Decay chain

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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-λt, 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 radioisotopes have decayed to their daughters. Half-lives have been determined in laboratories for thousands of radioisotopes (or, radionuclides). These can range from nearly instantaneous to as much as 1019 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[2].


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 minus decay, 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 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=4n, A=4n+2 and A=4n+3, respectively. The long lived starting isotopes 232Th, 238U and 235U of these three have existed since the formation of the earth; the precursor 244Pu has also been found in minute amounts on earth[1]. The fourth chain, the neptunium series with A=4n+1, due to quite short half life time of its starting isotope 237Np, is already extinct, except for the final rate-limiting step. The ending isotope of this chain is 205Tl. Some older sources give the final isotope as 209Bi, but it was recently discovered that 209Bi is radioactive with half-life of 1.9×1019 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 (electrons, alpha particles, gamma quanta, neutrinos, Auger electrons and X-rays) 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

The 4n chain of Th-232 is commonly called the "thorium series". In the following charts, the letter 'a' represents a year.

nuclide historic name (short) historic name (long) decay mode half life energy released, MeV product of decay
252Cf α 2.645 a 6.1181 248Cm
248Cm α 3.4×105 a 6.260 244Pu
244Pu α 8×107 a 4.589 240U
240U β- 14.1 h .39 240Np
240Np β- 1.032 h 2.2 240Pu
244Cm α 18 a 5.8048 240Pu
240Pu α 6561 a 5.1683 236U
236U α 2.3·107 a 4.494 232Th
232Th Th Thorium α 1.405·1010 a 4.081 228Ra
228Ra MsTh1 Mesothorium 1 β- 5.75 a 0.046 228Ac
228Ac MsTh2 Mesothorium 2 β- 6.25 h 2.124 228Th
228Th RdTh Radiothorium α 1.9116 a 5.520 224Ra
224Ra ThX Thorium X α 3.6319 d 5.789 220Rn
220Rn Tn Thoron α 55.6 s 6.404 216Po
216Po ThA Thorium A α 0.145 s 6.906 212Pb
212Pb ThB Thorium B β- 10.64 h 0.570 212Bi
212Bi ThC Thorium C β- 64.06%
α 35.94%
60.55 min 2.252
212Po ThC' Thorium C' α 299 ns 8.955 208Pb
208Tl ThC" Thorium C" β- 3.053 min 4.999 208Pb
208Pb . stable . .

Radium series (aka Uranium series)

The 4n+2 chain of U-238 is commonly called the "radium series" (sometimes "uranium series").

nuclide historic name (short) historic name (long) decay mode half life MeV product of decay
238U U Uranium α 4.468·109 a 4.270 234Th
234Th UX1 Uranium X1 β- 24.10 d 0.273 234Pa
234Pa UZ Uranium Z β- 6.70 h 2.197 234U
234U UII Uranium two α 245500 a 4.859 230Th
230Th Io Ionium α 75380 a 4.770 226Ra
226Ra Ra Radium α 1602 a 4.871 222Rn
222Rn Rn Radon α 3.8235 d 5.590 218Po
218Po RaA Radium A α 99.98 %
β- 0.02 %
3.10 min 6.115
218At α 99.90 %
β- 0.10 %
1.5 s 6.874
218Rn α 35 ms 7.263 214Po
214Pb RaB Radium B β- 26.8 min 1.024 214Bi
214Bi RaC Radium C β- 99.98 %
α 0.02 %
19.9 min 3.272
214Po RaC' Radium C' α 0.1643 ms 7.883 210Pb
210Tl RaC" Radium C" β- 1.30 min 5.484 210Pb
210Pb RaD Radium D β- 22.3 a 0.064 210Bi
210Bi RaE Radium E β- 99.99987%
α 0.00013%
5.013 d 1.426
210Po RaF Radium F α 138.376 d 5.407 206Pb
206Tl β- 4.199 min 1.533 206Pb
206Pb - stable - -

Actinium series

The 4n+3 chain of U-235 is commonly called the "actinium series".

This images gives the detailled actinium decay scheme.
nuclide historic name (short) historic name (long) decay mode half life energy released, MeV product of decay
239Pu α 2.41·104 a 5.244 235U
235U AcU Actin Uranium α 7.04·108 a 4.678 231Th
231Th UY Uranium Y β- 25.52 h 0.391 231Pa
231Pa α 32760 a 5.150 227Ac
227Ac Ac Actinium β- 98.62%
α 1.38%
21.772 a 0.045
227Th RdAc Radioactinium α 18.68 d 6.147 223Ra
223Fr AcK Actinium K β- 22.00 min 1.149 223Ra
223Ra AcX Actinium X α 11.43 d 5.979 219Rn
219Rn An Actinon α 3.96 s 6.946 215Po
215Po AcA Actinium A α 99.99977%
β- 0.00023%
1.781 ms 7.527
215At α 0.1 ms 8.178 211Bi
211Pb AcB Actinium B β- 36.1 min 1.367 211Bi
211Bi AcC Actinium C α 99.724%
β- 0.276%
2.14 min 6.751
211Po AcC' Actinium C' α 516 ms 7.595 207Pb
207Tl AcC" Actinium C" β- 4.77 min 1.418 207Pb
207Pb . stable . .

Neptunium series

4n + 1 chain:

nuclide decay mode half life energy released, MeV product of decay
249Cf α 351 a 5.813+.388 245Cm
245Cm α 8500 a 5.362+.175 241Pu
241Pu β- 14.4 a 0.021 241Am
241Am α 432.7 a 5.638 237Np
237Np α 2.14·106 a 4.959 233Pa
233Pa β- 27.0 d 0.571 233U
233U α 1.592·105 a 4.909 229Th
229Th α 7.54·104 a 5.168 225Ra
225Ra β- 14.9 d 0.36 225Ac
225Ac α 10.0 d 5.935 221Fr
221Fr α 4.8 min 6.3 217At
217At α 32 ms 7.0 213Bi
213Bi α 46.5 min 5.87 209Tl
209Tl β- 2.2 min 3.99 209Pb
209Pb β- 3.25 h 0.644 209Bi
209Bi α 1.9·1019 a 3.14 205Tl
205Tl . stable . .

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 decays 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:

Nuclide Halflife
99Y 1.470(7) s
99Zr 2.1(1) s
99Nb 15.0(2) s
99Mo 2.7489(6) d
99Tc 2.111(12)E+5 a
99Ru Stable
Nuclide Halflife
135I 6.57(2) h
135Xe 9.14(2) h
135Cs 2.3(3)E+6 a
135Ba Stable


C.M. Lederer, J.M. Hollander, I. Perlman, Table of Isotopes, 6th ed., Wiley & Sons, New York 1968

  1. 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

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