Neutron bombardment can also be used to increase the rate of nuclear fission in nuclei that undergo this process spontaneously, such as uranium-235 (see Figure). Nuclei that are susceptible to this “induced nuclear fission” include plutonium-239 and uranium-233. For example, a slow-moving or “thermal” neutron can be absorbed into the nucleus of an atom, rendering it unstable and causing it to fission instantly. In addition to nuclei that fission spontaneously, many other atomic nuclei can be induced to undergo nuclear fission by bombarding them with sub-atomic particles. The periodic table of the elements (from Heavy elements such as the actinoids are prone to nuclear fission. Despite being a spontaneous process, fission of these nuclei occurs very slowly in nature: the fission half-life of uranium-235 is 1.2 x 1017 years, which corresponds to one fission per gram per hour. Examples of materials in nature that spontaneously undergo nuclear fission are uranium-235 (an atomic nucleus containing 92 protons and 143 neutrons) and thorium-232 (90 protons and 142 neutrons). Nuclear fission only occurs in the heaviest elements of the periodic table: principally, the actinoid series, but more generally, those with atomic weights greater than 232 atomic mass units. This is often accompanied by the emission of neutrons, charged fragments such as alpha particles, and energy in the form of gamma or X-rays.
Nuclear fission is a naturally occurring phenomenon during which an atomic nucleus is divided into two or more parts.
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