  Solar flares are some of the most energetic events in the solar system and
can be studied to investigate the physics of plasmas and stellar processes. One
interesting aspect of solar flares is the presence of accelerated (nonthermal)
particles, whose signatures appear in solar flare hard X-ray emissions. Debate
has been ongoing since the early days of the space age as to how these
particles are accelerated, and one way to probe relevant acceleration
mechanisms is by investigating short-timescale (tens of milliseconds)
variations in solar flare hard X-ray flux. The Impulsive Phase Rapid Energetic
Solar Spectrometer (IMPRESS) CubeSat mission aims to measure these fast hard
X-ray variations. In order to produce the best possible science data from this
mission, we characterize the IMPRESS scintillator detectors using Geant4 Monte
Carlo models. We show that the Geant4 Monte Carlo detector model is consistent
with an analytical model. We find that Geant4 simulations of X-ray and optical
interactions explain observed features in experimental data, but do not
completely account for our measured energy resolution. We further show that
nonuniform light collection leads to double-peak behavior at the 662 keV
$^{137}$Cs photopeak and can be corrected in Geant4 models and likely in the
lab.
