The nuclei of atoms of fissionable isotopes fission if they are struck by a neutron. One product of such fissions is one or more neutrons, which, potentially, can cause other nuclei to fission.
If every fission event produced one neutron, and every new neutron caused another nucleus to fission, the reaction would go on at the same rate until all available fissionable nuclei had fissioned. A self-sustaining chain reaction, such as this hypothetical one, in which the number of neutrons in flight neither increases nor decreases, is said to have achieved “criticality.”
In the real world, however, many neutrons escape or are absorbed by nonfissionable isotopes, and do not cause another nucleus to fission. One way of producing a self-sustaining chain reaction is to use a larger chunk of fissionable material. The volume increases faster than the surface area, reducing the proportion of neutrons that escape. The longer the path a new neutron has within the material, the greater the chance that it will hit a fissionable nucleus. The critical mass of an isotope is that mass of the material (usually considered as shaped as a sphere) in which the number of neutrons in flight remains constant.
Some fissionable isotopes, such as uranium-238, do not possess the critical mass property. No matter how large a piece is assembled, it will not reach criticality.
Assembling a critical mass often does not result in an explosion like that of an atomic weapon, because the material blows itself apart before the full-scale explosion can occur. However, workers have died from exposure to the flash of radiation produced in such events.
Some examples of critical mass:
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Last revised: 1 November 2002.