Neutron slowing
Neutronics basics - Neutron migration

These are the scatterings that slow down the neutrons. A distinction is made between elastic and inelastic scattering.

A process in which kinetic energy is conserved is termed "elastic". In the case of an elastic shock, a neutron is nevertheless slowed down (except possibly in the thermal domain) because the target nucleus, which is practically immobile initially, takes a certain amount of recoil during the shock; we'll see that elastic slowing down is particularly effective if collisions take place on light nuclei.

A process in which kinetic energy is not conserved is called "inelastic". For sufficiently energetic incident neutrons, the residual nucleus, after re-emission of a neutron, may remain...