Fast neutron
A fast neutron is a free neutron with a kinetic energy level of more than 1MeV. They are named 'fast neutrons' to distinguish them from lower-energy 'thermal neutrons,' which have a kinetic energy level similar to the average kinetic energy of a room-temperature gas. Fast neutrons are produced by nuclear processes such as nuclear fission.
Fast neutrons have a much smaller effective capture cross-section than thermal neutrons, and are therefore less likely to be absorbed by atomic nuclei that they collide with. If efficient neutron capture is desired, fast neutrons must be slowed down by using a neutron moderator, a substance which will slow them down to thermal neutron velocities. Typically heavy water, light water, or graphite can be used to moderate neutrons.
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