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From Citizendium, the Citizens' Compendium
Nuclear fission takes place when the nucleus of a "heavy" (i.e., high atomic number) element splits into two or more nuclei of lighter (lower atomic number) elements, with the release of one or more neutrons and substantial energy. Not all heavy element nuclei will split under neutron bombardment; it is specific isotopes of heavy elements that are fissionable. The original heavy element nucleus can be referred to as the parent nucleus. The lighter element nuclei resulting from fission can be referred to as the daughter nuclei, and these resulting atoms or material are called fission products. The parent nuclei are of such high atomic number that they are radioactive, although their decay, typically alpha particle emission, may have very long half-lives. The daughter nuclei are almost always highly radioactive, commonly undergoing beta-minus decay with much shorter half-lives. Almost all nuclear fissions are initiated by collision of a neutron with the parent nucleus. A very few fissions occur without an initial neutron collision and are called spontaneous fissions. A repeated cycle of neutrons causing fissions, which in turn release more neutrons causing more fissions and so on, is called a nuclear fission chain reaction.