Slowing Down

Abstract

As a rule, neutrons produced in nuclear reactions have energies far above the thermal energy range. When such fast neutrons collide with the atoms of a scattering medium, loss of energy occurs simultaneously with the diffusion process. The collisions can be either elastic or inelastic. As long as the neutron energy is greater than about 1 ev, the struck atoms can be considered free and at rest before the collision. This is no longer the case at lower energies, where the chemical binding of the atoms of the scatterer and their thermal motion affect the slowing-down process. It is the aim of slowing-down theory to determine the space and energy distribution of the neutron flux arising from a given distribution of sources. In what follows, we shall treat three distinct aspects of this problem: To begin with, in this chapter we shall consider slowing down in the space-independent case, i.e., in an infinite medium with uniformly distributed sources. We shall take into account only elastic collisions with free atoms that are initially at rest, neglecting both the inelastic scattering of fast neutrons and the effects associated with the chemical binding and thermal motion of the atoms of the scatterer. In Chapter 8, we shall treat the space-dependent case under the same limitations, i.e., we shall study the simultaneous diffusion and slowing down of neutrons in a finite medium containing an arbitrary distribution of sources.