution for their abundances became the guiding principle for a new discipline of galactic abundance evolution that focuses on nucleosynthesis near the solar interstellar cloud during the billion years preceding solar birth. In 1983, at a time when astrophysicists were relying on only a uniform model of a well mixed interstellar gas,  Clayton introduced a new aspect of the ISM that has proven to be essential for understanding the abundances of the extinct radioactivities; namely the time required for isotopic mixing between freshly synthesized atoms ejected from supernovae with distinct physical phases of interstellar gas. He showed that owing to those time delays allowing more interstellar decay of radioactive nuclei, each phase of interstellar gas contains a distinctly different concentration of each of the extinct radioactive nuclides; but that the early solar system radioactivities measured specifically only abundances in the dense molecular-cloud phase in which the solar system had been born. In the 21st century many researchers have begun to present their own calculations of the effect of interstellar inter-phase mixing, often unaware of Clayton's (1983) paper owing to intervening decades. These aspects of interstellar phase mixing will remain important for decades to come while astronomers probe the circumstances of solar birth using accurate meteoritic data revealing the abundances of the extinct radioactive nuclei. Clayton gave emphasis to extinct radioactivity in the Glossary of his 2003 book on isotopes in the cosmos. Chapters 16 and 17 of "Catch a Falling Star".

text: Isotopes are different forms of elements; they have a different number of neutrons in the nucleus, meaning they have very similar chemical properties, but different mass.  The weight difference means that some isotopes are discriminated against in chemical processes – for example, plants find it easier to incorporate the lighter C than heavy C. Other isotopes are only produced as a result of the radioactive decay of other elements, such as Sr, the daughter isotope of Rb.  Rb, and therefore Sr, is common in the crust, so abundance of Sr in a sample of sediment (relative to Sr) is related to the amount of sediment which originated in the crust, as opposed to from the oceans.

text: Isotopes

text: are different forms of elements; they have a different number of neutrons in the nucleus, meaning they have very similar chemical properties, but different mass.  The weight difference means that some isotopes are discriminated against in chemical processes – for example, plants find it easier to incorporate the lighter

text: C

text: than heavy

text: C

text: . Other isotopes are only produced as a result of the radioactive decay of other elements, such as Sr, the daughter isotope of

text: Rb

text: .  Rb, and therefore Sr, is common in the crust, so abundance of Sr in a sample of sediment (relative to Sr) is related to the amount of sediment which originated in the crust, as opposed to from the oceans.

text: In order for a terrorist organization to construct and detonate a dirty bomb, it must acquire radioactive material. Possible RDD material could come from the millions of radioactive sources used worldwide in the industry, for medical purposes and in academic applications mainly for research. Of these sources, only nine reactor produced isotopes stand out as being suitable for radiological terror: americium-241, californium-252, caesium-137, cobalt-60, iridium-192, plutonium-238, polonium-210, radium-226 and strontium-90, and even from these it is possible that radium-226 and polonium-210 do not pose a significant threat. Of these sources the U.S. Nuclear Regulatory Commission has estimated that within the U.S., approximately one source is lost, abandoned or stolen every day of the year. Within the European Union the annual estimate is 70. There exist thousands of such "orphan" sources scattered throughout the world, but of those reported lost, no more than an estimated 20 percent can be classified as a potential high security concern if used in a RDD. Especially Russia is believed to house thousands of orphan sources, which were lost following the collapse of the Soviet Union. A large but unknown number of these sources probably belong to the high security risk category. Noteworthy are the  beta emitting strontium-90 sources used as radioisotope thermoelectric generators for beacons in lighthouses in remote areas of Russia. In December 2001, three Georgian woodcutters stumbled over such a power generator and dragged it back to their camp site to use it as a heat source. Within hours they suffered from acute radiation sickness and sought hospital treatment. The International Atomic Energy Agency (IAEA) later stated that it contained approximately  of strontium, equivalent to the amount of radioactivity released immediately after the Chernobyl accident (though the total radioactivity release from Chernobyl was 2500 times greater at around ).

