Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units (u), spontaneus breakdown into smaller nuclei and single particles becomes possible at heavier masses. Because of constraints in constructing the daughter fission product nuclei, spontaneus fission into known nuclides becomes theoretically possible (that is, energetically possible) for many atomic nuclei (nuclide) with a mass greater than or equal to 93 atomic mass units (u), with the possibility increasing as mass number grows above this boundary. The lightest nuclides found naturally which are in theory subject to spontaneous fission, are niobium-93 and molybdenum-94 (elements 41 and 42 respectively). Spontaneus fission has never been observered in naturally-occuring isotopes of these ligher elements, however. Operationally they are stable isotopes.
In practice, spontaneous fission is feasible over practical observation times, only for atomic masses above 232 u. These are elements heavier than thorium-232, which has a half life approximately equal to the age of the universe, but is the lightest [[[primordial nuclide]] that has been observed to undergo spontaneous fission. The elements most susceptible to spontaneous fission are the artificially-produced high-atomic-number actinide elements, such as mendelevium and lawrencium, and the trans-actinide elements, such as rutherfordium.
For uranium and thorium, spontaneous fission mode of decay does occur, as noted, but it is not seen for the majority of radioactive decay which is by alpha decay, and so it is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements. Mathematically, the criterion for whether spontaneous fission can occur in a time short enough to be observed by present methods, is approximately:
where Z is the atomic number and A is the mass number (e.g., 235 for U-235).
As the name suggests, spontaneous fission gives much the same result as induced nuclear fission. However, like other forms of radioactive decay, it occurs due to quantum tunneling, without the atom having been struck by a neutron or other particle as in induced nuclear fission. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a self-sustaining chain reaction. Also, radioisotopes for which spontaneous fission is a nonnegligible decay mode may be used as neutron sources; californium-252 (half-life 2.645 years, SF branch ratio 3.09%) is often used for this purpose. The neutrons may then be used to inspect airline luggage for hidden explosives, to gauge the moisture content of soil in the road construction and building industries, to measure the moisture of materials stored in silos, and in other applications.
As long as the fissions give a negligible reduction of the amount of nuclei that can spontaneously fission, this is a Poisson process: for very short time intervals the probability of a spontaneous fission is proportional to the length of time.
The spontaneous fission of uranium-238 leaves trails of damage in uranium-bearing minerals as the fission fragments recoil through the crystal structure. These trails, or fission tracks, provide the basis for the radiometric dating technique known as fission track dating.
