Neutron moderation, the reduction of the kinetic energy of neutrons as a result of their repeated collisions with atomic nuclei. The mechanism of neutron moderation depends on the energy of the neutrons. Sufficiently fast neutrons spend their energy chiefly on exciting nuclei. As the energy falls, the collisions of a neutron with a nucleus become elastic. In a single elastic collision a neutron loses on average a fraction of its energy that is the greater the lighter the nucleus (for hydrogen, half). The last stage of neutron moderation, called thermalisation, ends with the establishment of equilibrium between the neutron gas and the moderating medium. The thermal neutrons that are formed play an important part in science and technology, and above all in the building of nuclear reactors (see Nuclear reactor).