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A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design. == Explanation of criticality== When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a ''critical'' state in which there is no increase or decrease in power, temperature, or neutron population. A numerical measure of a critical mass is dependent on the effective neutron multiplication factor , the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. When , the mass is critical, and the chain reaction is barely self-sustaining. A ''subcritical'' mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, . A steady rate of spontaneous fissions causes a proportionally steady level of neutron activity. The constant of proportionality increases as increases. A ''supercritical'' mass is one where there is an increasing rate of fission. The material may settle into equilibrium (''i.e.'' become critical again) at an elevated temperature/power level or destroy itself, by which equilibrium is reached. In the case of supercriticality, . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「critical mass」の詳細全文を読む スポンサード リンク
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