MODERATOR NEUTRON PULSE-SHAPE STUDY FOR THE NXGENS LONG-PULSE SPALLATION SOURCE

摘要

We present results of MCNPX modeling of thernneutron pulse time structure delivered by moderatorsrnunder investigation for the proposed NxGENS long-pulsernspallation neutron source at the Los Alamos NeutronrnScience Center. As envisioned at the time of this work,rnlow-energy neutrons would be produced for the NxGENSrnflight path by a single "backscattering" moderator ofrneither water or liquid hydrogen in proximity to arntungsten-reflected tungsten target bombarded by 800 MeVrnprotons. Specifically, our results here pertain to a 9.2 cmrnthick water moderator, and a 5.2 cm thick liquid hydrogenrnmoderator with ortho:para ratio of 1:1. It was found thatrnthe Ikeda-Carpenter model accurately represents thernstorage of neutrons in these moderators, but that therernare inaccuracies in the time region dominated by slowing-down.rnWe suspect that inhomogeneity of the moderators,rntheir limited size, and neutronic coupling of thernmoderators to their surroundings may be responsible forrnthe observed disparities with this model.