As discussed, inelastic scattering of neutrons is limited to certain (high or low) energy ranges, but in elastic scattering collisions, there are no restrictions on the exchange of energy between the neutron and the target nucleus. Hence, elastic scattering is positive with all nuclei, free or bound, and neutrons of all energies. In elastic scattering, there is no change in the internal energy of the scattering system and kinetic energy of the neutron exceeds that of the scattering nucleus, some of the kinetic energy of the former may be transferred to the latter and vice versa.
Elastic collisions of neutrons with nuclei are of two types. One of the major interesting nuclear reactor systems is called potential scattering; for nuclei of low mass number, scattering occurs with neutrons that have energies up to a few MeV. In this type of scattering, there is no compound nucleus formation and scattering results from short-range forces acting on the neutron as it approaches the nucleus. In the other type of elastic scattering, known as resonance (or compound nucleus) scattering, a compound nucleus is formed when the scattering nucleus absorbs the neutron; the compound nucleus then expels a neutron leaving the target nucleus in its ground state but usually with a different (larger) kinetic energy. As a general rule in situations of present interest, elastic scattering can be treated as a 'billiard ball' type of collision. The behaviour can then be analyzed by means of the conservation of kinetic energy and of momentum.
After a sufficient number of scattering collisions, the speed of a neutron is reduced to such an extent that it has approximately the same average kinetic energy as the atoms (or molecules) of the scattering medium. The energy depends on the temperature of the medium, and so it is called thermal energy. Thermal neutrons are neutrons which are in thermal equilibrium with the atoms (or molecules) of the medium in which they are present. A particular thermal neutron undergoes collisions with the nuclei of the ambient medium and may gain or lose energy in a non-absorbing medium. There is no net energy change for all the neutrons.