text: In order for a terrorist organization to construct and detonate a dirty bomb, it must acquire radioactive material. Possible RDD material could come from the millions of radioactive sources used worldwide in the industry, for medical purposes and in academic applications mainly for research. Of these sources, only nine reactor produced

text: isotope

text: s stand out as being suitable for radiological terror:

text: and

text: , and even from these it is possible that radium-226 and polonium-210 do not pose a significant threat. Of these sources the U.S.

text: Nuclear Regulatory Commission

text: has estimated that within the U.S., approximately one source is lost, abandoned or stolen every day of the year. Within the

text: European Union

text: the annual estimate is 70. There exist thousands of such "orphan" sources scattered throughout the world, but of those reported lost, no more than an estimated 20 percent can be classified as a potential high security concern if used in a RDD. Especially

text: Russia

text: is believed to house thousands of orphan sources, which were lost following the collapse of the

text: Soviet Union

text: . A large but unknown number of these sources probably belong to the high security risk category. Noteworthy are the  beta emitting strontium-90 sources used as

text: radioisotope thermoelectric generator

text: s for

text: beacon

text: s in lighthouses in remote areas of Russia. In December 2001, three

text: Georgian

text: woodcutters stumbled over such a power generator and dragged it back to their camp site to use it as a heat source. Within hours they suffered from acute radiation sickness and sought hospital treatment. The

text: International Atomic Energy Agency

text: (IAEA) later stated that it contained approximately  of strontium, equivalent to the amount of radioactivity released immediately after the

text: Chernobyl accident

text: (though the total radioactivity release from Chernobyl was 2500 times greater at around ).

text: Lead has four stable isotopes, lead-204, lead-206, lead-207, and lead-208. The high number of stable isotopes relies on the fact that lead's atomic number of 82 is even, and is a magic number. With its high atomic number, lead is the second-heaviest element that occurs naturally in the form of isotopes regarded as stable: bismuth has a higher atomic number of 83, but its only primordial isotope was found in 2003 to be very slightly radioactive. The four stable isotopes of lead could theoretically undergo alpha decay to isotopes of mercury with a release of energy, but this has not been observed for any of them: accordingly, their predicted half-lives are extremely long, ranging up to over 10 years. As such, lead is often quoted as the heaviest stable element.

text: Lead has four stable isotopes, lead-204, lead-206, lead-207, and lead-208. The high number of stable isotopes relies on the fact that lead's

text: atomic number

text: of 82 is even, and is a

text: magic number

text: . With its high atomic number, lead is the second-heaviest element that occurs naturally in the form of isotopes regarded as stable:

text: bismuth

text: has a higher atomic number of 83, but its only primordial isotope was found in 2003 to be very slightly radioactive. The four stable isotopes of lead could theoretically undergo

text: alpha decay

text: to isotopes of

text: mercury

text: with a release of energy, but this has not been observed for any of them: accordingly, their predicted half-lives are extremely long, ranging up to over

text: years. As such, lead is often quoted as the heaviest stable element.

text: The existence of isotopes was first suggested in 1913 by the radiochemist Frederick Soddy, based on studies of radioactive decay chains that indicated about 40 different species referred to as radioelements (i.e. radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements from uranium to lead.<ref>Others had also suggested the possibility of isotopes; for example:

text: The existence of isotopes was first suggested in 1913 by the

text: radiochemist

text: Frederick Soddy

text: , based on studies of radioactive

text: decay chain

text: s that indicated about 40 different species referred to as radioelements (i.e. radioactive elements) between uranium and lead, although the periodic table only allowed for 11 elements from uranium to lead.<ref>Others had also suggested the possibility of isotopes; for example:

text: First, radioactivity is expressed as the decay rate of a particular radionuclide with decay constant λ and the number of atoms N:

text: A useful parameter of the mortgage model is the "half-life" of the loan which is the time it takes for the balance on the loan to reach half its original value. To determine the "half-life" we may write:

