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st if a decay chain were the only cause of its presence that sample is said to be out of equilibrium an unintuitive consequence of this disequilibrium is that a sample of enriched material may occasionally increase in radioactivity as daughter products that are more highly radioactive than their parents accumulate both enriched and depleted uranium provide examples of this phenomenon history edit the chemical elements came into being in two phases the first commenced shortly after the big bang from ten seconds to 20 minutes after the beginning of the universe the earliest condensation of light atoms was responsible for the manufacture of the four lightest elements the vast majority of this primordial production consisted of the three lightest isotopes of hydrogen protium deuterium and tritium and two of the nine known isotopes of helium helium 3 and helium 4 trace amounts of lithium 7 and beryllium 7 were likely also produced so far as is known all heavier elements came into being starting around 100 million years later in a second phase of nucleosynthesis that commenced with the birth of the first stars 1 the nuclear furnaces that power stellar evolution were necessary to create large quantities of all elements heavier than helium and the r and s process es of neutron capture that occur in stellar cores are thought to have created all such elements up to iron and nickel atomic numbers 26 and 28 the extreme conditions that attend supernovae explosions are capable of creating the elements between oxygen and rubidium i e atomic numbers 8 through 37 the creation of heavier elements including those without stable isotopes all elements with atomic numbers greater than lead s 82 appears to rely on r process nucleosynthesis operating amid the immense concentrations of free neutrons released during neutron star mergers most of the isotopes of each chemical element present in the earth today were formed by such processes no later than the time of our planet s condensation from the solar protoplanetary disc around 4 5 billion years ago the exceptions to these so called primordial elements are those that have resulted from the radioactive disintegration of unstable parent nuclei as they progress down one of several decay chains each of which terminates with the production of one of the 251 stable isotopes known to exist aside from cosmic or stellar nucleosynthesis and decay chains the only other ways of producing a chemical element rely on atomic weapons nuclear reactors natural or manmade or the laborious atom by atom assembly of nuclei with particle accelerators unstable isotopes decay to their daughter products which may sometimes be even more unstable at a given rate eventually often after a series of decays a stable isotope is reached there are 251 stable isotopes in the universe in stable isotopes light elements typically have a lower ratio of neutrons to protons in their nucleus than heavier elements light elements such as helium 4 have close to a 1 1 neutron proton ratio the heaviest elements such as uranium have close to 1 5 neutrons per proton e g 1 587 in uranium 238 no nuclide heavier than lead 208 is stable these heavier elements have to shed mass to achieve stability mostly by alpha decay the other common way for isotopes with a high neutron to proton ratio n p to decay is beta decay in which the nuclide changes elemental identity while keeping the same mass number and lowering its n p ratio for some isotopes with a relatively low n p ratio there is an inverse beta decay by which a proton is transformed into a neutron thus moving towards a stable isotope however since fission almost always produces products which are neutron heavy positron emission or electron capture are rare compared to electron emission there are many relatively short beta decay chains at least two a heavy beta decay and a light positron decay for every discrete weight up to around 207 and some beyond but for the higher mass elements isotopes heavier than lead there are only four pathways which encompass all decay chains citation needed this is because there are just two main decay methods alpha radiation which reduces the mass number by 4 and beta which leaves it unchanged the four paths are termed 4n 4n 1 4n 2 and 4n 3 the remainder from dividing the atomic mass by four gives the chain the isotope will follow in its decay there are other decay modes but they invariably occur at a lower probability than alpha or beta decay it should not be supposed that these chains have no branches the diagram below shows a few branches of chains and in reality there are many more because there are many more isotopes possible than are shown in the diagram for example the third atom of nihonium 278 synthesised underwent six alpha decays down to mendelevium 254 followed by an electron capture a form of beta decay to fermium 254 and then a seventh alpha to californium 250 2 upon which it would have followed the 4n 2 