Charged-particle detection efficiencies of close-packed CsI arrays

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.