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In radioactive decays, one considers the alpha, beta, gamma and electronic capture scenarios. Why is the spontaneous nuclear fission not considered as a kind of radioactive decay ?

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    $\begingroup$ I don’t know what source you’re relying on, but I consider it a decay. That said, heavy fission fragment range out so quickly that they don’t show up in most detectors so they weren’t assigned a Greek-letter designation. $\endgroup$ Commented Nov 10, 2019 at 20:50
  • $\begingroup$ @dmckee Thank you. This makes sense. I Don't get what you mean by "quick". Do you mean the speed of the flight of the fission fragments ? Why woul it be more difficult to detect fragment that go fast in a detector ? I believe that this is what particle physics detector do very easily ? $\endgroup$ Commented Nov 10, 2019 at 20:55
  • $\begingroup$ "range" means how far they travel in a medium of the first detectors at the time they discovered the radiation. The α β γ designations came with primitive detectors, not the sophisticated of particle physics.. (The more charge the fragment has the faster it is stopped in a medium )orau.org/ptp/articlesstories/names.htm $\endgroup$
    – anna v
    Commented Nov 11, 2019 at 8:23
  • $\begingroup$ thank you @anna v $\endgroup$ Commented Nov 11, 2019 at 20:34

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At least in the field of radiation protection, spontaneous fission actually is considered as a decay mode. For example in ICRP, 2008. Nuclear Decay Data for Dosimetric Calculations. ICRP Publication 107. Ann. ICRP 38 (3).:

Spontaneous fission

Spontaneous fission is a decay mechanism resulting in splitting of the nucleus into lighter nuclei, referred to as ‘fission fragments’, with the emission of neutrons.

In the ICRP-07 collection, 28 radionuclies decay by spontaneous fission.

The decay modes are abbreviated as: A – alpha, B− – beta minus, B+ – beta plus, EC – electron capture, IT – isomeric transition, SF-spontaneous fission. The contribution of fission fragments and neutrons associated with spontaneous fission are included in the total energy.

Note that spontaneous fission has a half-life and a branching fraction like any other radioactive decay. For example, Cf-252 has a half life of 2.645 years and branching fractions of 96.908 % for alpha decay and 3.092 % for spontaneous fission decay. Excluding the contribution of spontaneous fission decay and only looking at the alpha decay would result in a wrong total half-life for Cf-252.

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