Sensitivity of X-ray image detectors: Space charge effects

A model for calculating the sensitivity of direct conversion X-ray image detectors is developed incorporating the space charge effect. The space charge perturbs the applied electric field across the thickness of the photoconductor, and thus the electric field becomes nonuniform. First, Poisson's equation is solved to obtain the electric field distribution. Then, the continuity equations for both holes and electrons are solved considering the perturbed electric field. Finally, the collected charge is determined from the charge carrier distributions and electric field profile using the Shockley–Ramo theorem. The developed X-ray sensitivity model is applied to polycrystalline mercuric iodide based detectors. The electric field collapses for a critical amount of trapped charge if the ratio of trapped electrons to holes is high (∼0.5). The pinch in electric field slows down the drift of free electrons and hence reduces the sensitivity of the detector. The developed model is fitted with the published experimental data, and the fitted values of effective mobility-lifetime products of charge carriers agree well with the published results. It is found that the space charge reduces the sensitivity in the screen-printed sample more significantly than in the vacuum deposited sample.