chain radium series as given in this article however the heaviest superheavy nuclides synthesised do not reach the four decay chains because they reach a spontaneously fissioning nuclide after a few alpha decays that terminates the chain this is what happened to the first two atoms of nihonium 278 synthesised 3 4 as well as to all heavier nuclides produced three of those chains have a long lived isotope or nuclide near the top this long lived nuclide is a bottleneck in the process through which the chain flows very slowly and keeps the chain below them alive with flow the three long lived nuclides are uranium 238 half life 4 463 billion years uranium 235 half life 704 million years and thorium 232 half life 14 1 billion years the fourth chain has no such long lasting bottleneck nuclide near the top so that chain has long since decayed down to the last before the end bismuth 209 this nuclide was long thought to be stable but in 2003 it was found to be unstable with a very long half life of 20 1 billion billion years 5 it is the last step in the chain before stable thallium 205 because this bottleneck is so long lived very small quantities of the final decay product have been produced and for most practical purposes bismuth 209 is the final decay product in the past during the first few million years of the history of the solar system there were more unstable high mass nuclides in existence and the four chains were longer as they included nuclides that have since decayed away notably 244 pu 237 np and 247 cm have half lives over a million years and would have then been bottlenecks higher in the 4n 4n 1 and 4n 3 chains respectively 6 244 pu and 247 cm have been identified as having been present there is no nuclide with a half life over a million years above 238 u in the 4n 2 chain today some of these formerly extinct isotopes are again in existence as they have been manufactured thus they again take their places in the chain plutonium 239 used in nuclear weapons is the major example decaying to uranium 235 via alpha emission with a half life 24 500 years there has also been large scale production of neptunium 237 resurrecting the extinct fourth chain 7 the tables below hence start the four decay chains at isotopes of californium with mass numbers from 249 to 252 summary of the four decay chain pathways name of series thorium neptunium uranium or radium actinium mass numbers 4 n 4 n 1 4 n 2 4 n 3 long lived nuclide 232 th 244 pu 209 bi 237 np 238 u 235 u 247 cm half life billions of years 14 1 0 0813 20 100 000 000 0 002144 4 463 0 704 0 0156 end of chain 208 pb 205 tl 206 pb 207 pb these four chains are summarised in the chart in the following section types of decay edit this diagram illustrates the four decay chains discussed in the text thorium 4n in blue neptunium 4n 1 in pink radium 4n 2 in red and actinium 4n 3 in green the four most common modes of radioactive decay are alpha decay beta decay inverse beta decay considered as both positron emission and electron capture and isomeric transition of these decay processes only alpha decay fission of a helium 4 nucleus changes the atomic mass number a of the nucleus and always decreases it by four because of this almost any decay will result in a nucleus whose atomic mass number has the same residue mod 4 this divides the list of nuclides into four classes each of which forms a main decay chain three of these are readily observed in nature commonly called the thorium series the radium or uranium series and the actinium series representing three of these four classes and ending in three different stable isotopes of lead the mass number of every isotope in the chain can be represented as a 4 n a 4 n 2 or a 4 n 3 respectively the long lived starting isotopes of these three isotopes respectively thorium 232 uranium 238 and uranium 235 have existed since the formation of the earth ignoring the artificial isotopes and their decays created since the 1940s due to the relatively short half life of its starting isotope neptunium 237 2 144 million years the fourth chain the neptunium series with a 4 n 1 is already extinct in nature except for the final rate limiting step decay of bismuth 209 traces of 237 np and its decay products do occur in nature however as a result of neutron reactions in uranium ore neutron capture by natural thorium to give 233 u is also possible 8 the ending isotope of this chain is now known to be thallium 205 some older sources give the final isotope as bismuth 209 but in 2003 it was discovered that it is very slightly radioactive with a half life of 2 01 10 19 years 9 there are also non transuranic decay chains of unstable isotopes of light elements for example those of magnesium 28 and chlorine 39 on earth most of the starting isotopes of these chains before 1945 were generated by cosmic radiation since 1945 the testing and use of nuclear weapons has also released numerous radioactive fission products almost all such isotopes decay by either β or β decay modes