  A new neutron SIMulation program based on the versatile GEANT4 toolkit,
neuSIM4, has been developed to describe interactions of neutrons in the NE213
liquid scintillator from 0.1 to 3000 MeV. neuSIM4 is designed to accommodate
complicated modern detector geometry setups with multiple scintillator
detectors, each of which can be outfitted with more than one photo-multiplier.
To address a broad spectrum of neutron energies, two new neutron interaction
physics models, KSCIN and NxQMD, have been implemented in GEANT4. For neutrons
with energy below 110 MeV, we incorporate a total of eleven neutron induced
reaction channels on hydrogen and carbon nuclei, including nine carbon
inelastic reaction channels, into KSCIN. Beyond 110 MeV, we implement a neutron
induced reaction model, NxQMD, in GEANT4. We use its results as reference to
evaluate other neutron-interaction physics models in GEANT4. We find that
results from an existing cascade physics model (INCL) in GEANT4 agree very well
with the results from NxQMD, and results from both codes agree with new and
existing light response data. To connect KSCIN to NxQMD or INCL, we introduce a
transition region where the contribution of neuSIM4 linearly decreases with
corresponding increased contributions from NxQMD or INCL. To demonstrate the
application of the new code, we simulate the light response and performance of
a 2 x 2 m2 neutron detector wall array consisting of 25 2m-long scintillation
bars. We are able to compare the predicted light response functions to the
shape of the experimental response functions and calculate the efficiency of
the neutron detector array for neutron energies up to 200 MeV. These simulation
results will be pivotal for understanding the performance of modern neutron
arrays with intricate geometries, especially in the measurements of neutron
energy spectra in heavy-ion reactions.
