Yield, variance and spatial distribution of electron–hole pairs in CsI

A Monte Carlo (MC) method previously developed has been applied to simulate the interaction of photons, with energies ranging from 50 eV to ∼1 MeV, with CsI and the subsequent electron cascades. The MC model has been employed to compute nano-scale spatial distributions of electron–hole pairs and important intrinsic properties, including W, the mean energy per electron–hole pair, and the Fano factor, F. W exhibits discontinuities at the shell edges that follow the photoionization cross-sections and decreases with increase in photon energy (from ∼19 to 15 eV), with an asymptotic value of 15.2 eV at high energy. This decrease may contribute to the initial rise in relative light yield with incident energy observed experimentally for CsI, thus suggesting that nonlinearity may be associated with intrinsic properties of the material at low energies. F is calculated to increase with increase in energy and has an asymptotic value of 0.28. A significant number of electron–hole pairs is produced through the different ionization channels of core shells and the corresponding relaxation processes, which may explain why F is larger for CsI than for Si or Ge. Finally, the calculated spatial distributions show that the electron–hole pairs are primarily distributed along fast electron tracks. These spatial distributions constitute important input for large-scale simulations of electron–hole pair transport.