Text: Consider an arrangement in which neutrons are being produced at energies much greater than thermal - say, by a source of fission neutrons. Also assume that the source of fission neutrons is surrounded by a moderator which slows the neutrons down until eventually they have energies in thermal equilibrium with the molecules of the moderator. At any location where complete thermal equilibrium has not been established, the distribution of neutron velocities will contain velocities which exceed any permitted by a Maxwell distribution for the temperature of the moderator. Such a distribution is called epithermal and the neutrons in it are called epithermal neutrons.

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