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.
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Moderated and other, non-thermal neutron energy distributions or ranges are listed in the table below:
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (100 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.
Neutrons from fusion reactions are usually considerably more energetic than 1 MeV; the extreme case is deuterium-tritium fusion which produces 14.1 MeV neutrons (1400 TJ/kg, moving at 52,000 km/s, 17.3% of the speed of light) that can easily fission uranium-238 and other non-fissile actinides.
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.
A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (approx. 4.0×10-21 J; 2.4 MJ/kg, hence a speed of 2.2 km/s) which is the most probable energy at a temperature of 290 K (17°C or 62°F), the mode of the Maxwell–Boltzmann distribution for this temperature.
The most probable energy is different from the mean energy, which as in any Maxwell–Boltzmann distribution is 50% greater than the mode. After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed.
Thermal neutrons have a different and often much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus, creating a heavier - and often unstable - isotope of the chemical element as a result. (neutron activation)
Most fission reactors are thermal reactors that use a neutron moderator to slow down, or thermalize the neutrons produced by nuclear fission. This does not increase the fission cross section for fissile nuclei such as uranium-235 or plutonium-239, but uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and continue the chain reaction, rather than be captured by 238U. This allows thermal reactors to use lower-enriched uranium, or even natural uranium with an efficient moderator like heavy water.
An increase in fuel temperature will raise U-238's neutron absorption by Doppler broadening, providing negative feedback to help control the reactor. Also, when the moderator is also a circulating coolant (light water or heavy water), boiling of the coolant will reduce the moderator density and provide negative feedback. (negative void coefficient)
Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels except possibly the uranium-233 of the thorium cycle.
Fast reactors use unmoderated fast neutrons to sustain the reaction and require the fuel to contain a higher concentration of fissile material. However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel than it consumes. But reactor control is more difficult because of decreased Doppler broadening and lack of negative void coefficient from a moderator. Once expected to be the wave of the future, fast breeder development has been nearly dormant with only a handful of reactors in the decades since the Chernobyl accident (and because of low prices in the uranium market) although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.