Quantitative analysis of signal formation in a-Se based diagnostic imagers

Research into the use of amorphous Selenium (a-Se) based detectors in digital radiography has seen a dramatic increase in the past few years due to the development of thin film transistor technology. Direct detection Active matrix flat-panel imagers (AMFPIs), which use a-Se to directly convert a-rays into charge, have been studied for use in mammography, chest radiography and fluoroscopy and have shown great promise due to their high intrinsic resolution, Before these detectors can be optimized to fulfill their maximum potential, however, the physics of the interaction of a-rays with a-Se must be understood in a quantitative manner. Although the interaction of visible light with a-Se is adequately understood, the interaction with x-rays has been under debate for some time. In this work, we develop a formalism to directly analyze the signal produced due to the interaction of a-rays with a-Se. We simulate the creation of electron-hole pairs, their transport within the detector, and their recombination along the secondary electron tracks. We show that our analysis agrees with recently published experimental results of the pair creation energy W+/- as a function of both the applied electric field and the incident x-ray energy.