Many of the nuclides in the actinide family—U, Np, Pu, etc.—fission spontaneously as one of the modes of radioactive decay. Usually, for a nuclide with multiple modes of radioactive decay, the half-life of the nuclide is determined from the total decay rate, representing all the decay processes for that nuclide. However, in the case of spontaneous fission, a separate half-life for that process alone is used. Examples of nuclides that undergo spontaneous fission are given in Table 2.5.

The neutrons from spontaneous fission are emitted with average energies of a few million electron volts. Because the neutron carries no electrical charge, these fission neutrons penetrate quite readily through solids and liquids. They are stopped or slowed down only when they

collide with nuclei of the material through which they are traveling. A neutron loses the greatest amount of energy per collision when it collides with a hydrogen nucleus, whose mass is almost identical with the neutron mass. Consequently, hydrogenous materials are used to degrade, or “moderate,” energies of fission neutrons to energies in the few electron volt or kiloelectron volt range, where they are more easily absorbed by nuclear reactions. When energetic neutrons pass through animal tissue, the protons (hydrogen nuclei) recoiling from neutron collisions cause ionization within the tissue and can result in biological damage. Radionuclides with appreciable spontaneous fission, e. g., 252Cf, must be shielded with mixtures of hydrogenous materials and neutron absorbers (e. g., boron) to protect against external hazards.