changing from one element to another at the same atomic mass the later daughter products in such a chain being closer to beta stability generally have the longer half lives heavy nuclei actinide decay chains edit actinides and fission products by half life v t e actinides 10 by decay chain half life range a fission products of 235 u by yield 11 4 n thorium 4 n 1 neptunium 4 n 2 radium 4 n 3 actinium 4 5 7 0 04 1 25 0 001 228 ra 4 6 a 155 eu þ 248 bk 12 9 a 244 cm ƒ 241 pu ƒ 250 cf 227 ac 10 29 a 90 sr 85 kr 113m cd þ 232 u ƒ 238 pu ƒ 243 cm ƒ 29 97 a 137 cs 151 sm þ 121m sn 249 cf ƒ 242m am ƒ 141 351 a no fission products have a half life in the range of 100 a 210 ka 241 am ƒ 251 cf ƒ 13 430 900 a 226 ra 247 bk 1 3 1 6 ka 240 pu 229 th 246 cm ƒ 243 am ƒ 4 7 7 4 ka 245 cm ƒ 250 cm 8 3 8 5 ka 239 pu ƒ 24 1 ka 230 th 231 pa 32 76 ka 236 np ƒ 233 u ƒ 234 u 150 250 ka 99 tc 126 sn 248 cm 242 pu 327 375 ka 79 se 1 33 ma 135 cs 237 np ƒ 1 61 6 5 ma 93 zr 107 pd 236 u 247 cm ƒ 15 24 ma 129 i 244 pu 80 ma nor beyond 15 7 ma 14 232 th 238 u 235 u ƒ 0 7 14 1 ga has thermal neutron capture cross section in the range of 8 50 barns ƒ fissile primarily a naturally occurring radioactive material norm þ neutron poison thermal neutron capture cross section greater than 3k barns in the four tables below very minor branches of decay branching probability less than one in a million are omitted spontaneous fission is also omitted though larger than this for the heaviest even nuclei and detectable down to thorium all nuclear data is taken from 9 unless otherwise noted the historical names of isotopes are recorded in 15 the energy release includes the total kinetic energy of all the emitted particles electrons alpha particles gamma quanta neutrinos auger electrons and x rays and the recoiling decay product nucleus this corresponds to that calculated from atomic masses the letter a represents a year from the latin annus in the tables except for the neptunium series the historical names of the naturally occurring nuclides are also given such names were used at the time when the decay chains were first discovered and investigated the system listed was only finalized in the 1920s but it would be too confusing to give earlier names also from these historical names one can thus find the modern isotopic designation the three primordial chains given below thorium uranium radium from uranium 238 and actinium from uranium 235 each ends with its own specific lead isotope lead 208 lead 206 and lead 207 respectively all the lead isotopes are stable and are also present in nature as primordial nuclides so their excess amounts in comparison with lead 204 which has only a primordial origin are required for accurate uranium lead dating of rocks correlating more than one results in lead lead dating capable of even greater accuracy thorium series edit the 4n chain of thorium 232 is commonly called the thorium series or thorium cascade the series terminates with lead 208 6 alpha decays and 4 beta decays from thorium plutonium 244 which appears several steps above thorium 232 was present in the early solar system 6 and is just long lived enough half life 81 1 myr that it should still survive in trace quantities today 16 though it probably has not been detected 17 the total energy released from thorium 232 to lead 208 including the energy lost to neutrinos is 42 65 mev from californium 252 71 11 mev the last is the largest of the four chains nuclide historic names decay mode half life a years energy released mev decay product short long 252 cf α 2 645 a 6 217 248 cm 248 cm α 3 48 10 5 a 5 162 244 pu 244 pu α 8 13 10 7 a 4 666 240 u 240 u β 14 1 h 0 382 240m np 18 240m np it 0 12 β 99 88 7 22 min 0 018 2 209 240 np 240 pu 240 np β 61 9 min 2 191 240 pu 240 pu α 6561 a 5 256 236 u 236 u thoruranium 19 α 2 342 10 7 a 4 573 232 th 232 th th thorium α 1 40 10 10 a 4 082 228 ra 228 ra msth 1 mesothorium 1 β 5 75 a 0 046 228 ac 228 ac msth 2 mesothorium 2 β 6 15 h 2 123 228 th 228 th rdth radiothorium α 1 9125 a 5 520 224 ra 224 ra thx thorium x α 3 632 d 5 789 220 rn 220 rn tn thoron thorium emanation α 55 6 s 6 405 216 po 216 po tha thorium a α 0 144 s 6 906 212 pb 212 pb thb thorium b β 10 627 h 0 569 212 bi 212 bi thc thorium c β 64 06 α 35 94 60 55 min 2 252 6 207 212 po 208 tl 212 po thc thorium c α 294 4 ns 8 954 208 pb 208 tl thc thorium c β 3 053 min 4 999 208 pb 208 pb thd thorium d stable neptunium series edit the 4n 1 chain of neptunium 237 is commonly called the neptunium series or neptunium cascade in this series only two of the isotopes involved are found naturally in significant quantities namely the final two bismuth 209 and thallium 205 some of the other isotopes have been detected in nature originating from trace quantities of 237 np produced by the n 2n knockout reaction in primordial 238 u 8 this series terminates with the stable isotope thallium 205 8 alpha decays and 4 beta decays from neptunium the neptunium series is fairly unique among the 4 for severa...