【作者】 徐向晏；

【导师】 牛憨笨；

【作者基本信息】 中国科学院西安光学精密机械研究所 ， 物理电子学， 2001， 博士

【摘要】 随着信息数字化的迅速发展，数字成像技术成为X射线透视诊断成像发展的潮流，非晶硒+薄膜晶体管（TFT）直接数字平板实时成像系统性能优越，分辨率高，全面满足成像器件固体化、数字化、平板化的发展潮流。本论文即以研制非晶硒+TFT平板数字成像系统为最终目的展开实验和理论研究工作。 论文首先从非晶硒合金膜的制备工艺和性能测试展开研究。选砷作为抑制非晶硒晶化的合金元素，采用先制备硒砷合金材料，再以该合金材料为蒸镀源材料的制膜方法，成功制备了稳定的非晶硒合金膜，其载流子迁移长度可达1300μm以上（10V/μm场强），达到厚膜应用要求。 在常用X光子能量范围内，分析了X光子与非晶硒发生各种作用的概率，给出了初级X光子作用中的能量吸收，重点讨论了次级X光子重吸收过程。 以X射线与非晶硒相互作用的物理过程为依据，通过建立模型，以光学传递函数为分析手段，采用理论分析和数值计算方法详细考查了探测X射线成像的非晶硒器件（不包含读出系统）的空间分辨率，给出了单色X射线入射时非晶硒器件空间分辨率表达式。在以能量吸收和X射线谱分布为权重求和的基础上，详细考查了入射X射线具有谱分布时非晶硒器件的空间分辨率。合理设计非晶硒厚度，尤其注意减小入射角有助于提高空间分辨率。 在分析电荷收集效率和能量吸收基础上，引入一灵敏度概念，全面分析了非晶硒对X射线的灵敏度与载流子迁移特性、光子能量、非晶硒厚度和场强的关系。非晶硒载流子迁移长度至少应为非晶硒厚度的2倍，提高场强是增加灵敏度的有效方法。论文根据成像过程详细分析了非晶硒对X射线的量子探测效率DQE，增加非晶硒厚度和减小入射角可以有效提高DQE。成像系统的设计应权衡分辨率、灵敏度和量子探测效率，使综合性能最佳。 在薄膜的基础上，经过多次实验，摸索出了制备非晶硒合金厚膜的成熟工艺。通过实验研究了非晶硒合金厚膜对X射线的光电响应特性。测量表明，光电流密度与照射率成线性关系，光电流密度强烈依赖于场强，在10V/μm场强下，激发一电子空穴对所需的X射线能量W_±约为45eV。 分析了重要转换屏CsI:Na（CsI:TI）的荧光逸出效率、能量吸收以及对X射线的荧光转换因子，建立了较为完整的荧光转换因子模型。计算表明，荧光光子等效自由程至少应在转换屏厚度的10倍以上。 在考查非晶硒+薄膜晶体管（TFT）直接数字X射线成像器件的基础上，针对非晶硒原子序数偏低，对高能X光了吸收小的缺点，论文提出了一种将非晶硒和高原子序数转换屏结合起来的联合靶平板数字固体成像器件方案。联合靶转换屏首选Q：Na，分析了X光子人射联合靶时的作用概率、能量吸收和灵敏度。aSe(sl们a联合靶有效提高了对高能X光子的响应，可望在不增加非晶硒厚度、无额外复杂工艺条件下拓宽其能量应用范围。更多还原

【Abstract】 Digital x-ray imaging diagnosis is currently of great interest compared to traditional screen-film system. A large area, flat-panel, direct digital x-ray imaging detector, which uses amorphous selenium (a-Se) alloy layer for detecting x-rays and thin film transistor (TFT) for reading out signal, stands out among digital x-ray imaging detectors. Its excellent properties such as high resolution can meet almost all cases in diagnosing x-ray imaging. Experimental and theoretical studies are carried out for the goal of developing such a flat-panel digital x-ray imaging detector in this thesis. The stress is first put on the preparation of stabilized a-Se alloy film/layer. Arsenic is selected as alloying element for inhibiting crystallization of a-Se film. After long-playing probe the stabilized a-Se alloy films is successfully prepared by vacuum thermal evaporation, its carriers?transport properties are measured by time of flight technique, and about 1300 jim of carriers?mean ranges are achieved at the electric field of 1 OV/ pm The interaction probability of x-ray with a-Se is discussed in the medical used x-ray energy range, the absorbed energy in the primary interaction and the reabsorption of secondary x-rays in a-Se are studied in detail. According to physical interaction process of x-ray with a-Se, an expression of the optical transfer function (OTF) for a-Se layer is obtained after about ten facts related to the spatial resolution is studied theoretically, and the modulation transfer function (MTF), which is the absolute value of the OTF, is provided numerically for the monoenergetic x-ray photons. Based on the weighted averaging method of energy absorption and x-ray spectra, MTF of a-Se layer is further calculated for the case of x-ray spectra. Some important conclusion, such as decreasing a-Se layer thickness and the angle of oblique to improve the spatial resolution, and some useful parameters are obtained through above studies. After charge collection efficiency of a-Se layer is studied, a sensitivity model of a-Se to x-rays, related to charge carrier mean ranges, x-ray photon energy, layer thickness and electric field, is introduced, and the calculation has lead to some useful results. We also investigate the detective quantum efficiency (DQE) of a-Se layer to x-rays in detail, increasing a-Se layer thickness and decreasing the angle of oblique can improve DQE. Spatial resolution, sensitivity and detective quantum efficiency should be optimized while designing the imaging detectox. Based on the former preparation techniques, Thicker a-Se alloy layer is manufactured afler solving some new problems in techniques, and the photoelectric properties of the so-made a-Se alloy layer to x-rays are measured through experiments. The results show that there is linear relationship between x-ray photocurrent and x-ray exposure rate, and the sensitivity of a-Se strongly depends on the electric field. About 45 eV is needed for x-rays to release an electron-hole pair in a-Se at the electric field of 10 V/pin. The fluorescence escape efficiency of CsI:Na (CsI:Tl) phosphor is discussed, and a converting factor model of the phosphor to x-rays is proposed. The calculation shows that the fluorescence photon mean range should be above 10 times of phosphor layer thickness. A new x-ray imaging detector, which uses a-Se layer in combithation with high atomic number phosphor layer, is put forward on the basis of ti-Se + TFT imaging system. It has the advantage of higher更多还原