The '''neutron temperature''', also called the '''neutron energy''', indicates a free neutron's kinetic energy, usually given in
electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher is the kinetic energy of the free neutron. Kinetic energy, speed and
wavelength of the neutron are related through the De Broglie relation. Moderated and other, non-thermal neutron energy distributions or ranges are listed in the table below:
*'''Fast neutrons''' have an energy greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
A '''fast neutron''' is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg), hence a speed of 14,000 km/s. They are named ''fast''
neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as
nuclear fission.
Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a
neutron moderator
. In reactors, typically
heavy water,
light water, or
graphite are used to moderate neutrons.
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