text: Natural thorium is essentially isotopically pure Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its radioactive decay produces a significant amount of the Earth's internal heat; the other major contributors are the shorter-lived primordial nuclides, U, U, and K. The other natural thorium isotopes are much shorter-lived; of them, only Th is usually detectable, occurring in secular equilibrium with its parent U, and making up at most 0.04% of natural thorium.{{efn|Other isotopes may occur alongside Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be Th, which occurs in the decay chain of Th (the thorium series): the ratio of Th to Th would be under 10. If uranium is present, tiny traces of several other isotopes will also be present: Th and Th from the decay chain of U (the actinium series), and slightly larger but still tiny traces of Th and Th from the decay chain of U (the uranium series). Th is also been produced in the decay chain of Np (the neptunium series): while all primordial Np is extinct, it is still produced today as a result of nuclear reactions in uranium ores. Th is mostly produced as a daughter of artificial U, itself produced from neutron irradiation of Th, due to its extreme rarity in nature.}}

text: Natural thorium is essentially isotopically pure Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its

text: radioactive decay

text: produces a significant amount of the

text: Earth's internal heat

text: ; the other major contributors are the shorter-lived primordial nuclides, U, U, and K. The other natural thorium isotopes are much shorter-lived; of them, only Th is usually detectable, occurring in

text: secular equilibrium

text: with its parent U, and making up at most 0.04% of natural thorium.{{efn|Other isotopes may occur alongside Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be Th, which occurs in the

text: decay chain

text: of Th (the

text: thorium series

text: ): the ratio of Th to Th would be under 10. If uranium is present, tiny traces of several other isotopes will also be present: Th and Th from the decay chain of U (the

text: actinium series

text: ), and slightly larger but still tiny traces of Th and Th from the decay chain of U (the

text: uranium series

text: ). Th is also been produced in the decay chain of Np (the

text: neptunium series

text: ): while all primordial Np is

text: extinct

text: , it is still produced today as a result of nuclear reactions in uranium ores. Th is mostly produced as a

text: daughter

text: of artificial U, itself produced from

text: neutron irradiation

text: of Th, due to its extreme rarity in nature.}}

text: Although it is estimated that about 32 primordial nuclides are radioactive (list below), it becomes very difficult to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There exist many extremely long-lived nuclides whose half-lives are still unknown. For example, it is known theoretically that all isotopes of tungsten, including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by alpha emission, but  this could only be measured experimentally for . Nevertheless, the number of nuclides with half-lives so long that they cannot be measured with present instruments—and are considered from this viewpoint to be stable nuclides—is limited. Even when a "stable" nuclide is found to be radioactive, the fact merely moves it from the stable to the unstable list of primordial nuclides, and the total number of primordial nuclides remains unchanged.

text: Although it is estimated that about 32 primordial nuclides are

text: radioactive

text: (list below), it becomes very difficult to determine the exact total number of radioactive primordials, because the total number of stable nuclides is uncertain. There exist many extremely long-lived nuclides whose half-lives are still unknown. For example, it is known theoretically that all

text: isotope

text: s of

text: tungsten

text: , including those indicated by even the most modern empirical methods to be stable, must be radioactive and can decay by

text: alpha emission

text: , but  this could only be measured experimentally for . Nevertheless, the number of nuclides with half-lives so long that they cannot be measured with present instruments—and are considered from this viewpoint to be

text: stable nuclides

text: —is limited. Even when a "stable" nuclide is found to be radioactive, the fact merely moves it from the stable to the unstable list of primordial nuclides, and the total number of primordial nuclides remains unchanged.

text: Radioactive isotopes can be produced by various nuclear reactions. For example, the isotope Co is produced by cyclotron irradiation of iron. The principal reaction involved is the (d,n) reaction   Fe + H → n + Co.

text: Radioactive isotopes can be produced by various

text: nuclear reaction

text: s. For example, the isotope Co is produced by

text: cyclotron

text: irradiation of iron. The principal reaction involved is the (d,n) reaction   Fe + H → n + Co.

text: which is indeed the correct form for radioactive decay.

text: Types of radioactive decay include

text: Types of radioactive decay include

text: An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969. IUPAC defines an element to exist if its lifetime is longer than 10 seconds, which is the time it takes for the nucleus to form an electron cloud. No elements in this region have been synthesized or discovered in nature. The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9.

text: An eight-period table containing this block was suggested by

text: Glenn T. Seaborg

text: in 1969.

