slow neutron

A neutron in thermal equilibrium with the surrounding medium, especially one produced by fission and slowed by a moderator. Also called thermal neutron.

Neutrons whose energy distribution is governed primarily by the kinetic energy distribution of molecules of the material in which the neutrons are found. See also Neutron.

The molecules of the material usually have a kinetic energy distribution very close to a Maxwell-Boltzmann distribution. This distribution shows a peak at an energy equal to half the product of the temperature and the Boltzmann constant. At high energies it decreases exponentially, and at low energies it is proportional to the square root of the energy. When the material is large and very weakly absorbing, the neutron energy distribution closely approaches this maxwellian. See also Boltzmann statistics; Kinetic theory of matter.

The most common way of generating thermal neutrons is to allow neutrons from a source—reactor, accelerator, or spontaneous fission neutron emitter—to diffuse outward through a large block or tank of very weakly absorbing moderator. See also Reactor physics.