fast neutron

noun

1. physics
   1. a neutron produced by nuclear fission that has lost little energy by collision; a neutron with a kinetic energy in excess of 0.1 MeV
   2. a neutron with a kinetic energy in excess of 1.5 MeV, the fission threshold of uranium-238

1. A neutron that is not in thermal equilibrium with the surrounding medium, especially one produced by fission. Compare slow neutron. See also fast-breeder reactor.