text: IUPAC

text: defines an element to exist if its lifetime is longer than 10 seconds, which is the time it takes for the nucleus to form an electron cloud. No elements in this region have been synthesized or discovered in nature. The first element of the g-block may have

text: atomic number

text: 121, and thus would have the

text: systematic name

text: unbiunium

text: . Elements in this region are likely to be highly unstable with respect to

text: radioactive decay

text: , and have extremely short

text: half lives

text: , although element 126 is hypothesized to be within an

text: island of stability

text: that is resistant to fission but not to

text: alpha decay

text: . It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9.

text: The radioactivity of all radioactive waste diminishes with time. All radionuclides contained in the waste have a half-life—the time it takes for half of the atoms to decay into another nuclide—and eventually all radioactive waste decays into non-radioactive elements (i.e., stable nuclides). Certain radioactive elements (such as plutonium-239) will remain hazardous to humans and other creatures for hundreds or thousands of years. Other radionuclides remain radioactive for millions of years (though most of these products have so little activity as a result of their long half-lives that their radiation is lost in the background level). Thus, these wastes must be shielded for centuries and isolated from the living environment for millennia. Since radioactive decay follows the half-life rule, the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like iodine-129 will be much less intense than that of a short-lived isotope like iodine-131. The two tables show some of the major radioisotopes, their half-lives, and their radiation yield as a proportion of the yield of fission of uranium-235.

text: The radioactivity of all radioactive waste diminishes with time. All radionuclides contained in the waste have a

text: half-life

text: —the time it takes for half of the atoms to decay into another

text: nuclide

text: —and eventually all radioactive waste decays into non-radioactive elements (i.e.,

text: stable nuclide

text: s). Certain radioactive elements (such as

text: ) will remain hazardous to humans and other creatures for hundreds or thousands of years. Other radionuclides remain radioactive for millions of years (though most of these products have so little activity as a result of their long half-lives that their radiation is lost in the

text: background level

text: ). Thus, these wastes must be shielded for centuries and isolated from the living

text: environment

text: for millennia. Since radioactive decay follows the half-life rule, the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like

text: will be much less intense than that of a short-lived isotope like

text: . The two tables show some of the major radioisotopes, their half-lives, and their

text: radiation yield

text: as a proportion of the yield of fission of uranium-235.

text: SYERS previous version has also been used with the Lockheed U-2 reconnaissance aircraft.

text: SYERS previous version has also been used with the

text: Lockheed U-2

text: reconnaissance aircraft

text: Radioactive decay is the process of emission of particles and energy from the unstable nucleus of an atom to form a stable product. This is done via the tunnelling of a particle out of the nucleus (an electron tunnelling into the nucleus is electron capture). This was the first application of quantum tunnelling and led to the first approximations. Radioactive decay is also a relevant issue for astrobiology as this consequence of quantum tunnelling is creating a constant source of energy over a large period of time for environments outside the circumstellar habitable zone where insolation would not be possible (subsurface oceans) or effective.

text: Radioactive decay is the process of emission of particles and energy from the unstable nucleus of an atom to form a stable product. This is done via the tunnelling of a particle out of the nucleus (an electron tunnelling into the nucleus is

text: electron capture

text: ). This was the first application of quantum tunnelling and led to the first approximations. Radioactive decay is also a relevant issue for

text: astrobiology

text: as this consequence of quantum tunnelling is creating a constant source of energy over a large period of time for environments outside the

text: circumstellar habitable zone

text: where insolation would not be possible (

text: subsurface oceans

text: ) or effective.

text: There was a release of 740 PBq of fission products, approximately 10% of which was dispersed into the atmosphere. Cerium-144 and Zirconium-95 (both relatively short lived isotopes with a half life of 285 and 64 days respectively) made up 91% of the release. There was 1 PBq of Sr-90, and 13 TBq of Cs-137. The contaminated zone,  called East Urals Radioactive Trace (EURT), measuring 300 x 50 km was contaminated by more than 4 kBq/m² of Sr-90. The global fallout of Sr-90 was about 2 kBq/m². An area measuring 17 km² was contaminated by about 100 MBq Sr-90/m².

text: There was a release of 740

text: PBq

text: of

text: fission product

text: s, approximately 10% of which was dispersed into the atmosphere.

