Decay chain

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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-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 a leading cause of cancer in humans.

Contents

[edit] Types

 This diagram illustrates the four decay chains: thorium (in blue), radium (in red), actinium (in green), and neptunium (in purple).
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 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.

[edit] 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
Th 232 α 1.405·1010 a 4.081 Ra 228
Ra 228 β- 5.75 a 0.046 Ac 228
Ac 228 β- 6.25 h 2.124 Th 228
Th 228 α 1.9116 a 5.520 Ra 224
Ra 224 α 3.6319 d 5.789 Rn 220
Rn 220 α 55.6 s 6.404 Po 216
Po 216 α 0.145 s 6.906 Pb 212
Pb 212 β- 10.64 h 0.570 Bi 212
Bi 212 β- 64.06%
α 35.94%
60.55 min 2.252
6.208
Po 212
Tl 208
Po 212 α 299 ns 8.955 Pb 208
Tl 208 β- 3.053 min 4.999 Pb 208
Pb 208 . stable . .

[edit] Radium series

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

nuclide decay mode half life MeV product of decay
U 238 α 4.468·109 a 4.270 Th 234
Th 234 β- 24.10 d 0.273 Pa 234
Pa 234 β- 6.70 h 2.197 U 234
U 234 α 245500 a 4.859 Th 230
Th 230 α 75380 a 4.770 Ra 226
Ra 226 α 1602 a 4.871 Rn 222
Rn 222 α 3.8235 d 5.590 Po 218
Po 218 α 99.98 %
β- 0.02 %
3.10 min 6.115
0.265
Pb 214
At 218
At 218 α 99.90 %
β- 0.10 %
1.5 s 6.874
2.883
Bi 214
Rn 218
Rn 218 α 35 ms 7.263 Po 214
Pb 214 β- 26.8 min 1.024 Bi 214
Bi 214 β- 99.98 %
α 0.02 %
19.9 min 3.272
5.617
Po 214
Tl 210
Po 214 α 0.1643 ms 7.883 Pb 210
Tl 210 β- 1.30 min 5.484 Pb 210
Pb 210 β- 22.3 a 0.064 Bi 210
Bi 210 β- 99.99987%
α 0.00013%
5.013 d 1.426
5.982
Po 210
Tl 206
Po 210 α 138.376 d 5.407 Pb 206
Tl 206 β- 4.199 min 1.533 Pb 206
Pb 206 - stable - -

[edit] 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
Pu 239 α 2.41·104 a 5.244 U 235
U 235 α 7.04·108 a 4.678 Th 231
Th 231 β- 25.52 h 0.391 Pa 231
Pa 231 α 32760 a 5.150 Ac 227
Ac 227 β- 98.62%
α 1.38%
21.772 a 0.045
5.042
Th 227
Fr 223
Th 227 α 18.68 d 6.147 Ra 223
Fr 223 β- 22.00 min 1.149 Ra 223
Ra 223 α 11.43 d 5.979 Rn 219
Rn 219 α 3.96 s 6.946 Po 215
Po 215 α 99.99977%
β- 0.00023%
1.781 ms 7.527
0.715
Pb 211
At 215
At 215 α 0.1 ms 8.178 Bi 211
Pb 211 β- 36.1 m 1.367 Bi 211
Bi 211 α 99.724%
β- 0.276%
2.14 min 6.751
0.575
Tl 207
Po 211
Po 211 α 516 ms 7.595 Pb 207
Tl 207 β- 4.77 min 1.418 Pb 207
Pb 207 . stable . .

[edit] Neptunium series

4n + 1 chain:

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

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