A Study of the Interaction of Neutrons with Moderating Materials

In the theoretical part of this paper we consider the time dependence of the slowing-down process for certain simple cases: (1) For free nuclei at rest an approximate expression is found for the velocity distribution. (2) The existence of a bond in one direction while the motion is unhindered in other directions is found to have little influence on the slowing-down process. (3) The slowing-down process in a monatomic gas is discussed quantitatively on the assumption of a Maxwellian neutron velocity distribution. The experimental part of the paper reports measurements with a pulsed neutron source and a time analyzer, in which the variations of the neutron velocity spectrum with time were studied by transmission measurements. Measurements on water show that the neutrons approach thermal equilibrium with the moderator at a rate which is in agreement with the theoretical predictions. The equilibrium temperature of the neutrons is found to agree with the moderator temperature over a wide temperature range. At low temperatures evidence is found for the effect of the lattice forces on the slowing-down process. Measurements on the initial slowing-down process for neutrons in heavy water indicate satisfactory agreement with the theory for slowing down by free deuterons at rest. The various factors which determine the rate of decay of the neutron intensity in a moderator are discussed, and the feasibility of using measurements of the decay rate to determine the diffusion and absorption properties of the moderating material as well as the geometrical buckling of complicated moderating systems is considered. In an appendix tables are given for the absorption of thermal neutrons in $\frac{1}{v}$ absorbers.