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

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Abstract
The photoconductor amorphous selenium (a-Se) has been used in medical imaging since the 1950s, and is currently under study for use in digital radiography. The quantity of interest for radiographic applications is the energy required to create a detectable electron-hole pair, W±, which is governed by the recombination of electron-hole pairs. Both geminate and columnar recombination theories have been invoked to describe recombination in a-Se, without success. In this work, we develop a Monte Carlo code which follows individual collisions along the x-ray/electron track structure in order to determine the initial positions of the electron-hole pairs within a-Se. We subsequently simulate the transport of the pairs within an electric field to calculate the amount of recombination. We use these simulations to calculate W± as a function of applied electric field and incident x-ray energy. We find excellent agreement between our simulations and experimental results. This indicates that the recombination occurs not only between geminate pairs but also between other pairs created along the track. This inter-track recombination leads to an energy dependent recombination probability which could not be explained with previous recombination theories.