[en] A cascaded linear system model that includes incomplete charge collection and interaction-depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency, DQE(0), of a direct conversion x-ray image detector. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) addition of electronic noise. The primary x-ray photon interaction and also the secondary K-fluorescent photon interaction are included in determining the interaction-depth dependent conversion gain across the photoconductor. We examine DQE(0) of a-Se detectors for fluoroscopic applications as a function of photoconductor thickness with varying amounts of electronic noise and x-ray exposure under (a) constant field, and (b) constant voltage operating conditions. We show that there is an optimum photoconductor thickness, which maximizes DQE(0) under a constant voltage operation. The optimum thickness depends on the added electronic noise, x-ray exposure, charge collection efficiency and bias voltage. For the quantities mentioned above that are appropriate for a-Se detectors and fluoroscopic applications, the optimum a-Se thickness is ∼700 μm and the corresponding DQE is ∼0.4. It is shown that the DQE depends strongly on the charge transport properties of the photoconductors. With the radiation-receiving electrode negatively biased, the DQE is more dependent on electron lifetime (τ_e) than hole lifetime (τ_h). Full electron trapping, (τ_e=0) reduces the DQE by about 73.3% at the detector thickness of 1000 μm whereas full hole trapping (τ_h=0) reduces the DQE by about 43.7%. The DQE for the negative bias is lower than for the positive bias, and the difference in DQE, as expected, increases with the photoconductor thickness because of the asymmetric transport properties of holes and electrons in a-Se. The present results show that the DQE generally does not continue to improve with greater photoconductor thickness because of charge carrier trapping effects. The DQE of a polyenergetic x-ray beam is only slightly lower than a monoenergetic x-ray beam with the same average photon energy. The theoretical model shows a very good agreement with the experimental DQE versus exposure characteristics published in the literature

[en] Sensitivity reduction in amorphous Se-based photoconductive x-ray image detectors due to previous exposures is studied by Monte Carlo simulation. Collected charge, hence x-ray sensitivity, is calculated by considering deep carrier trapping, taking into account the effects of trap filling, recombination between trapped and drifting carriers and the generation of x-ray induced new deep trap centers. Space charge effects on the electric field, and hence, the effects of electric field on electron hole pair generation and charge transport are also considered. The comparison of the model with the experimental data reveals that the recombination between trapped and oppositely charged drifting carriers and x-ray induced new deep trap centers are mainly responsible for the sensitivity reduction in biased a-Se-based x-ray detectors

[en] The charge collection and absorption-limited x-ray sensitivity of a direct conversion pixellated x-ray detector operating in the presence of deep trapping of charge carriers is calculated using the Shockley-Ramo theorem and the weighting potential of the individual pixel. The sensitivity of a pixellated x-ray detector is analyzed in terms of normalized parameters; (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness), (b) normalized pixel width (pixel size/thickness), and (c) normalized carrier schubwegs (schubweg/thickness). The charge collection and absorption-limited sensitivity of pixellated x-ray detectors mainly depends on the transport properties (mobility and lifetime) of the charges that move towards the pixel electrodes and the extent of dependence increases with decreasing normalized pixel width. The x-ray sensitivity of smaller pixels may be higher or lower than that of larger pixels depending on the rate of electron and hole trapping and the bias polarity. The sensitivity of pixellated detectors can be improved by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes

[en] A generalized expression for charge carrier transport and absorption-limited sensitivity of x-ray photoconductors is derived by analytically solving the continuity equation for both holes and electrons considering the drift of electrons and holes in the presence of deep traps. The normalized sensitivity equation is applied to stabilized a-Se and HgI₂. In the latter case, the sensitivity model is fitted to published data to determine the electron and hole ranges (6.4x10^-6 cm²/V and 7x10^-8 cm²/V, respectively) in screen-printed polycrystalline HgI₂

[en] An analytical expression for calculating the modulation transfer function (MTF) due to distributed carrier trapping in the bulk of the photoconductor of a direct conversion pixellated x-ray image detector is derived using the trapped charge distribution across the photoconductor. The analytical expressions of trapped charge distributions are also derived by solving the continuity equation for both types of carriers (electrons and holes). The MTF of photoconductive x-ray detectors is analysed in terms of normalized parameters, namely (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness) and (b) normalized carrier schubwegs (schubweg/thickness). Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the carriers that move away from the pixels improves the sharpness of the x-ray image. The MTF model is applied to polycrystalline CdZnTe detectors and is fitted to recently published experimental results. The theoretical model shows very good agreement with reported experimental data

[en] Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient C_r between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the concentration of x-ray-induced negative centers increases almost linearly with the x-ray exposure. As a corollary, in terms of fundamental physics of amorphous semiconductors, we can also conclude that the influence of potential fluctuations in the noncrystalline structure in shielding a charged center in a-Se is relatively small