Spontaneous Fission

Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. Because the nuclear binding energy of the elements reaches its maximum at an atomic mass number greater than about 58 atomic mass units (u), spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at heavier masses.

Because of the 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 (a.m.u.), with the probability of spontaneous fission increasing as the atomic mass number increases above this value.

The lightest natural nuclides that are hypothetically subject to spontaneous fission are niobium-93 and molybdenum-94 (elements #41 and #42, respectively). Spontaneous fission has never been observed in the naturally-occurring isotopes of these elements, however. In practice, these are stable isotopes.

Spontaneous fission is feasible over practical observation times only for atomic masses of 232 a.m.u. or more. These are elements at least as heavy as thorium-232 – which has a half-life somewhat longer than the age of the Universe. Thorium-232 is the lightest primordial nuclide that has left evidence of undergoing spontaneous fission in its minerals.

The known elements most susceptible to spontaneous fission are the synthetic high-atomic-number actinide elements with odd atomic numbers: mendelevium and lawrencium, and also some of the transactinide very-heavy elements, such as rutherfordium.

For naturally occurring thorium, uranium-235, and uranium-238, spontaneous fission does occur rarely, but in the vast majority of the radioactive decay of these atoms, alpha decay or beta decay occurs instead. Hence, the spontaneous fission of these isotopes is usually negligible, except in using the exact branching ratios when finding the radioactivity of a sample of 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 not negligible can be used as neutron sources. For example, californium-252 (half-life 2.645 years, SF branch ratio about 3.1 percent) can be used for this purpose. The neutrons released can be used to inspect airline luggage for hidden explosives; to gauge the moisture content of soil in highway and building and construction; or to measure the moisture of materials stored in silos, for example.

As long as the spontaneous fission gives a negligible reduction of the number of nuclei that can undergo such fission, this process can be approximated closely as a Poisson process. In this situation, for short time intervals the probability of a spontaneous fission is directly proportional to the length of time.

The spontaneous fission of uranium-238 and uranium-235 does leave trails of damage in the crystal structure of uranium-containing minerals when the fission fragments recoil through them. These trails, or fission tracks, are the foundation of the radiometric dating method called fission track dating.