  Fission events from Special Nuclear Material (SNM), such as highly enriched
uranium or plutonium, can produce simultaneous emission of multiple neutrons
and high energy gamma-rays. The observation of time correlations between any of
these particles is a significant indicator of the presence of fissionable
material. Cosmogenic processes can also mimic these types of correlated
signals. However, if the background is sufficiently low and fully
characterized, significant changes in the correlated event rate in the presence
of a target of interest constitutes a robust signature of the presence of SNM.
Since fission emissions are isotropic, adequate sensitivity to these
multiplicities requires a high efficiency detector with a large solid angle
with respect to the target. Water Cerenkov detectors are a cost-effective
choice when large solid angle coverage is required. In order to characterize
the neutron detection performance of large-scale water Cerenkov detectors, we
have designed and built a 3.5 kL water Cerenkov-based gamma-ray and neutron
detector, and modeled the detector response in Geant4 [1]. We report the
position-dependent neutron detection efficiency and energy response of the
detector, as well as the basic characteristics of the simulation.
