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

fast neutron

1. A neutron that is not in thermal equilibrium with the surrounding medium, especially one produced by fission.

2. Compare slow neutron See also fast-breeder reactor