Optimization of Coupled Hydrogen Moderator for a Short Pulse Spallation Source

Abstract

Extensive neutronic design studies for the Japanese pulsed spallation neutron source are under progress in the high- intensity proton accelerator project by JAERI and KEK. The development of a high-performance pulsed cold-neutron source with a coupled hydrogen (H2) moderator is one of the key issues. At least, two viewed surfaces are indispensable for this moderator to satisfy the requirements from neutron instruments. The use of two coupled H2 moderators sharing a backside premoderator was originally proposed aiming at a higher cold neutron intensity. Various studies were carried out by Monte Carlo calculations based on the proposed concept. An idea of the premoderator extension brought about an appreciable intensity enhancement. With this concept we concluded that the two coupled H2 moderators could be replaced by one without sacrificing the neutron intensity, because the intensity enhancement by the extension was more effective in the one moderator case. We also compared the neutronic performance of an H2 moderator in a lead and a beryllium reflectors optimized for each systems. It was found that the lead reflected system could provide a higher peak intensity with a narrower pulse width and a faster decay time than the beryllium one although a time-integrated intensity was slightly lower.