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Spontaneous fission

Nuclear processes
Radioactive decay processes

Spontaneous Fission is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for Uranium and Thorium, or indeed for any atomic nucleus whose mass is greater than or equal to 100 amu (Ruthenium). In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 amu (Thorium).

For Uranium and Thorium, the spontaneous fission mode of decay 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. Mathematically, Z2/A ≈ 45 is the criterion for whether spontaneous fission can occur.

As the name suggests, spontaneous fission follows the exact same process as nuclear fission, only it is not self-sustaining and does not generate the neutron flux necessary to "go critical" and continue such fissions.

The elements most susceptible to spontaneous fission are the trans-actinide elements, such as Rutherfordium.