COMPARISON OF COUPLED LIQUID-HYDROGEN AND SOLID METHANE MODERATORS FOR PULSED-NEUTRON SOURCES

A coupled liquid-hydrogen moderator with a premoderator can provide a much higher cold-neutron beam intensity than a traditional decoupled m oderator. A premoderator is being considered also for the solid-methan e moderator system, mainly aimed at reducing the radiation damage of t he methane. We have studied experimentally the neutronics of two diffe rent cryogenic moderators of liquid hydrogen and solid methane, with v ariable premoderator thickness. The optimal premoderator thicknesses w ere found for 2 and 5 cm thick cryogenic moderators. The optimal premo derator thickness is thinner for solid methane moderators than for liq uid hydrogen moderators, due to the larger hydrogen number density. Th e pulse characteristics of cold neutrons from these moderators were al so measured. It was found that a coupled solid-methane moderator with premoderator cannot exceed a coupled liquid-hydrogen moderator with pr emoderator in cold neutron intensity. If solid methane is utilized, th e thickness of both premoderator and solid methane moderator must be s mall.