Neutron moderation
= Neutron moderation
<Neutron moderation> reduces the <kinetic energy> of fast <neutrons> through repeated collisions in a <neutron moderator>. In a simple <elastic collision> model, <hydrogen> is especially effective because a <proton> and a <neutron> have nearly equal <masses>: the <maximum elastic recoil energy fraction> can approach one. The <mean logarithmic energy decrement> describes typical accumulated slowing, whereas the maximum transfer describes a single endpoint.
Slowing, absorption and escape compete. A material can remove <neutrons> by <neutron capture> before they become <thermal neutrons>, and some <neutrons> escape its boundaries. Describing a <neutron moderator> therefore requires more than collision kinematics. Near <thermal equilibrium>, target thermal motion can also give <energy> back to a <neutron>; the initially stationary target approximation must then be replaced.