针对基于感应位敏阳极的光子计数成像探测器中非晶态Ge(α-Ge)膜的方块电阻对探测器成像性能的影响, 研究了方块电阻的选配范围和方法。由于方块电阻的大小会影响Ge膜上的电子云的扩散特性从而影响探测器的计数率和分辨率, 故本文根据菲克(Fick)扩散定律分析了吸收边界条件下非晶态薄膜上电子云的扩散特性。确定了电子云扩散时间与Ge膜方块电阻之间的数学关系, 推导获得了探测器高质量成像时非晶态Ge膜方块电阻的阻值为30~2 700 MΩ/□。采用具有不同方块电阻的感应位敏阳极进行了实际成像实验, 结果表明: 当Ge膜方块电阻在上述范围时, 光子计数探测器在计数率为53 kc/s时分辨率可以达到0.5 mm。实验结果证明了推导得出的方块电阻选配范围的正确性。

In consideration of the great influence of the sheet resistance of Ge thin film in a single-photon counting imaging detector on its imaging characteristics, this paper explores the selection of the resistance range. As the resistance values are relative to charge cloud diffusion on the Ge thin film, it will effect the counting rate and resolution ratio of the single-photon counting imaging detector. Therefore, the charge cloud diffusion characteristics on the Ge thin film under condition of absorbing boundary were analyzed based on FICK diffusion law and the relationship of the time of charge cloud diffusion on the thin film and the sheet resistance was calculated by the finite diffusion equation. The analysis results show that the photon counting detector has well spatial resolution and counting rate when the sheet resistance of Ge thin film is between 30 MΩ/□ and 2700 MΩ/□. The imaging experiments on Ge film anode with different resistance values were performed, and obtained data demonstrate that the resolution of the photo counting detector can reach 0.5 mm at counting rate of 53 kc/s when sheet resistance of Ge thin film is in the theoretical range, which proves the validity of resistance value selection range by the proposed method.