text: and

text: (both relatively short lived isotopes with a half life of 285 and 64 days respectively) made up 91% of the release. There was 1 PBq of

text: , and 13 TBq of

text: . The contaminated zone,  called East Urals Radioactive Trace (EURT), measuring 300 x 50 km was contaminated by more than 4 kBq/m² of Sr-90. The global fallout of Sr-90 was about 2 kBq/m². An area measuring 17 km² was contaminated by about 100 MBq Sr-90/m².

text: The most stable nuclear isomer occurring in nature is , which is present in all tantalum samples at about 1 part in 8,300. Its half-life is at least 10 years, markedly longer than the age of the universe. This persistence results from the fact that the excitation energy of the isomeric state is low, and both gamma de-excitation to the  ground state (which itself is radioactive by beta decay, with a half-life of only 8 hours), and direct beta decay to hafnium or tungsten are all suppressed, owing to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed in supernovae (as are most other heavy elements). When it relaxes to its ground state, it releases a photon with an energy of 75 keV.

text: The most stable nuclear isomer occurring in nature is

text: , which is present in all

text: tantalum

text: samples at about 1 part in 8,300. Its half-life is at least 10 years, markedly longer than the

text: age of the universe

text: . This persistence results from the fact that the excitation energy of the isomeric state is low, and both gamma de-excitation to the  ground state (which itself is radioactive by beta decay, with a half-life of only 8 hours), and direct

text: beta decay

text: to

text: hafnium

text: or

text: tungsten

text: are all suppressed, owing to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed in

text: supernova

text: e (as are most other heavy elements). When it relaxes to its ground state, it releases a

text: photon

text: with an energy of 75

text: keV

text: Harrison's observational research showed that the afterglows of gamma-ray bursts exhibit breaks in their decay rate due to collimation of the ejecta. Scientific highlights from the NuSTAR mission include mapping the radioactive debris in the Cassieopeia A supernova remnant to constrain the core collapse explosion mechanism, measurement of the spin of supermassive and stellar mass black holes, the discovery of a magnetar in the Galactic Center, and the discovery of an ultra luminous pulsar.

text: Harrison's observational research showed that the afterglows of gamma-ray bursts exhibit breaks in their decay rate due to collimation of the ejecta. Scientific highlights from the NuSTAR mission include mapping the radioactive debris in the

text: Cassieopeia A supernova remnant

text: to constrain the core collapse explosion mechanism, measurement of the spin of supermassive and stellar mass

text: black holes

text: , the discovery of a

text: magnetar

text: in the

text: Galactic Center

text: , and the discovery of an ultra luminous

text: pulsar

text: The decay energy is the energy released by a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide.

text: The decay energy is the

text: energy

text: released by a

text: radioactive decay

text: . Radioactive decay is the process in which an unstable

text: atomic nucleus

text: loses energy by emitting ionizing particles and

text: radiation

text: . This decay, or loss of energy, results in an atom of one type, called the parent

text: nuclide

text: transforming to an atom of a different type, called the daughter nuclide.

text: Radium is a chemical element with symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily combines with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (RaN). All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1600 years and decays into radon gas (specifically the isotope radon-222). When radium decays, ionizing radiation is a product, which can excite fluorescent chemicals and cause radioluminescence.

text: Radium is a

text: chemical element

text: with symbol Ra and

text: atomic number

text: 88. It is the sixth element in

text: group 2

text: of the

text: periodic table

text: , also known as the

text: alkaline earth metal

text: s. Pure radium is silvery-white, but it readily combines with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (RaN). All isotopes of radium are highly

