Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function ͑MTF͒ of amorphous selenium ͑a-Se͒ based active matrix flat panel imagers ͑AMFPIs͒ are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions:The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons ͑electrons remain in deep traps due to their long release times compared with the time scale of the experiments͒ and the deep trapping of drifting holes and electrons.
Direct conversion flat panel detectors (FPD) often experience a loss of sensitivity to x rays caused by previous exposure of the panel to radiation-a phenomenon known as ghosting. Bulk charge trapping and recombination, collectively referred to as incomplete collection of charges, are one of the major causes of ghosting in FPDs. In our investigation, the effects of incomplete charge collection on the modulation transfer function (MTF) of an a-Se direct conversion FPD were studied. The approach used was to repeatedly ghost the panel by uniformly exposing it to a high dose of radiation to force bulk trapping of charges, measure the MTF of the panel after each exposure, and compare this MTF to that of a non-ghosted panel by taking their ratio. MTF ratios allow us to isolate charge collection dependent components of the MTF from all other components. Our results show that ghosting brings about an increase in the MTF of the panel which we interpret as being due to an increase in the incomplete collection of holes. This involves the recombination of free holes with electrons that were trapped from prior exposures, as well as additional trapping of holes caused by the formation of new radiation induced trap states.
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