Monte Carlo simulations of x-ray induced recombination in amorphous selenium

Abstract

The photoconductor amorphous selenium (a-Se) has been used inmedical imaging since the 1950s, and is currently under studyfor use in digital radiography. The quantity of interest forradiographic applications is the energy required to create a detectable electron-hole pair, W±, which is governed by therecombination of electron-hole pairs. Both geminate and columnarrecombination theories have been invoked to describerecombination in a-Se, without success. In this work, we develop aMonte Carlo code which follows individual collisions along thex-ray/electron track structure in order to determine the initialpositions of the electron-hole pairs within a-Se. Wesubsequently simulate the transport of the pairs within anelectric field to calculate the amount of recombination. We usethese simulations to calculate W± as a function ofapplied electric field and incident x-ray energy. We findexcellent agreement between our simulations and experimentalresults. This indicates that the recombination occurs not onlybetween geminate pairs but also between other pairs created alongthe track. This inter-track recombination leads to an energydependent recombination probability which could not be explainedwith previous recombination theories.