Theneutron detection temperature, also called theneutron energy, indicates afree neutron'skinetic energy, usually given inelectron volts. The termtemperature is used, since hot, thermal and cold neutrons aremoderated in a medium with a certain temperature. The neutron energy distribution is then adapted to theMaxwell distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. Themomentum andwavelength of the neutron are related through thede Broglie relation. The long wavelength of slow neutrons allows for the large cross section.^[1]

The precise boundaries of neutron energy ranges are not well defined, and differ between sources,^[2] but some common names and limits are given in the following table.

The following is a detailed classification:

Athermal neutron is a free neutron with a kinetic energy of about 0.025eV (about 4.0×10⁻²¹J or 2.4 MJ/kg, hence a speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F), themode of theMaxwell–Boltzmann distribution for this temperature, E_peak =k T.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, thoseneutrons which are not absorbed reach about this energy level.

Thermal neutrons have a different and sometimes much larger effectiveneutron absorptioncross-section for a givennuclide than fast neutrons, and can therefore often be absorbed more easily by anatomic nucleus, creating a heavier, oftenunstableisotope of thechemical element as a result. This event is calledneutron activation.

Epithermal neutrons are those with energies above the thermal energy at room temperature (i.e. 0.025 eV). Depending on the context, this can encompass all energies up to fast neutrons (as in e.g.^[6][7]).

This includes neutrons produced by conversion of accelerated protons in a pitcher-catcher geometry^[8]

Cold neutrons are thermal neutrons that have been equilibrated in a very cold substance such as liquiddeuterium. Such acold source is placed in the moderator of a research reactor or spallation source. Cold neutrons are particularly valuable forneutron scattering experiments.^[9]

Ultracold neutrons are produced by inelastic scattering of cold neutrons in substances with a low neutron absorption cross section at a temperature of a few kelvins, such as soliddeuterium^[10] or superfluidhelium.^[11] An alternative production method is the mechanical deceleration of cold neutrons exploiting the Doppler shift.^[12][13]

Ultra-cold neutrons reflect at all angles of incidence. This is because their momentum is comparable to the optical potential of materials. This effect is used to store them in bottles and study their fundamental properties^[5][14] e.g. lifetime, neutron electrical-dipole moment etc... The main limitations of the use of slow neutrons is the low flux and the lack of efficient optical devices (in the case of CN and VCN). Efficient neutron optical components are being developed and optimized to remedy this lack.^[15]

Afast 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 or higher. They are namedfastneutrons 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:

• Nuclear fission: thermal fission of²³⁵
  U produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s),^[16] which qualifies as "fast". However, the energy spectrum of these neutrons approximately follows aright-skewed Watt distribution${\displaystyle N(E)\propto \exp (-aE)\sinh (\sqrt{bE})}$,^[17][18] with a range of 0 to about 17 MeV,^[16] amedian of 1.6 MeV,^[19] and amode of 0.75 MeV.^[16] A significant proportion of fission neutrons do not qualify as "fast" even by the 1 MeV criterion.
• Spontaneous fission is a mode of radioactive decay for some heavy nuclides. Examples includeplutonium-240 andcalifornium-252.
• Nuclear fusion:deuterium–tritium fusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% of thespeed of light) that can easily fissionuranium-238 and other non-fissileactinides.
• Neutron emission occurs in situations in which a nucleus contains enough excess neutrons that theseparation energy of one or more neutrons becomes negative (i.e. excess neutrons "drip" out of the nucleus). Unstable nuclei of this sort will often decay in less than one second.

Fast neutrons are usually undesirable in a steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it. Ideally, a room temperatureneutron moderator is used for this process. In reactors,heavy water,light water, orgraphite are typically used to moderate neutrons.

Afast neutron is a free neutron with a kinetic energy level close to1 MeV (1.6×10⁻¹³ J), hence a speed of ~14000 km/s (~ 5% of the speed of light). They are namedfission energy orfast 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 asnuclear fission. Neutrons produced in fission, as noted above, have aMaxwell–Boltzmann distribution of kinetic energies from 0 to ~14 MeV, a mean energy of 2 MeV (for²³⁵U fission neutrons), and amode of only 0.75 MeV, which means that more than half of them do not qualify as fast (and thus have almost no chance of initiating fission infertile materials, such as²³⁸U and²³²Th).

