The neutron detection 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 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 distribution ranges The precise boundaries of neutron energy ranges are not well defined, and differ between sources, but some common names and limits are given in the following table.
The following is a detailed classification:
Thermal A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10−21 J 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), the mode of the Maxwell–Boltzmann distribution for this temperature, Epeak = k T. After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, those neutrons 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 of the chemical element as a result. This event is called neutron activation.
Epithermal 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.). This includes neutrons produced by conversion of accelerated protons in a pitcher-catcher geometry
Cold (slow) neutrons
Cold neutrons are thermal neutrons that have been equilibrated in a very cold substance such as liquid deuterium. Such a cold source is placed in the moderator of a research reactor or spallation source. Cold neutrons are particularly valuable for neutron scattering experiments. 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.
Fast 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 or higher. 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:
Nuclear fission: thermal fission of 235U produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as "fast". However, the energy spectrum of these neutrons approximately follows a right-skewed Watt distribution N ( E ) ∝ exp ( − a E ) sinh ( b E ) {\displaystyle N(E)\propto \exp(-aE)\sinh({\sqrt {bE}})} , with a range of 0 to about 17 MeV, a median of 1.6 MeV, and a mode of 0.75 MeV. 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 include plutonium-240 and californium-252. Nuclear fusion: deuterium–tritium fusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% of the speed of light) that can easily fission uranium-238 and other non-fissile actinides. Neutron emission occurs in situations in which a nucleus contains enough excess neutrons that the separation 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 temperature neutron moderator is used for this process. In reactors, heavy water, light water, or graphite are typically used to moderate neutrons.
Fission energy neutrons
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (1.6×10−13 J), hence a speed of ~14000 km/s (~ 5% of the speed of light). They are named fission energy or 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 produced in fission, as noted above, have a Maxwell–Boltzmann distribution of kinetic energies from 0 to ~14 MeV, a mean energy of 2 MeV (for 235U fission neutrons), and a mode 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 in fertile materials, such as 238U and 232Th). 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.
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