Diffusion cooling of thermal neutrons in the outer Plexiglas layer of a two-zone system with non-hydrogenous material inside

Abstract

Diffusion cooling of thermal neutron flux in a finite volume of a medium results from the leakage of neutrons. The cooling effect is particularly complex when the system consists of zones with different neutron scattering properties. A study is presented on the diffusion cooling in Plexiglas (hydrogenous material), which constitutes an outer cylindrical shell surrounding an inner cylinder made of a non-hydrogenous substance. The pulsed neutron method has been used. Numerous series of the desired pulsed neutron experiments in two-zone cylindrical systems have been substituted by Monte Carlo simulations. The decay constant of the fundamental mode in the time distribution of the pulsed thermal neutron flux has been determined in each experiment. The so-called theoretical decay constants (defined under a particular assumption) have been calculated as a function of the system geometry and neutron dynamic parameters including the diffusion cooling coefficient of the outer Plexiglas shell. The variability of this parameter has been determined from a combination of experimental and theoretical results. For a system with a theoretical decay constant varying between 12,000 and 34,000 s −1, the diffusion cooling coefficient of Plexiglas in the outer layer varies between 9000 and 4800 cm 4 s −1 with the latter being close to the value for homogeneous material. A function has been obtained and successfully applied in interpreting real experiments using Czubek’s method of measuring the absorption cross-section of small samples.