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with aneutron moderator. In reactors, typicallyheavy water,light water, orgraphite are used to moderate neutrons.

D–T (deuterium–tritium) fusion is thefusion reaction that produces the most energetic neutrons, with 14.1 MeV ofkinetic energy and traveling at 17% of thespeed of light. D–T fusion is also the easiest fusion reaction to ignite, reaching near-peak rates even when the deuterium and tritium nuclei have only a thousandth as much kinetic energy as the 14.1 MeV that will be produced.

14.1 MeV neutrons have about 10 times as much energy as fission neutrons, and they are very effective at fissioning even non-fissileheavy nuclei. These high-energy fissions also produce more neutrons on average than fissions by lower-energy neutrons. D–T fusion neutron sources, such as proposedtokamak power reactors, are therefore useful fortransmutation of transuranic waste. 14.1 MeV neutrons can also produce neutrons byknocking them loose from nuclei.

On the other hand, these very high-energy neutrons are less likely to simplybe captured without causing fission or spallation. For these reasons,nuclear weapon design extensively uses D–T fusion 14.1 MeV neutrons tocause more fission. Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such asdepleted uranium (uranium-238), and these materials have been used in the jackets ofthermonuclear weapons. Fusion neutrons also can cause fission in substances that are unsuitable or difficult to make into primary fission bombs, such asreactor grade plutonium. This physical fact thus causes ordinary non-weapons grade materials to become of concern in certainnuclear proliferation discussions and treaties.

Other fusion reactions produce much less energetic neutrons. D–D fusion produces a 2.45 MeV neutron andhelium-3 half of the time and producestritium and a proton but no neutron the rest of the time. D–³He fusion produces no neutron.

A fission energy neutron that has slowed down but not yet reached thermal energies is called an epithermal neutron.

Cross sections for bothcapture andfission reactions often have multipleresonance peaks at specific energies in the epithermal energy range. These are of less significance in afast-neutron reactor, where most neutrons are absorbed before slowing down to this range, or in a well-moderatedthermal reactor, where epithermal neutrons interact mostly with moderator nuclei, not with eitherfissile orfertileactinide nuclides. But in a partially moderated reactor with more interactions of epithermal neutrons with heavy metal nuclei, there are greater possibilities fortransient changes inreactivity that might make reactor control more difficult.

Ratios of capture reactions to fission reactions are also worse (more captures without fission) in mostnuclear fuels such asplutonium-239, making epithermal-spectrum reactors using these fuels less desirable, as captures not only waste the one neutron captured but also usually result in anuclide that is notfissile with thermal or epithermal neutrons, though stillfissionable with fast neutrons. The exception isuranium-233 of thethorium cycle, which has good capture-fission ratios at all neutron energies.

High-energy neutrons have much more energy than fission energy neutrons and are generated as secondary particles byparticle accelerators or in the atmosphere fromcosmic rays. These high-energy neutrons are extremely efficient ationization and far more likely to causecell death thanX-rays or protons.^[20][21]

Mostfission reactors arethermal-neutron reactors that use aneutron moderator to slow down ("thermalize") the neutrons produced bynuclear fission. Moderation substantially increases the fissioncross section forfissile nuclei such asuranium-235 orplutonium-239. In addition,uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by²³⁸U. The combination of these effects allowslight water reactors to uselow-enriched uranium.Heavy water reactors andgraphite-moderated reactors can even usenatural uranium as these moderators have much lowerneutron capturecross sections than light water.^[22]

An increase in fuel temperature also raises uranium-238's thermal neutron absorption byDoppler broadening, providingnegative feedback to help control the reactor. When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negativevoid coefficient), depending on whether the reactor is under- or over-moderated.

Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception is theuranium-233 of thethorium cycle, which has a good fission/capture ratio at all neutron energies.

Fast-neutron reactors use unmoderatedfast neutrons to sustain the reaction, and require the fuel to contain a higher concentration offissile material relative tofertile material (uranium-238). However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so afast breeder reactor can potentially "breed" more fissile fuel than it consumes.

Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only a handful of reactors built in the decades since theChernobyl accident due to low prices in theuranium market, although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.^[when?]

• Neutron
• Temperature

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