Avalanche Gain and Energy Resolution of Semiconductor X-ray Detectors

Realistic Monte Carlo simulations for the avalanche gain of absorbed X-ray photons were carried out in a study of the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes (APDs). The work explored how the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and, hence, the photon energy), the relationship between ionization coefficients, and the mean gain itself. We showed that the conventional notion of APD gains significantly degrading energy resolution is incomplete. If the X-ray photons are absorbed outside the avalanche region, then high avalanche gains with little energy resolution penalty can be achieved using dissimilar ionization coefficients. However, absorption of X-ray photons within the avalanche region will always result in broad gain distribution (degrading energy resolution), unless electrons and holes have similar ionization coefficients.