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In [[nuclear science]], the '''decay chain''' refers to the [[radioactive decay]] of different discrete radioactive [[decay products]] as a chained series of transformations.
Most radioactive elements do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable [[isotope]] is reached. 

Decay stages are referred to by their relationship to previous or subsequent stages. A ''parent isotope'' is one that undergoes decay to form a ''daughter isotope''. The daughter isotope may be stable or it may decay to form a daughter isotope of its own. The daughter of a daughter isotope is sometimes called a ''granddaughter isotope''.  

The time it takes for a single parent atom to decay to an atom of its daughter isotope can vary widely, not only for different parent-daughter chains, but also for identical pairings of parent and daughter isotopes. While the decay of a single atom occurs spontaneously, the decay of an initial population of identical atoms over time, ''t'', follows a decaying exponential distribution, ''e<sup>-λt</sup>'', where λ is called a [[decay constant]]. Because of this exponential nature, one of the properties of an isotope is its [[half-life]], the time by which half of an initial number of identical parent [[radioisotope]]s have decayed to their daughters.  Half-lives have been determined in laboratories for thousands of radioisotopes (or, [[radionuclide]]s).  These can range from nearly instantaneous to as much as 10<sup>19</sup> years or more.

The intermediate stages often emit more radioactivity than the original radioisotope: when equilibrium is achieved, a granddaughter isotope is present in proportion to its half-life; but since its activity is inversely proportional to its half-life, any nucleid in the decay chain finally contributes as much as the head of the chain. For example, natural [[uranium]] is not significantly radioactive, but samples of [[pitchblende]], a uranium ore, are 13 times more radioactive because of the [[radium]] and other daughter isotopes they contain. Not only are unstable [[radium]] isotopes significant radioactive emitters, but they also generate gaseous [[radon]] as the next stage in the decay chain. Thus, radon is a naturally occurring radioactive gas, which is the leading cause of [[lung cancer]] in non-smokers[http://www.epa.gov/radon/].

==Types ==
[[Image:Radioactive decay chains diagram.svg|thumb|right| This diagram illustrates the four decay chains: thorium (in blue), radium (in red), actinium (in green), and neptunium (in purple).]]
The four most common modes of radioactive decay are: [[alpha decay]], [[beta decay|beta minus decay]], [[positron emission|beta plus decay]] (considered as both [[positron emission]] and [[electron capture]]) and [[isomeric transition]]. Of these decay processes, alpha decay changes the [[atomic mass]] number 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 [[modular arithmetic|residue]] mod 4, dividing all nuclides into four classes. The members of any possible decay chain must be drawn entirely from one of these classes.

Three main decay chains (or families) are observed in nature, commonly called the [[thorium]] series, the [[radium]] series (not [[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 these chains can be represented as ''A''=4''n'', ''A''=4''n''+2 and A=4''n''+3, respectively. The long lived starting isotopes <sup>232</sup>Th, <sup>238</sup>U and <sup>235</sup>U of these three have existed since the formation of the earth; the precursor <sup>244</sup>Pu has also been found in minute amounts on earth<ref>D.C . Hoffman, F. O. Lawrence, J. L. Mewheter, F. M. Rourke: ''Detection of Plutonium-244 in Nature.'' In: ''Nature'', Nr. 34, 1971, pp. 132–134</ref>. The fourth chain, the [[neptunium]] series with ''A''=4''n''+1, due to quite short half life time of its starting isotope [[neptunium|<sup>237</sup>Np]], is already extinct, except for the final rate-limiting step. The ending isotope of this chain is [[thallium|<sup>205</sup>Tl]]. Some older sources give the final isotope as [[bismuth|<sup>209</sup>Bi]], but it was recently discovered that <sup>209</sup>Bi is radioactive with half-life of 1.9×10<sup>19</sup> years. 

There are also many shorter chains, for example [[carbon-14]].  On the earth, most of the starting isotopes of these chains are generated by [[cosmic radiation]].  

==Actinide alpha decay chains==
In the tables below, the minor branches of decay (with the branching ratio of less than 0.0001%) are omitted. The energy release includes the total kinetic energy of all the emitted particles ([[electron]]s, [[alpha particle]]s, [[Gamma rays|gamma quanta]], [[neutrino]]s, [[Auger electron]]s and [[X-ray]]s) and the recoil nucleus, assuming that the original nucleus was at rest.

In the tables below, the historic names of the naturally occurring nuclides are also given. These names were used at the time when the decay chains were first discovered and investigated. From these names one can infer the particular chain to which the nuclide belongs. Also, the names indicate similarities: for example, Tn, Rn and An are all inert gases.

{{Thorium series}}

{{Radium series}}

{{Actinium series}}

{{Neptunium series}}

==Beta decay chains==

Since heavy nuclei have a greater proportion of neutrons, [[fission product]] nuclei almost always start out with a neutron/proton ratio greater than what is stable for their mass range; therefore they undergo multiple [[beta decay]]s in succession, each converting a neutron to a proton. The first decays tend to have higher decay energy and shorter half-life; the last decays may have low decay energy and/or long half-life.

For example, [[uranium-235]] has 92 protons and 143 neutrons. Fission takes one more neutron, then produces two or three more neutrons; assume that 92 protons and 142 neutrons are available for the two fission product nuclei. Suppose they have mass 99 with 39 protons and 60 neutrons ([[yttrium]]-99), and mass 135 with 53 protons and 82 neutrons ([[iodine]]-135); then the decay chains are:


{| class="wikitable"
!Nuclide!!Halflife
|-
|<sup>99</sup>Y||1.470(7) s
|-
|<sup>99</sup>Zr||2.1(1) s
|-
|<sup>99</sup>Nb||15.0(2) s
|-
|<sup>99</sup>Mo||2.7489(6) d
|-
|[[Tc-99|<sup>99</sup>Tc]]||2.111(12)E+5 a
|-
|<sup>99</sup>Ru||Stable
|}

{| class="wikitable"
!Nuclide!!Halflife
|-
|<sup>135</sup>I||6.57(2) h
|-
|[[Xe-135|<sup>135</sup>Xe]]||9.14(2) h
|-
|[[Cs-135|<sup>135</sup>Cs]]||2.3(3)E+6 a
|-
|<sup>135</sup>Ba||Stable
|}

== Literature ==
C.M. Lederer, J.M. Hollander, I. Perlman, Table of Isotopes, 6th ed., Wiley & Sons, New York 1968
{{reflist|2}}

== External links ==
{{Commons|Category:Decay chain|Decay chain}}

* [http://www.epa.gov/radiation/understand/chain.html Decay chains]
* [http://www.atral.com/U2381.html Uranium-238 decay chain]
* [http://ie.lbl.gov/education/isotopes.htm Government website listing isotopes and decay energies]

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