Formation and Decay
The sum of the atomic weight of the two atoms produced by the fission of one fissile atom is always less than the atomic weight of the original atom. This is because some of the mass is lost as free neutrons, and once kinetic energy of the fission products has been removed (i.e., the products have been cooled to extract the heat provided by the reaction), then the mass associated with this energy is lost to the system also, and thus appears to be "missing" from the cooled fission products.
Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are almost always more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable ruthenium-100 is 56% neutrons; stable xenon-134 is 60%). The initial fission products therefore may be unstable and typically undergo beta decay towards stable nuclei, converting a neutron to a proton with each beta emission. (Fission products do not emit alpha particles.)
A few neutron-rich and short-lived initial fission products first decay by emitting a neutron. This is the source of delayed neutrons which play an important role in control of a nuclear reactor.
The first beta decays are rapid and may release high energy beta particles or gamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a long half-life and release less energy. There are a few exceptions with relatively long half-lives and high decay energy, such as:
- Strontium-90 (high energy beta, half-life 30 years)
- Caesium-137 (high energy gamma, half-life 30 years)
- Tin-126 (even higher energy gamma, but long half-life of 230,000 years means a slow rate of radiation release, and the yield of this nuclide per fission is very low)
Read more about this topic: Nuclear Fission Product
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