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*[http://en.wikipedia.org/wiki/1919 1919]
==Definition==
*1: a size, [[number]], or amount large enough to produce a particular result
==Description==
A '''critical mass''' is the smallest amount of [http://en.wikipedia.org/wiki/Fissile fissile] material needed for a sustained [http://en.wikipedia.org/wiki/Nuclear_chain_reaction nuclear chain reaction]. The critical mass of a fissionable material depends upon its [[nuclear]] properties (e.g. the nuclear fission cross-section), its [[density]], its shape, its enrichment, its [[purity]], its [[temperature]] and its [[surroundings]].
When a [http://en.wikipedia.org/wiki/Nuclear_chain_reaction 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 [http://en.wikipedia.org/wiki/Neutron neutron] population.
A numerical measure of a critical mass is dependent on the [http://en.wikipedia.org/wiki/Nuclear_chain_reaction#Effective_neutron_multiplication_factor effective neutron multiplication factor k], 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 k = 1, 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, k < 1. A steady rate of [[spontaneous]] fissions causes a [[proportionally]] steady level of neutron activity. The constant of proportionality increases as k increases.
A '''''supercritical''''' mass is one where there is an increasing [[rate]] of [http://en.wikipedia.org/wiki/Nuclear_fission 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'', k > 1.[http://en.wikipedia.org/wiki/Critical_mass]
[[Category: Physics]]
*[http://en.wikipedia.org/wiki/1919 1919]
==Definition==
*1: a size, [[number]], or amount large enough to produce a particular result
==Description==
A '''critical mass''' is the smallest amount of [http://en.wikipedia.org/wiki/Fissile fissile] material needed for a sustained [http://en.wikipedia.org/wiki/Nuclear_chain_reaction nuclear chain reaction]. The critical mass of a fissionable material depends upon its [[nuclear]] properties (e.g. the nuclear fission cross-section), its [[density]], its shape, its enrichment, its [[purity]], its [[temperature]] and its [[surroundings]].
When a [http://en.wikipedia.org/wiki/Nuclear_chain_reaction 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 [http://en.wikipedia.org/wiki/Neutron neutron] population.
A numerical measure of a critical mass is dependent on the [http://en.wikipedia.org/wiki/Nuclear_chain_reaction#Effective_neutron_multiplication_factor effective neutron multiplication factor k], 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 k = 1, 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, k < 1. A steady rate of [[spontaneous]] fissions causes a [[proportionally]] steady level of neutron activity. The constant of proportionality increases as k increases.
A '''''supercritical''''' mass is one where there is an increasing [[rate]] of [http://en.wikipedia.org/wiki/Nuclear_fission 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'', k > 1.[http://en.wikipedia.org/wiki/Critical_mass]
[[Category: Physics]]
