Weapons-grade plutonium contains no more than 7% Pu-240. Pu-240's high rate of spontaneous fission events makes it an undesirable contaminant. In practice Pu-239 will invariably contain a certain amount of Pu-240 due to the tendency of Pu-239 to absorb an additional neutron during production. These trails, or fission tracks provide the basis for the radiometric dating technique: fission track dating. The spontaneous fission of uranium-238 leaves trails of damage in uranium containing minerals as the fission fragments recoil through the crystal structure. 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.Īs 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. 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. Spontaneous fissions release neutrons as all fissions do, so if a critical mass is present, a spontaneous fission can initiate a chain reaction. Where Z is the atomic number and A is the mass number (e.g., 235 for U-235).Īs the name suggests, spontaneous fission follows exactly the same process as nuclear fission, except that it occurs without the atom having been struck by a neutron or other particle. Mathematically, the criterion for whether spontaneous fission can occur is approximately: For uranium and thorium, the spontaneous fission mode of decay does occur, but is not seen for the majority of radioactive breakdowns and is usually neglected except for the exact considerations of branching ratios when determining the activity of a sample containing these elements.

Spontaneous fission rate u238