  Detector efficiency determination is essential to correct the measured yields
and extract reliable cross sections of particles emitted in nuclear reactions.
We investigate the efficiencies for measuring the full energies of light
charged particle in arrays of CsI crystals employed in particle detection
arrays such as HiRA, LASSA and MUST2. We perform these simulations with a
GEANT4 Monte Carlo transport code implemented in the NPTool framework. Both
Coulomb multiple scattering and nuclear reactions within the crystal can
significantly reduce the efficiency of detecting the full energy of high energy
particles. The calculated efficiencies decrease exponentially as a function of
the range of the particle and are quite similar for both the hydrogen ($p, d,
t$) and helium ($^3$He, $\alpha$) isotopes. The use of a close-packed array
introduces significant position dependent efficiency losses at the interior
boundaries between crystals that need to be considered in the design of an
array and in the efficiency corrections of measured energy spectra.
