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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.
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 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.
<center>For lessons on the [[topic]] of '''''Critical Mass''''', follow [http://nordan.daynal.org/wiki/index.php?title=Category:Critical_Mass '''''this link'''''].</center>
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 '''''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.
