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Neutron temperatureThe 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, nonthermal neutron energy distributions or ranges are listed in the table below:
Additional recommended knowledgeA 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 lowerenergy thermal neutrons, and highenergy 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 deuteriumtritium 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 uranium238 and other nonfissile 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. 4.0e21 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 (statistics) of the Maxwell–Boltzmann distribution for this temperature. The most probable energy is different from the mean (statistics) 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 crosssection 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 is not primarily to increase the fission cross section for fissile nuclei such as uranium235 or plutonium239; it is because uranium238 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 ^{238}U. Fast reactors use fast neutrons without moderation, but require more highly enriched fuel. See also
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Neutron_temperature". A list of authors is available in Wikipedia. 