3. Evolution of heavy nuclei

3.1 General information on the effects of evolution

Neutron irradiation will, on the one hand, cause fission (hence the accumulation of fission products we have just described) and, on the other hand, transmute heavy nuclei by neutron capture (sometimes followed by radioactive decay): it is this latter aspect that is dealt with now.

These heavy nuclei are all more or less fissile: as a general rule, those with an odd number of neutrons are fissile by all neutrons, and preferentially thermal neutrons, whereas those with an even number of neutrons can, in practice, only be fissioned by fast neutrons.

Capture (in the absence of fission) can transform a nucleus of the second type into a nucleus of the first type, and vice versa. In general,...