Neutrons, Moderation of

Neutrons, Moderation of


the reduction of the kinetic energy of neutrons as a result of repeated collisions with the atomic nuclei of a substance. In nuclear reactions, which are neutron sources, fast neutrons (having an energy > 1 MeV) are generally formed. On collision with atomic nuclei, fast neutrons lose their energy in large portions, expending it primarily to excite or split the nuclei. As a result of one or several collisions, the energy of the neutron be-comes less than the minimum excitation energy of the nucleus (from tens of keV to several MeV, depending on the properties of the nucleus). After this, the scattering of the neutron by the nucleus becomes elastic, that is, the neutron expends energy to impart a velocity to the nucleus without changing its internal state. During one elastic collision a neutron loses a fraction of its energy that on the average is equal to 2A/(A + I)2, where A is the mass number of the target nucleus. This fraction is small for heavy nuclei (1/100 for lead) and large for light nuclei (1/7 for carbon and 1/2 for hydrogen). Therefore, the moderation of neutrons takes place much faster in light than heavy nuclei (see Table 1).

Table 1. Average number of collisions N, mean moderation time f, and mean square distance LF of neutron from source when neutron is slowed down in an unbounded medium from an energy of 1 MeV to 0.1 eV
SubstanceNt,μsecL,, cm
Lead ......................................1,6001,300200
Graphite ......................................1107043
Water ......................................23313

The moderation of neutrons ultimately leads to the formation of so-called thermal neutrons—a neutron gas in thermal equilibrium with the medium in which the moderation of neutrons transpires. The mean energy of a thermal neutron at room temperature is 0.04 eV.

In the process of moderation some of the neutrons are lost, being absorbed by nuclei on collision or flying out of the medium. In neutron moderators—substances that contain light nuclei, which capture neutrons poorly—the losses are small and many of the neutrons emitted by the source are converted into thermal neutrons (for this purpose the size of the moderator must be great in comparison with the size LF of the region in which the neutrons diffuse during moderation; see Table 1).

Water, heavy water, beryllium, and graphite are among the best moderators and are widely used in nuclear physics and nuclear technology to convert fast neutrons into thermal neutrons.