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What is critical mass, as defined for a fissionable material?
AThe maximum amount that can ever be assembled safely
BThe exact mass of a single nucleus of that nuclide
CThe mass at which all fission instantly stops
DThe minimum amount needed for self-sustained fission
Answer & Solution
Correct answer: D. The minimum amount needed for self-sustained fission
1. Not every neutron released by fission goes on to cause another fission, since some escape the material entirely.
2. A large enough sample is needed so that, on average, each fission triggers at least one more, and that minimum amount is the critical mass.
3. It is a minimum requirement, not a maximum safe limit or a stopping point for fission.
4. It also has nothing to do with the mass of a single nucleus.
5. So the minimum amount needed for self-sustained fission is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.4 Nuclear Fission and Fusion_
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