Time and exposure dependent x-ray sensitivity in multilayer amorphous selenium detectors

Abstract

A numerical model is developed to study the time and exposure dependent x-ray sensitivity of multilayer a-Se x-ray imaging detectors on repeated x-ray exposures by considering accumulated trapped charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron–hole pair creation), the carrier transport in the blocking layers, x-ray induced metastable deep trap centre generations and the effects of charge injection. The time-dependent carrier detrapping and structural relaxation (recovery of metastable trap centres) are also considered. The continuity equations for both holes and electrons, trapping rate equations, and Poisson's equation across the photoconductor for a step x-ray exposure are simultaneously solved by the backward Euler finite difference method. The electric field distribution across the multilayer detector and the dark current density under repeated exposures are also estimated. The sensitivity in a rested sample is recovered mainly by the carrier detrapping and the recombination of the injected carriers with the existing trapped carriers. The sensitivity is expected to recover almost fully by resting the sample longer than the recovery time constant of the metastable trap centres (the structural relaxation time constant), which is in the range of several hours. The simulation result is fitted with the experimental data. The proposed theoretical model shows very good agreement with the experimental relative sensitivity versus time and accumulative x-ray exposure characteristics.