PNNL

Abstract

The interaction of X-ray and gamma-ray photons with materials is of fundamental interest to many fields. The interaction of these primary photons with atoms results in the creation of fast electrons. Electron-solid interaction can be determined experimentally by measuring physical quantities, such as scattering cross section, but the partitioning of energy loss into different quantum mechanical processes is important to determine the mean energy required to create an electron-hole pair, W, and the intrinsic variance (or Fano factor, F) for radiation detectors. In the present work, a Monte Carlo (MC) method previously developed has been employed to simulate the interaction of photons with Ge over the energy range from 50 eV to ~1 MeV and the subsequent electron cascades. Various quantum mechanical processes for energy loss of fast electrons, which control the broadening of Fano factor, are investigated in detail. At energies lower than 1 keV, W generally decreases with increasing photon energy from 2.95 to 2.75 in Ge, whereas it has a constant value of 2.64 eV for higher energies. Also, the function, F, decreases with increasing photon energy. Above the L shell edge, F has a value of 0.11 that is smaller than that in Si (0.14). However, F exhibits a sawtooth variation, and discontinuities at the shell edges follow the photoionization cross section. These results are in good agreement with experimental measurements. The simulated distribution indicates that the interband transition and plasmon excitation are the most important mechanisms of electron-hole pair creation in Ge, while core shell ionization appears to be significant only at high energies.