Spontaneous fission

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Nuclear processes
Radioactive decay processes
Alpha decay
Beta decay
Cluster decay
Double beta decay
Double electron capture
Electron capture
Gamma radiation
Internal conversion
Isomeric transition
Neutron emission
Positron emission
Proton emission
Spontaneous fission
Nucleosynthesis
Neutron Capture
* The R-process
* The S-process
Proton capture:
* The P-process
* The Rp-process
Spallation

Nuclear processes
Radioactive decay processes Alpha decay Beta decay Cluster decay Double beta decay Double electron capture Electron capture Gamma radiation Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission Nucleosynthesis Neutron Capture * The R-process * The S-process Proton capture: * The P-process * The Rp-process Spallation

Spontaneous fission (SF) is a form of radioactive decay characteristic of very heavy isotopes, and is theoretically possible for any atomic nucleus whose mass is greater than or equal to 100 amu (elements near ruthenium). In practice, however, spontaneous fission is only energetically feasible for atomic masses above 230 amu (elements near thorium). The elements most susceptible to spontaneous fission are the trans-actinide elements, such as rutherfordium.

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. Mathematically, the criterion for whether spontaneous fission can occur is:

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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. However, spontaneous fissions release neutrons as all fissions do, so 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 containing minerals as the fission fragments recoil through the crystal structure. These trails, or fission tracks provide the basis for the radiometric dating technique: fission track dating.

Spontaneous fission rates

Spontaneous fission rates:

U-235: 0.2 - 0.3 fissions/s-kg
U-238: 7 fissions/s-kg
Pu-239: 10 fission/s-kg
Pu-240: 415,000 fission/s-kg (ca. 1,000,000 neutrons/s-kg)

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. Pu-240's high rate of spontaneous fission events makes it an undesirable contaminant. Weapons-grade plutonium must contain no more than 7% Pu-240.

The gun-type fission weapon has a critical insertion time of ca. 1ms, and the probability of a fission during this time interval should be small. Therefore only U-235 is suitable.

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