Fast neutrons

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 adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation.

Neutron energy distribution ranges

Neutron energy range names [1]
Neutron energy Energy range
0.0 eV-0.025 eV Cold neutrons
0.025 eV Thermal neutrons
0.025 eV-0.4 eV Epithermal neutrons
0.4 eV-0.6 eV Cadmium neutrons
0.6 eV-1 eV EpiCadmium neutrons
1 eV-10 eV Slow neutrons
10 eV-300 eV Resonance neutrons
300 eV-1 MeV Intermediate neutrons
1 MeV-20 MeV Fast neutrons
> 20 MeV Relativistic neutrons

But different ranges with different names are observed in other sources. For example,

Epithermal neutrons have energies between 1 eV and 10 keV and smaller nuclear cross sections than thermal neutrons.
—H. Tomita, C. Shoda, J. Kawarabayashi, T. Matsumoto, J. Hori, S. Uno, M. Shoji, T. Uchida, N. Fukumotoa and T. Iguchia, "Development of epithermal neutron camera based on resonance-energy-filtered imaging with GEM" (2012)

or the following classification:


  • Neutrons in thermal equilibrium with their surroundings
  • Most probable energy at 20 degrees (C) - 0.025 eV; Maxwellian distribution of 20 degrees (C) extends to about 0.2 eV.


  • Neutrons of energy greater than thermal
  • Greater than 0.2 eV


  • Neutrons which are strongly absorbed by cadmium
  • Less than 0.4 eV


  • Neutrons which are not strongly absorbed by cadmium
  • Greater than 0.6 eV


  • Neutrons of energy slightly greater than thermal
  • Less than 1 to 10 eV (sometimes up to 1 keV)


  • In pile neutron physics, usually refers to neutrons which are strongly captured in the resonance of U-238, and of a few commonly used detectors (e.g., Indium, Gold, etc.)
  • 1 eV to 300 eV


  • Neutrons that are between slow and fast
  • Few hundred eV to 0.5 MeV


  • Greater than 0.5 MeV


  • Relativistic
  • Greater than 20 MeV


  • Neutrons of all energies present in nuclear reactors
  • 0.001 eV to 15 MeV


  • Neutrons formed during fission
  • 100 keV to 15 MeV (Most probable - 0.8 Mev; Average - 2.0 MeV)

—IEM - Consulting and Services,

Ultracold neutrons (UCN)

Ultracold neutrons are free neutrons which can be stored in traps made from certain materials.

Thermal neutrons

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 2.2 km/s), which is the energy corresponding to the most probable velocity at a temperature of 290 K (17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this temperature.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed.

Thermal neutrons have a different and often 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 (neutron activation).

Fast neutrons

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 produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), 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 disintigration, and the mode of the energy is only 0.75 MeV, meaning that less than half of fission neutrons qualify as "fast" even by the 1 MeV criterion.[2]

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.

Fast reactor and thermal reactor compared

Most fission reactors are thermal reactors that use a neutron moderator to slow down ("thermalize") 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 capture cross sections than light water.[3]

An increase in fuel temperature also raises U-238's thermal 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 (a negative void coefficient).

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 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.

See also


External links

  • Language of the Nucleusfr:Température neutronique

ja:高速中性子 pl:Neutrony prędkie ru:Спектр нейтронов zh:中子温度

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