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1、华中科技大学 博士学位论文 基于碘化汞的X射线成像控测器理论与实验研究 姓名：桂建保 申请学位级别：博士 专业：光学工程 指导教师：牛憨笨 20071104 I 摘 要* 硬 X 射线相位衬度成像是一门极具前景的新技术， 主要用于观察弱吸收材料内部 的精细结构，同传统的吸收成像不同，它提取结构细节信息不依赖于 X 射线曝光样品 吸收强度的差异，而是通过重构物体内部不同区域的折射引起的相位变化信息，从而 获得更好的成像对比度。但是它的应用要求高灵敏度、高空间分辨率的大面积数字 X 射线成像探测器，为此，本文将研制一种全面符合该要求的成像探测器。 本文提出了一种直接转换 X 射线成像探测器的研制方
2、案， 该方案采用碘化汞(HgI2) 光电导材料和基于绿光半导体激光器的激光诱导放电(PID)信号读出方式， 采用极细的 线激光束扫描和窄的并行条形电极读出，从而可获得高灵敏度、高空间分辨率、大面 积数字 X 射线成像探测器。 本论文从总体方案设计入手，分析了本方案的特色和难点，给出了膜层和电极的 设计，探测器的制作工艺流程，并给出了读出电路、光机扫描和系统同步控制的初步 方案。 对探测器的主要性能参数进行了深入的理论分析和数值模拟，包括灵敏度、空间 分辨率、像素放电时间和零频探测量子效率 DQE(0)，给出了每个性能参数的计算模 型和计算方法，其中对空间分辨率和像素放电时间的计算分别用到蒙特卡
3、罗和迭代算 法，对有关影响因素作了数值模拟研究，给出了影响关系曲线和设计探测器时的一些 优化值。 根据 HgI2所特有的性质， 自行特殊设计并研制了 HgI2物理气相沉积(PVD)实验装 置，该装置不容易被 HgI2腐蚀，可以维持高的真空和长的寿命，并使周围环境不被污 染。利用该装置和低纯度(普通分析纯，99.5)廉价的 HgI2原料，对成膜工艺参数进 行了实验研究，获得了优化的工艺参数，在氧化铟锡(ITO)玻璃衬底上，制备了直径为 130 mm 的大面积高质量 HgI2多晶膜。该装置可用于生长更大面积的多晶膜，不同 膜厚，晶粒大小和晶体性能的样品可通过调整生长工艺参数获得。 _ *本课题得到
4、国家自然科学基金重点项目支持(批准号：60532090) II 对膜的基本物理特性和暗电流， X 射线响应特性进行了测试。 实验测试结果表明： 该多晶膜具有良好的(00l)生长取向， 暗电流密度小于 10 pA/mm2， 电阻率达 1013 cm， X 射线响应灵敏度为 16 C/cm2R 左右。这些结果与国外获得的性能相当。采用低纯 度原料制得高性能多晶膜，大大降低了探测器制作成本。 关键词：关键词：X 射线相位衬度成像 X 射线探测器 碘化汞多晶膜 激光诱导放电读出 空间分辨率 灵敏度 探测量子效率 III ABSTRACT Hard X-ray phase-contrast radiog
5、raphy is a promising novel technique for the observation of internal structures of low absorbing materials. By this technique, the extraction of structural details does not rely anymore on the analysis of the X-ray intensity fluctuations due to the samples absorption but on the reconstruction of the
6、 phase changes induced by the refraction of different domains inside the object and that causes a better imaging contrast than conventional absorption imaging. But applications of this technique rely on digital X-ray imaging detectors with large area, high sensitivity and high spatial resolution. In
7、 ordr to satisfy above all the imaging requirements, this paper will deal with the development of a high performance X-ray imaging detector. A direct conversion X-ray imaging detector based on mercuric iodide (HgI2) and photoinduced discharge (PID) readout by green wavelength semiconductor lasers wa
8、s put forward. Two-dimension images were achieved by ultra-thin linear laser beam scanning and the parallel signal readout based on multiple narrow strip electrodes, so it is possible getting an X-ray imaging detector with large area, high sensitivity and high spatial resolution. The overall plan of
9、 this detector system was given and also its operating principle, features and difficulties were analyzed. Further, the designs of films and electrodes and processing flow of this detector fabrication were given, and also the preliminary schemes of readout circuits, opticmechanical scanning and syst
10、em synchro control were given. Several primary performance parameters of this detector, sensitivity, spatial resolution, pixel discharge time and Detective Quantum Efficiency DQE, were analyzed in detail from theory and simulated numerically. The computing model and method of every performance param
11、eter were given, Monte Carlo algorithm and iterative algorithm was applied respectively for computing the spatial resolution and pixel discharge time, relative affecting factors were investigated and their relation curves were given, and also some optimized values for the design of this detector wer
12、e given. A unique physical vapor deposition (PVD) device for HgI2 deposition was developed and it can endure the corrosion of HgI2, keep high vacuum and long life, and also prevent the surroundings pollution since HgI2 is highly toxic. Depending on this device and low purity (99.5%) low-cost HgI2 so
13、urce material, polycrystalline HgI2 films have been grown IV with dimensions 130 mm in diameter onto ITO-coated glass substrate. The growth techniques used can be easily extended to produce much larger film areas and the thickness of the grown layers, size of the grains and crystallinity can be regu
14、lated in a controlled way by adjusting the growth parameters. The basic physics characteristic, dark current and response characteristic to X-ray were tested and the results show that the films have preferred crystalline orientation (00l), dark current density less than 10 pA/mm2, volume resistivity
15、 in the order of 1013 cm and X-ray response sensitivity of about 16 C/cm2R. The results can comparative with the foreign level and the fabrication of high performance HgI2 polycrystalline films grown from the low purity source material is very significative to decrease the development cost of detect
16、ors. Key words: X-ray phase-contrast imaging X-ray detector mercuric iodide polycrystalline films photoinduced discharge readout spatial resolution sensitivity Detective Quantum Efficiency(DQE) 独创性声明 本人声明所呈交的学位论文是我个人在导师指导下进行的研究工作及取得的研 究成果。尽我所知，除文中已经标明引用的内容外，本论文不包含任何其他个人或集 体已经发表或撰写过的研究成果。对本文的研究做出贡献的个人和集体，均已在文中 以明确方式标明
