fast neutron

A fast neutron is a neutron resulting from nuclear fission that has lost little of its energy by collision and therefore travels at high speed. It is usual to describe neutrons with energies in excess of 0.1 MeV (see electron volt) as "fast". However, fission induced by fast neutrons is often described as "fast fission" and in this context the neutrons are so described if they have energies in excess of the fission threshold of uranium-238, i.e. above 1.5 MeV.