But different ranges with different names are observed in other sources. The following is a detailed classification:
Thermal
A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV, which is the most probable energy at a temperature of 290 K, the mode of the Maxwell–Boltzmann distribution for this temperature. After a number of collisions with nuclei in a medium 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 absorptioncross-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
Cadmium
Epicadmium
Slow
Resonance
Intermediate
Fast
Fast neutrons are produced by nuclear processes:
Nuclear fission produces neutrons with a mean energy of 2 MeV, which qualifies as "fast". However the range of neutrons from fission follows a Maxwell–Boltzmann distribution from 0 to about 14 MeV in the center of momentum frame of the disintegration, and the mode of the energy is only 0.75 MeV, meaning that fewer than half of fission neutrons qualify as "fast" even by the 1 MeV criterion.
Nuclear fusion: deuterium-tritium fusion produces neutrons of 14.1 MeV 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. 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 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, or graphite are typically used to moderate neutrons. es at a temperature of 298.15 K. An explanation of the vertical axis label appears on the image page. Similar speed distributions are obtained for neutrons upon moderation.
Most fission reactors are thermal-neutron reactors that use a neutron moderator to slow down the neutrons produced by nuclear fission. Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-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 by 238U. The combination of these effects allows light water reactors to use low-enriched uranium. Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capturecross sections than light water. An increase in fuel temperature also raises U-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, boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback, 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 fast neutrons to sustain the reaction and require the fuel to contain a higher concentration of fissile material relative to fertile material U-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.
