Comparison of coupled liquid hydrogen and solid methane moderators for pulsed neutron sources

Abstract

A coupled liquid-hydrogen moderator with a premoderator can provide a much higher cold-neutron beam intensity than a traditional decoupled moderator. A premoderator is being considered also for the solid-methane moderator system, mainly aimed at reducing the radiation damage of the methane. We have studied experimentally the neutronics of two different cryogenic moderators of liquid hydrogen and solid methane, with variable premoderator thickness. The optimal premoderator thicknesses were found for 2 and 5 cm thick cryogenic moderators. The optimal premoderator thickness is thinner for solid methane moderators than for liquid hydrogen moderators, due to the larger hydrogen number density. The pulse characteristics of cold neutrons from these moderators were also measured. It was found that a coupled solid-methane moderator with premoderator cannot exceed a coupled liquid-hydrogen moderator with premoderator in cold neutron intensity. If solid methane is utilized, the thickness of both premoderator and solid methane moderator must be small.