Thermal Neutron

a neutron having a kinetic energy of between 5 × 10^–3 and 0.5 electron volt (eV). Such neutrons are called thermal because they are obtained when neutrons are moderated, or slowed down, to thermal equilibrium with the atoms of the moderating medium; the process of reducing the energy of a neutron to the thermal region is called thermalization. In English the term “slow neutron” is often used synonymously with “thermal neutron.” It should be noted, however, that “slow neutron” is also used, particularly in the Soviet literature, in a broader sense (seeNEUTRONS, SLOW).

The velocity distribution of thermal neutrons in a moderator is determined by the moderator’s temperature in accordance with the Maxwellian distribution. The energy corresponding to the most probable velocity of thermal neutrons is 8.6 × 10^–5T eV, where T is the absolute temperature in degrees Kelvin. The average kinetic energy of thermal neutrons at room temperature is 0.025 eV, which corresponds to a velocity of 2,200 m/sec and a de Broglie wavelength λ of 1.8 angstroms (seeNEUTRON OPTICS).

Since λ is close to the interatomic distances in solids, the structure of solids can be studied by means of the diffraction of thermal neutrons. The neutron’s possession of a magnetic moment means that the magnetic structure of solids can be investigated through coherent magnetic scattering of thermal neutrons. Since the changes in energy occurring in the inelastic scattering of thermal neutrons in condensed media are comparable with the neutrons’ original energies, the motion of atoms and molecules in solids and liquids can be studied by means of the inelastic scattering of thermal neutrons (seeNEUTRON DIFFRACTION ANALYSIS). Thermal neutrons are of immense importance in the operation of nuclear reactors, since such neutrons are used to produce the chain reaction involving the fission of U and Pu. An important role is also played by thermal neutrons in the production of radioactive isotopes.