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The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in . 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 adapted to the Maxwell distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. The momentum and wavelength of the neutron are related through the de Broglie relation. The long wavelength of slow neutrons allows for the large cross section.
| + Neutron energy range names | |||
| Ultracold neutrons | Moderation by liquid helium or solid deuterium | Storage times >15 minutes in bottles. | |
| Cold (slow) neutrons | Moderation by liquid hydrogen or liquid methane | ||
| Thermal neutrons (at 20 °C) | Room temperature moderators | Room temperature | |
| Epithermal neutrons | Reduced moderation | Above room temperature | |
| Resonance neutrons | Susceptible to non-fission capture by 238U. | ||
| Fast neutrons | Nuclear fission reactions, nuclear fusion reactions | ||
| Ultrafast neutrons | Nuclear spallation from particle accelerator Ion | Relativistic |
The following is a detailed classification:
After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, those which are not absorbed reach about this energy level.
Thermal neutrons have a different and sometimes 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, often unstable isotope isotope of the chemical element as a result. This event is called neutron activation.
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Lower flux and lack in optical components limit the applications of cold and very cold neutrons. New holographic gratings based on nanodiamond polymers have been shown to provide higher optical efficiencies.
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 solid deuterium or superfluid helium. An alternative production method is the mechanical deceleration of cold neutrons exploiting the Doppler shift.
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 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. (2022). 9781510651784, SPIE. ISBN 9781510651784
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 temperature neutron moderator is used for this process. In reactors, heavy water, light water, or graphite are typically used to moderate neutrons.
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.
14.1 MeV neutrons have about 10 times as much energy as fission neutrons, and they are very effective at fissioning even non-fissile actinides. These high-energy fissions also produce more neutrons on average than fissions by lower-energy neutrons. D–T fusion neutron sources, such as proposed tokamak power reactors, are therefore useful for transmutation of transuranic waste. 14.1 MeV neutrons can also produce neutrons by knocking them loose from nuclei.
On the other hand, these very high-energy neutrons are less likely to simply neutron capture. For these reasons, nuclear weapon design extensively uses D–T fusion 14.1 MeV neutrons to fusion boosting. Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such as depleted uranium (uranium-238), and these materials have been used in the jackets of thermonuclear weapons. Fusion neutrons also can cause fission in substances that are unsuitable or difficult to make into primary fission bombs, such as reactor grade plutonium. This physical fact thus causes ordinary non-weapons grade materials to become of concern in certain nuclear proliferation discussions and treaties.
Other fusion reactions produce much less energetic neutrons. D–D fusion produces a 2.45 MeV neutron and helium-3 half of the time and produces tritium and a proton but no neutron the rest of the time. D–3He fusion produces no neutron.
Cross sections for both neutron capture and nuclear fission reactions often have multiple resonance peaks at specific energies in the epithermal energy range. These are of less significance in a fast-neutron reactor, where most neutrons are absorbed before slowing down to this range, or in a well-moderated thermal reactor, where epithermal neutrons interact mostly with moderator nuclei, not with either fissile or fertile material actinide nuclides. But in a partially moderated reactor with more interactions of epithermal neutrons with heavy metal nuclei, there are greater possibilities for transient changes in reactivity that might make reactor control more difficult.
Ratios of capture reactions to fission reactions are also worse (more captures without fission) in most such as plutonium-239, making epithermal-spectrum reactors using these fuels less desirable, as captures not only waste the one neutron captured but also usually result in a nuclide that is not fissile with thermal or epithermal neutrons, though still fissionable with fast neutrons. The exception is uranium-233 of the thorium cycle, which has good capture-fission ratios at all neutron energies.
An increase in fuel temperature also raises uranium-238's thermal neutron absorption by Doppler broadening, providing negative 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 negative void 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 the uranium-233 of the thorium cycle, which has a good fission/capture ratio at all neutron energies.
Fast-neutron reactors use unmoderated to sustain the reaction, and require the fuel to contain a higher concentration of fissile material relative to fertile 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 a fast 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 the Chernobyl accident due to 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.
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