text: radioactive

text: , with the most stable

text: isotope

text: being

text: , which has a

text: half-life

text: of 1600 years and

text: decays

text: into

text: radon

text: gas (specifically the isotope

text: ). When radium decays,

text: ionizing radiation

text: is a product, which can excite

text: fluorescent

text: chemicals and cause

text: radioluminescence

text: For the NAA procedure to be successful, the specimen or sample must be selected carefully. In many cases small objects can be irradiated and analysed intact without the need of sampling. But, more commonly, a small sample is taken, usually by drilling in an inconspicuous place. About 50 mg (one-twentieth of a gram) is a sufficient sample, so damage to the object is minimised. It is often good practice to remove two samples using two different drill bits made of different materials. This will reveal any contamination of the sample from the drill bit material itself. The sample is then encapsulated in a vial made of either high purity linear polyethylene or quartz. These sample vials come in many shapes and sizes to accommodate many specimen types. The sample and a standard are then packaged and irradiated in a suitable reactor at a constant, known neutron flux. A typical reactor used for activation uses uranium fission, providing a high neutron flux and the highest available sensitivities for most elements. The neutron flux from such a reactor is in the order of 10 neutrons cm s. The type of neutrons generated are of relatively low kinetic energy (KE), typically less than 0.5 eV. These neutrons are termed thermal neutrons. Upon irradiation, a thermal neutron interacts with the target nucleus via a non-elastic collision, causing neutron capture. This collision forms a compound nucleus which is in an excited state. The excitation energy within the compound nucleus is formed from the binding energy of the thermal neutron with the target nucleus. This excited state is unfavourable and the compound nucleus will almost instantaneously de-excite (transmutate) into a more stable configuration through the emission of a prompt particle and one or more characteristic prompt gamma photons. In most cases, this more stable configuration yields a radioactive nucleus. The newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons. This decay process is at a much slower rate than the initial de-excitation and is dependent on the unique half-life of the radioactive nucleus. These unique half-lives are dependent upon the particular radioactive species and can range from fractions of a second to several years. Once irradiated, the sample is left for a specific decay period, then placed into a detector, which will measure the nuclear decay according to either the emitted particles, or more commonly, the emitted gamma rays.

text: For the NAA procedure to be successful, the specimen or sample must be selected carefully. In many cases small objects can be irradiated and analysed intact without the need of sampling. But, more commonly, a small sample is taken, usually by drilling in an inconspicuous place. About 50 mg (one-twentieth of a

text: gram

text: ) is a sufficient sample, so damage to the object is minimised. It is often good practice to remove two samples using two different drill bits made of different materials. This will reveal any contamination of the sample from the drill bit material itself. The sample is then encapsulated in a vial made of either high purity linear

text: polyethylene

text: or

text: quartz

text: . These sample vials come in many shapes and sizes to accommodate many specimen types. The sample and a standard are then packaged and irradiated in a suitable reactor at a constant, known neutron

text: flux

text: . A typical reactor used for activation uses

text: uranium

text: fission

text: , providing a high neutron flux and the highest available sensitivities for most elements. The neutron flux from such a reactor is in the order of 10 neutrons cm s. The type of neutrons generated are of relatively low

text: kinetic energy

text: (KE), typically less than 0.5

text: eV

text: . These neutrons are termed thermal neutrons. Upon irradiation, a thermal neutron interacts with the target nucleus via a non-elastic collision, causing neutron capture. This collision forms a compound nucleus which is in an excited state. The excitation energy within the compound nucleus is formed from the

text: binding energy

text: of the thermal neutron with the target nucleus. This excited state is unfavourable and the compound nucleus will almost instantaneously de-excite (transmutate) into a more stable configuration through the emission of a prompt particle and one or more characteristic prompt gamma photons. In most cases, this more stable configuration yields a radioactive nucleus. The newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons. This decay process is at a much slower rate than the initial de-excitation and is dependent on the unique half-life of the radioactive nucleus. These unique half-lives are dependent upon the particular radioactive species and can range from fractions of a second to several years. Once irradiated, the sample is left for a specific decay period, then placed into a detector, which will measure the nuclear decay according to either the emitted particles, or more commonly, the emitted gamma rays.

text: For example, the DNA fragment of interest may be PCR amplified using a P 5' labeled primer, with the result being many DNA molecules with a radioactive label on one end of one strand of each double stranded molecule. Cleavage by DNase will produce fragments. The fragments which are smaller with respect to the P-labelled end will appear further on the gel than the longer fragments. The gel is then used to expose a special photographic film.

text: For example, the DNA fragment of interest may be

text: PCR

text: amplified using a P 5' labeled primer, with the result being many DNA molecules with a radioactive label on one end of one strand of each double stranded molecule. Cleavage by DNase will produce fragments. The fragments which are smaller with respect to the P-labelled end will appear further on the gel than the longer fragments. The gel is then used to expose a special photographic film.