Characteristics and improvement scheme of dark-field imaging of high energy electron radiography
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摘要: 基于EGS5与PARMELA模拟软件组成的高能电子成像系统，对暗场成像的模拟研究发现，通过调节光阑位置实现的暗场成像结果存在失真现象。针对该失真现象提出的改进方案，消除了暗场成像结果的失真。通过对40 MeV电子透射7~224 μm的铝样品开展的成像模拟结果表明：40 MeV高能电子暗场成像技术在铝样品厚度小于25 μm情况下具有明显的面密度分辨优势，且空间分辨率达到μm量级，非常适用于高能量密度物质诊断。Abstract: The simulations of dark-field imaging of high energy electron radiography show that the dark field imaging is distorted in the case of large angle selected. In order to eliminate distortion, an optimized scheme is proposed in this paper. The results of high Energy Electron Radiography (HEER) simulations by optimization show that the dark-field image has better areal density resolution when the thickness of aluminum target is less than 25 μm, and the spatial resolution of dark-field imaging is about several microns. In summary, dark-field imaging of high energy electron radiography is ideal to thin warm dense matter specimen diagnosis.
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图 1 高能电子成像原理示意图(在线彩图)
Figure 1. The schematic of high energy electron radiography (color on line)
图 2 暗场成像原理示意图。将光阑位置设定为绿色区域，则为明场成像; 设定为红色区域，则为暗场成像
Figure 2. Schematic of dark-field image. When the aperture position is set in the green area, the image is bright-field; when the aperture is set to the red area, the dark field image can be got
图 5 40 MeV电子束透射不同厚度铝样品，电子数密度在Fourier面处沿x轴分布图
Figure 5. Distribution of electrons along x-axis at Fourier plane in the case of the 40 MeV electron beam passing through the aluminum specimen with different thickness
图 6 不同光阑中心位置对应的像平面上电子数密度分布图
Figure 6. Results of HEER for different aperture position setting
图 7 劈裂尺寸模拟值与理论值对比图
Figure 7. Comparison of theoretical and simulated values of the crack size
图 11 不同狭缝位置与对应入射角度情况下电子数分布拟合曲线对比图
Figure 11. Fitting curve of electron number distribution at image plane for different aperture position and different incident angles setting
图 12 不同入射角度情况下，厚度分辨本领随厚度变化关系图
Figure 12. Capabilities of areal density resolution under different incident angle
表 1 靶平面至Fourier面处的传输矩阵参数
Table 1. Matrix parameters of transport from object plane to Fourier plane
R11F R12F/(mm·mrad-1) R33F R34F/(mm·mrad-1) -3.0×10-3 1.1 -6.7×10-3 -5.3 
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表 2 成像系统传输矩阵参数
Table 2. Matrix parameters of transport from object plane to image plane
R11 R12/(mm·mrad-1) R21/(mm·mrad-1) R22 R33 R34/(mm·mrad-1) R43/(mm·mrad-1) R44 -1.0 -6.67×10-3 4.8×10-3 -1.0 -1.0 -7.10×10-3 2.35×10-2 -1.0
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表 3 不同光阑位置对应的角度筛选区间
Table 3. Angle range selected for different aperture position setting
xaperture/mm angle selected/mrad xaperture/mm angle selected/mrad 0 －0.23~0.23 10 8.86~9.32 5 4.32~4.78 15 13.40~13.87
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表 4 不同入射角度情况下，对于不同厚度铝样品的空间分辨
Table 4. Spatial resolution for different steps and different incident angle
incident angle/mrad RMS spatial resolution for different steps/μm 7 μm 14 μm 28 μm 56 μm 112 μm 224 μm 0 5 6 5 4 2 3 4.54 3 2 10 9 14 5 9.09 3 1 7 5 2 3 13.64 1 5 2 10 3
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