A Review of the Cold Neutron Moderator Materials: Neutronic Performance and Radiation Effects

Abstract

Liquid hydrogen, ice and solid methane are common neutron moderating materials. More recently methane hydrate and mesitylene have been proposed to improve performance of these materials.Liquid hydrogen is used in high-power spallation neutron sources since it is the only material that withstands such high power. Despite this, it is affected by radiation leading to spontaneous release of energy. It can be controlled by programmed annealings. Solid methane generally shows the highest neutron flux but, it is affected by radiation leading to formation of organic contaminants. In addition, spontaneous release of stored energy, that could damage the moderator container, was detected. Mesitylene is less affected by radiation and therefore, no spontaneous release of energy is observed or accumulation of any major contaminant. However, mesitylene shows a lower neutron energy spectrum than solid methane.Methane hydrate was considered as an alternative. Nevertheless, reduced intensity of neutrons has been observed compared to solid methane.This contribution provides a review of the main characteristics of these cold neutron moderator materials including the study of their respective neutron fluxes, the accumulation of energy in “frozen radicals” with the subsequent spontaneous release of energy and their thermal conductivities.