Principle
Using U-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit two or three free neutrons (2.43 on average), called prompt neutrons. A subsequent fission fragment occasionally undergoes a stage of radioactive decay that yields an additional neutron, called a delayed neutron. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
Delayed neutrons are associated with the beta decay of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission occurs.
Delayed Neutron Data for Thermal Fission in U-235
| Group | Half-Life (s) | Decay Constant (s−1) | Energy (keV) | Yield, Neutrons per Fission | Fraction |
|---|---|---|---|---|---|
| 1 | 55.72 | 0.0124 | 250 | 0.00052 | 0.000215 |
| 2 | 22.72 | 0.0305 | 560 | 0.00546 | 0.001424 |
| 3 | 6.22 | 0.111 | 405 | 0.00310 | 0.001274 |
| 4 | 2.30 | 0.301 | 450 | 0.00624 | 0.002568 |
| 5 | 0.614 | 1.14 | - | 0.00182 | 0.000748 |
| 6 | 0.230 | 3.01 | - | 0.00066 | 0.000273 |
Read more about this topic: Prompt Neutron
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