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Decay chain

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. For example, natural uranium is not significantly radioactive, but samples of pitchblende, a uranium ore, are 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[1].



  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 starting isotopes of these three have existed since the formation of the earth. 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.

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.

Thorium series

The 4n chain of Th-232 is commonly called the "thorium series". In this chart, the letter 'a' represents a Julian year (365.25 days).

nuclide 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 8×108 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 α 1.405·1010 a 4.081 228Ra
228Ra β- 5.75 a 0.046 228Ac
228Ac β- 6.25 h 2.124 228Th
228Th α 1.9116 a 5.520 224Ra
224Ra α 3.6319 d 5.789 220Rn
220Rn α 55.6 s 6.404 216Po
216Po α 0.145 s 6.906 212Pb
212Pb β- 10.64 h 0.570 212Bi
212Bi β- 64.06%
α 35.94%
60.55 min 2.252
212Po α 299 ns 8.955 208Pb
208Tl β- 3.053 min 4.999 208Pb
208Pb . stable . .

Radium series

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

nuclide decay mode half life MeV product of decay
238U α 4.468·109 a 4.270 234Th
234Th β- 24.10 d 0.273 234Pa
234Pa β- 6.70 h 2.197 234U
234U α 245500 a 4.859 230Th
230Th α 75380 a 4.770 226Ra
226Ra α 1602 a 4.871 222Rn
222Rn α 3.8235 d 5.590 218Po
218Po 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 β- 26.8 min 1.024 214Bi
214Bi β- 99.98 %
α 0.02 %
19.9 min 3.272
214Po α 0.1643 ms 7.883 210Pb
210Tl β- 1.30 min 5.484 210Pb
210Pb β- 22.3 a 0.064 210Bi
210Bi β- 99.99987%
α 0.00013%
5.013 d 1.426
210Po 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".

nuclide decay mode half life energy released, MeV product of decay
239Pu α 2.41·104 a 5.244 235U
235U α 7.04·108 a 4.678 231Th
231Th β- 25.52 h 0.391 231Pa
231Pa α 32760 a 5.150 227Ac
227Ac β- 98.62%
α 1.38%
21.772 a 0.045
227Th α 18.68 d 6.147 223Ra
223Fr β- 22.00 min 1.149 223Ra
223Ra α 11.43 d 5.979 219Rn
219Rn α 3.96 s 6.946 215Po
215Po α 99.99977%
β- 0.00023%
1.781 ms 7.527
215At α 0.1 ms 8.178 211Bi
211Pb β- 36.1 m 1.367 211Bi
211Bi α 99.724%
β- 0.276%
2.14 min 6.751
211Po α 516 ms 7.595 207Pb
207Tl 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 m 6.3 217At
217At α 32 ms 7.0 213Bi
213Bi α 46.5 m 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 . .
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Decay_chain". A list of authors is available in Wikipedia.
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