Neutron temperature
The neutron 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 is the kinetic energy of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation. Moderated and other, non-thermal neutron energy distributions or ranges are listed in the table below:
- Fast neutrons have an energy greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
- Slow neutrons have an energy less than 1 eV.
- Epithermal neutrons have an energy from 0.025 to 1 eV.
- Hot neutrons have an average energy of about .2 eV.
- Thermal neutrons have an "average" (most probable) energy of about 0.025 eV.
- Cold neutrons have an average energy from 5x10-5 eV to 0.025 eV.
- Very cold neutrons have an energy from 2x10-7 eV to 5x10-5 eV.
- Ultra cold neutrons have an energy less than 2x10-7 eV.
- Continuum region neutrons have an energy from 0.01 MeV to 25 MeV.
- Resonance region neutrons have an energy from 1 eV to 0.01 MeV.
- Low energy region neutrons have an energy less than 1 eV.
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg), hence a speed of 14,000 km/s. 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 such as nuclear fission.
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.
A thermal neutron is a free neutron with a most probable kinetic energy level of ca. 0.025 eV (approx. 4.0e-21 J; 2.4 MJ/kg, hence a speed of 2.2 km/s). They are named 'thermal' as this level of kinetic energy kT is the most probable (i.e. the top of the distribution peak) kinetic energy of molecules in an ideal room-temperature monatomic gas or for atoms in a crystal (see Maxwellian distribution for most probable energies of molecules). After a number of collisions with nuclei, neutrons arrive at about this energy level, provided that they are not absorbed.
Thermal neutrons have a much larger effective cross-section than fast neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier - and often unstable - isotope of the chemical element as a result.
Most fission reactors use a neutron moderator to slow down, or thermalize the neutrons that are emitted by nuclear fission so that they are more easily captured, causing further fission: these are called thermal reactors. Others, called fast breeder reactors, use fast neutrons directly.