Sensitivity reduction in biased amorphous selenium photoconductors

Abstract

We have experimentally studied the reduction in x-ray sensitivity of individual biased amorphous selenium (a-Se) detectors as a function of radiation dose. This study was performed to understand the effects of detector parameters on the reduction of sensitivity in a-Se active matrix flat panel imagers, which results in latent `ghost' images. The sensitivity was measured for various x-ray dose rates, electric field strengths, and effective photon energies. The reduction of sensitivity has a weak dependence on the incident dose rate (reduces to 0.67 and 0.63 of original value after 100 cGy for dose rates of 2.73 cGy min−1 and 8.18 cGy min−1, respectively), is strongly affected by the applied electric field (reduces to 0.32 and 0.73 of original value after 100 cGy for electric fields of 0.6 V μm−1 and 5 V μm−1, respectively), and is greater for higher-energy photons. The measured sensitivity curves were fitted using a linear-exponential equation (reduced χ2 values averaging 0.73). Experiments demonstrated that a-Se recovers approximately 20% of its original sensitivity at 30 min post-irradiation. If a-Se is allowed to recover its sensitivity for 24 h between irradiation, the initial measured current is a linear function of both the dose rate and applied electric field.