Effects of High-Dose X-Ray Irradiation on the Hole Lifetime in Vacuum-Deposited Stabilized a-Se Photoconductive Films: Implications to the Quality Control of a-Se Used in X-Ray Detectors

Abstract

Stabilized amorphous selenium (a-Se)-based digital flat-panel X-ray imaging detectors are widely used in modern mammography. The dependence of the hole lifetime, $\tau _{h}$ , in a-Se on high-dose X-ray irradiation under high electric fields is examined. The X-ray-induced effects on the samples were studied using conventional time-of-flight (TOF) and interrupted-field time-of-flight (IFTOF) techniques. The samples were placed under a high electric field (5–10 V/ $\mu \text{m}$ ) and exposed to X-ray irradiation, after which TOF and IFTOF measurements were taken to find the hole drift mobility and lifetime after X-ray exposure. There was no observed change in the hole drift mobility but there was a large drop in the hole lifetime. The reduction in the hole lifetime depended only on the total or accumulated dose and the applied field. There was no dependence on the dose rate, over a range of 0.12–2.5 Gy/s, or on the X-ray photon energy, over a range of 50–90 keVp (corresponding to a mean photon energy of 31.9–44.7 keV). The implications of these findings can be seen through the calculation of the charge collection efficiency (CCE). The results show that the effects of X-ray irradiation on a-Se X-ray detectors are considerable when the detector is under an applied field during exposure, but are minimized when the operating field is high (corresponding to a high CCE) and the a-Se is of high-quality electronic grade material.