Charge transport and trapping-limited sensitivity and resolution of pixellated x-ray image detectors

Charge transport and trapping-limited sensitivity and signal spreading over neighboring pixels of a direct conversion pixellated x-ray image detector are calculated by using the final. trapped charge distributions across the photoconductor and the weighting potential of the individual pixel. The analytical expressions for the final trapped charge distributions across the photoconductor are derived by analytically solving the continuity equation for both types of carriers (electrons and holes). We calculate collected charges at different pixels by considering square pixels. arranged in a - two dimensional array. We calculate the amount of collected charge per unit incident radiation, the x-ray sensitivity, in terms of normalized parameters.; (a) the normalized absorption depth (= absorption depth / photoconductor thickness), (b) normalized electron schubweg (schubweg / thickness), (c) normalized hole schubweg, and (d) normalized pixel pitch (pixel size/thickness). The composite (finely sampled) line spread function (LSF) is calculated by calculating collected charges at different pixels. and by considering diagnostic x-ray irradiation along a line. The modulation transfer function (MTF) due to distributed carrier trapping is calculated by taking Fourier transform of composite LSF and correcting for the square sampling aperture. The charge transport and trapping-limited sensitivity and resolution of pixellated x-ray detectors mostly depend on the mobility and lifetime product of charges that move towards the pixel electrodes and the extent of dependence increases with decreasing normalized pixel pitch. The polarity (negative or positive signal) and the quantity of induced signals in the surrounding pixels depend on the bias polarity and the rate of trapping of both types of carriers. Optimal sensitivity and resolution can be attained by ensuring that the carriers which drift towards the pixel electrodes have a schubweg much longer than the sample thickness.