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Spontaneous fission
Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 58; spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at greater atomic mass numbers.
Because of constraints in forming the daughter fission-product nuclei, spontaneous fission into known nuclides becomes ''theoretically'' possible (that is, energetically possible) for some atomic nuclei with atomic masses greater than 92 atomic mass units (amu), with the probability of spontaneous fission increasing as the atomic mass increases above this value.
== History ==
The first nuclear fission process discovered was the fission induced by neutrons. Because cosmic rays produce some neutrons, it was difficult to distinguish between induced and spontaneous events. Cosmic rays can be reliably shielded by a thick layer of rock or water. The spontaneous fission was identified in 1940 by Soviet physicists Georgy Flyorov and Konstantin Petrzhak〔(Igor Sutyagin: The role of nuclear weapons and its possible future missions )〕 by their observations of uranium in the Moscow Metro Dinamo station, underground.〔(K. Petrzhak: How the spontaneous fission was discovered ) (in Russian).〕
Cluster decay was shown to be a superasymmetric spontaneous fission process.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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