Neutron temperature 3770784 222745103 2008-06-30T22:09:46Z JWB 273993 /* Fast reactor and thermal reactor compared */ rewrite - previous had some wrong points [[Image:MaxwellBoltzmann.gif|right|thumb|350px|A chart displaying the speed probability density functions of the speeds of a few [[noble gas]]es at a temperature of 298.15 K (25 C). An explanation of the y-axis label appears on the image page (click to see). Similar speed distributions are obtained for [[neutron_radiation|neutrons]] upon [[Neutron_moderator|moderation]].]] The '''neutron temperature''', also called the '''neutron energy''', indicates a [[free neutron|free neutron's]] [[kinetic energy]], usually given in [[electron volt]]s. The term ''temperature'' is used, since hot, thermal and cold neutrons are [[Neutron_moderator|moderated]] in a medium with a certain temperature. The neutron energy distribution is then adopted to the [[Maxwell–Boltzmann distribution|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_hypothesis|De Broglie relation]]. ==Neutron energy distribution ranges== 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. *'''Slow neutrons''' have an energy less than 1 eV. *'''Epithermal neutrons''' have an energy from 0.025 to 1 eV. *'''Hot neutrons''' have an energy of about .2 eV. *'''Thermal neutrons''' have an energy of about 0.025 eV. *'''Cold neutrons''' have an energy from 5x10<sup>-5</sup> eV to 0.025 eV. *'''Very cold neutrons''' have an energy from 3x10<sup>-7</sup> eV to 5x10<sup>-5</sup> eV. *'''[[Ultracold Neutrons|Ultra cold neutrons]]''' have an energy less than 3x10<sup>-7</sup> eV. *'''Continuum region neutrons''' have an energy from 0.01 MeV to 25 MeV. *'''Resonance region neutrons''' have an energy from 1 eV to 0.01 MeV. *'''Low energy region neutrons''' have an energy less than 1 eV. ==Fast neutrons== A '''fast neutron''' is a free neutron with a kinetic energy level close to 1 [[Mega|M]][[electronvolt|eV]] (100 [[Tera|T]][[Joule|J]]/[[Kilogram|kg]]), hence a speed of 14,000 [[Kilometre|km]]/[[second|s]]. They are named ''fast'' [[neutron]]s 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 [[nuclear fusion#Criteria and candidates for terrestrial reactions|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. ==Thermal neutrons== A '''thermal neutron''' is a free neutron with a kinetic energy of about 0.025 [[electron volt|eV]] (approx. 4.0×10<sup>-21</sup> [[Joule|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.<br> 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, [[neutron]]s arrive at about this energy level, provided that they are not absorbed. Thermal neutrons have a different and often much larger effective [[neutron absorption]] [[Neutron_cross-section|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|unstable]] - [[isotope]] of the [[chemical element]] as a result. ([[neutron activation]]) ==Fast reactor and thermal reactor compared== Most [[nuclear reactor|fission reactor]]s are [[thermal reactor]]s 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 <sup>238</sup>U. 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 reactor]]s 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. == See also == * [[List of particles]] * [[Neutron source]] * [[Nuclear reaction]] * [[Thermal reactor]] * [[Fast neutron reactor]] ==References== *[http://www.nuclearglossary.com/suites/nuclearglossary_neutrons.html Language of the Nucleus] [[Category:Neutron]] [[he:נייטרון איטי]] [[ja:高速中性子]] [[pl:Neutrony prędkie]] [[ru:Спектр нейтронов]]