Spontaneous Fission Rates
Spontaneous fission rates:
| Nuclide | Half-life | Fission prob. per decay | Neutrons per fission | Neutrons per gram-second |
|---|---|---|---|---|
| 235U | 7.04x108 years | 7.0x10−11 | 1.86 | 1.0x10−5 |
| 238U | 4.47x109 years | 5.4x10−7 | 2.07 | 0.0136 |
| 239Pu | 2.41x104 years | 4.4x10−12 | 2.16 | 2.2x10−2 |
| 240Pu | 6569 years | 5.0x10−8 | 2.21 | 920 |
| 250Cm | 8300 years | 0.80 | 3.3 | 2x1010 |
| 252Cf | 2.638 years | 3.09x10−2 | 3.73 | 2.3x1012 |
In practice 239Pu will invariably contain a certain amount of 240Pu due to the tendency of 239Pu to absorb an additional neutron during production. 240Pu's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium contains no more than 7.0% 240Pu.
The rarely-used gun-type atomic bomb has a critical insertion time of about one millisecond, and the probability of a fission during this time interval should be small. Therefore only 235U is suitable. Almost all nuclear bombs use some kind of implosion method.
Spontaneous fission can occur much more rapidly when the nucleus of an atom undergoes superdeformation.
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