Design options for coupled cold moderators on a low power spallation source

The paper describes the results of Monte-Carlo calculations for a coupled moderator on a low power pulsed neutron spallation source. Various options were compared including hydrogen, solid methane, grooving the solid methane and compound moderators made of hydrogen in front of solid methane. To maximize the neutron current at low energies, two strategies appear to emerge from the calculations. For instruments that view a large area of moderator surface, a layer of hydrogen in front of a thin solid-methane moderator is optimum, giving a gain of about a factor 10 relative to the current liquid hydrogen moderator on the existing ISIS tantalum target. For instruments that only view a restricted area, a higher flux corresponding to a gain of 13.5, can be achieved with the use of a single groove or re-entrant hole in the moderator.