We present results of MCNPX modeling of the neutron pulse time structure delivered by moderators under investigation for the proposed NxGENS long-pulse spallation neutron source at the Los Alamos Neutron Science Center. As envisioned at the time of this work, low-energy neutrons would be produced for the NxGENS flight path by a single "backscattering" moderator of either water or liquid hydrogen in proximity to a tungsten-reflected tungsten target bombarded by 800 MeV protons. Specifically, our results here pertain to a 9.2 cm thick water moderator, and a 5.2 cm thick liquid hydrogen moderator with ortho:para ratio of 1:1. It was found that the Ikeda-Carpenter model accurately represents the storage of neutrons in these moderators, but that there are inaccuracies in the time region dominated by slowing-down. We suspect that inhomogeneity of the moderators, their limited size, and neutronic coupling of the moderators to their surroundings may be responsible for the observed disparities with this model.
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