微结构阳极X射线源中电子与靶作用过程的蒙特卡罗研究

发布时间：2018-07-05 04:51

本文选题：X射线相衬成像 + 微结构X射线源　；参考：《深圳大学》2017年硕士论文

【摘要】：目前,限制X射线相衬成像向应用化发展的主要阻力是缺乏高亮度高相干的微结构X射线源,而新型微结构X射线源可以提供高空间相干的X射线光束,因此近年来它成为X射线相衬成像光源的研究热点。然而,由于国内外对入射电子在阳极靶内的电子沉积能量分布研究相对较少且研究不够深入,因此研究人员在设计微结构X射线源时,只能通过自身的经验结合电子射程经验公式值来确定具体参数,因而无法提高设计效率和避免较大的设计误差。更重要的是,微结构X射线源中微米级结构与尺寸对出射的X射线强度和角分布起到至关重要的影响,因此如何进一步研究高能电子与阳极靶作用的电子轨迹及能量沉积分布成为了亟待解决的问题。本文根据电子的多重散射理论,并基于蒙特卡罗方法进行数值计算,模拟了高能电子在钨、钼、铜、金刚石中的电子轨迹和沉积能量分布(代表电子入射到阳极靶后热能分布和初始的X射线强度分布)。计算结果表明:99%电子能量沉积在近似圆柱形区域内,相对于靶面上的电子束斑,当电子与靶原子作用时,产生X射线的区域更大(即X射线有效焦斑远大于电子束斑),其区域大小受到入射电子能量、靶材料等因素影响。同时,通过实验发现电子在入射方向上的沉积能量分布是会随着深度增加而加强,当到一定深度后才会出现递减,即沉积能量峰值点在距表面一定深度处。另外,在相同能量电子入射下,峰值点处沉积的能量不完全与靶材原子序数和密度呈正相关,如金刚石原子序数和密度远小于钼和铜,但金刚石的峰值点能量反而更高。然后,通过结合四种靶材的模拟结果和杜隆-柏替定理,我们得到不同阳极靶在20~100keV电子入射下所能承受的阳极临界电流值,它按高低排列依次是金刚石、钼、铜、钨。通过对比分析,我们发现5μm厚度的钨靶能够吸收能量为100keV的电子99%的能量,非常适合作为微结构靶,而金刚石膜的阳极临界电流最高,作为热沉材料有很大的潜力。在此基础上,本文深入研究了周期为3μm、占空比为1:3的微结构钨靶中出射X射线强度的角分布和光谱与入射电子能量、靶厚度的关系,发现微结构钨靶出射的X射线强度角分布呈高斯分布,X-ray主要集中在+Z方向±15o区域内,且Z=50cm处垂轴平面上的X射线分布类似于“M”型。因此,本文计算结果对于微结构X射线源和10μm以下、高亮度微焦斑X射线源的设计研究提供极为有价值的参考。
[Abstract]:At present, the main resistance to the development of X-ray phase contrast imaging is the lack of high brightness and high coherence microstructural X-ray sources, and the new microstructured X-ray sources can provide highly spatially coherent X-ray beams. Therefore, in recent years, it has become the research hotspot of X-ray phase contrast imaging light source. However, because the energy distribution of incident electron deposition in anode target is less studied at home and abroad, the researchers design microstructural X-ray source. The parameters can only be determined by combining their own experience with the empirical formula value of electron range, so the design efficiency can not be improved and large design errors can be avoided. More importantly, the micron structure and size of the microstructural X-ray source play a crucial role in the intensity and angular distribution of the emitted X-ray. Therefore, how to further study the electron trajectory and energy deposition distribution of high energy electron and anode target is an urgent problem to be solved. Based on the theory of multiple scattering of electrons and Monte Carlo method, the high energy electrons in tungsten, molybdenum and copper are simulated. The electron trace and deposition energy distribution in diamond (representing the thermal energy distribution and the initial X-ray intensity distribution after the electron incident to the anode target). The calculated results show that the electron energy of: 99% is deposited in an approximate cylindrical region, relative to the electron beam spot on the target surface, when the electron acts with the target atom, The region producing X-ray is larger (that is, the effective focal spot of X-ray is much larger than that of electron beam), and the size of the region is affected by the incident electron energy, target material and other factors. At the same time, it is found that the distribution of electron deposition energy in the incident direction will be strengthened with the increase of depth, and will decrease only after a certain depth, that is, the peak point of deposition energy is at a certain depth from the surface. In addition, at the same energy electron incidence, the energy deposited at the peak point is not completely correlated with the atomic number and density of the target, such as diamond atomic number and density is much smaller than that of molybdenum and copper, but the peak energy of diamond is higher. Then, by combining the simulation results of four kinds of targets and the Duron-Berti theorem, we obtain the anode critical current values of different anode targets under the electron incidence of 20 ~ 100keV, which are diamond, molybdenum, copper and tungsten in order of high and low. By comparison and analysis, we find that tungsten target with 5 渭 m thickness can absorb 99% of electron energy of 100keV, which is very suitable for microstructural target, while the anode critical current of diamond film is the highest, which has great potential as heat sink material. On this basis, the angular distribution of X-ray intensity and the relationship between X-ray intensity and electron energy and target thickness in micro-structured tungsten target with a period of 3 渭 m and a duty cycle of 1:3 have been studied. It is found that the X-ray intensity angle distribution of the micro-structure tungsten target is mainly in the Z direction 卤15o region, and the X ray distribution on the vertical plane at 50 cm is similar to that of "M" type. Therefore, the calculated results in this paper provide a valuable reference for the design and study of microstructural X-ray sources and X-ray sources with high brightness and microfocal spot below 10 渭 m.
【学位授予单位】：深圳大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O434.1

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