Spontanspaltung

Spontanspaltung, or spontaneous fission, is a mode of radioactive decay in which a heavy atomic nucleus splits into two lighter nuclei without any external particle triggering the reaction. This contrasts with induced fission, where a nucleus splits after absorbing a neutron or other particle. Spontaneous fission occurs only in certain very heavy nuclei and is exceedingly rare for most isotopes, but it becomes more probable as the atomic number and mass increase.

The process results from the internal structure of the nucleus and the balance of forces inside it. The nucleus must pass through a fission barrier in its potential-energy surface, either by quantum tunneling or via large-scale shape fluctuations that lead to a necked, elongated configuration and eventual scission. The rate depends on nuclear structure, deformation, and shell effects, and it is largely independent of external environmental factors at ordinary temperatures.

Typical features include a release of a large amount of energy—on the order of 170 MeV per fission event—and the emission of prompt neutrons and gamma rays. The fission fragments are usually two mid-weight nuclei with a wide distribution of masses, spanning roughly from about mass 90 to 140, and the exact fragment pattern is strongly isotope-dependent. The neutron yield per fission varies by isotope, with some heavy actinides producing multiple neutrons per event.

Notable isotopes involved are certain actinides such as uranium and californium isotopes. Californium-252, for example, undergoes spontaneous fission with a relatively short half-life and is widely used as a neutron source due to its high fission rate and neutron emission. Spontaneous fission has historical significance for the discovery of the phenomenon of fission itself and remains important in nuclear physics and reactor and source applications.