Hunter David M, Belev Gueorgi, DeCrescenzo Giovanni, Kasap Safa O, Mainprize James G, Rowlands J A, Smith Charles, Tümer Tümay, Verpakhovski Vladimir, Yin Shi, Yaffe Martin J
Sunnybrook Health Sciences Centre, University of Toronto, Toronto, Ontario, M4N 3M5, Canada.
Med Phys. 2007 Aug;34(8):3358-73. doi: 10.1118/1.2757002.
Blocking layers are used to reduce leakage current in amorphous selenium detectors. The effect of the thickness of the blocking layer on the presampling modulation transfer function (MTF) and on dark current was experimentally determined in prototype single-line CCD-based amorphous selenium (a-Se) x-ray detectors. The sampling pitch of the detectors evaluated was 25 microm and the blocking layer thicknesses varied from 1 to 51 microm. The blocking layers resided on the signal collection electrodes which, in this configuration, were used to collect electrons. The combined thickness of the blocking layer and a-Se bulk in each detector was approximately 200 microm. As expected, the dark current increased monotonically as the thickness of the blocking layer was decreased. It was found that if the blocking layer thickness was small compared to the sampling pitch, it caused a negligible reduction in MTF. However, the MTF was observed to decrease dramatically at spatial frequencies near the Nyquist frequency as the blocking layer thickness approached or exceeded the electrode sampling pitch. This observed reduction in MTF is shown to be consistent with predictions of an electrostatic model wherein the image charge from the a-Se is trapped at a characteristic depth within the blocking layer, generally near the interface between the blocking layer and the a-Se bulk.
阻挡层用于减少非晶硒探测器中的漏电流。在基于单线电荷耦合器件（CCD）的原型非晶硒（a-Se）X射线探测器中，通过实验确定了阻挡层厚度对采样前调制传递函数（MTF）和暗电流的影响。所评估探测器的采样间距为25微米，阻挡层厚度在1至51微米之间变化。阻挡层位于信号收集电极上，在这种配置下，信号收集电极用于收集电子。每个探测器中阻挡层和a-Se主体的总厚度约为200微米。正如预期的那样，随着阻挡层厚度的减小，暗电流单调增加。研究发现，如果阻挡层厚度与采样间距相比很小，它对MTF的降低可忽略不计。然而，当阻挡层厚度接近或超过电极采样间距时，在接近奈奎斯特频率的空间频率处，MTF会显著下降。观察到的MTF降低与静电模型的预测一致，在该模型中，来自a-Se的图像电荷被困在阻挡层内的一个特征深度处，通常靠近阻挡层与a-Se主体的界面。
