Prompt gamma activation imaging technology under idealized model
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摘要: 为了讨论PGAI技术分析的准确性，并验证冷中子和热中子应用于PGAI技术的可行性，通过蒙特卡罗模拟计算软件对PGAI技术理想化模型进行了研究，采用高准直的冷中子及热中子束和高纯锗探测器，对一块5 cm× 5 cm×1 cm均匀铁单质样品进行了模拟计算及图像重建，选取的等效体积大小为1 cm×1 cm×1 cm。结果显示：两种能量中子可以用于PGAI技术实现元素分布测量，但无论使用何种能量中子，由于物料体效应带来的中子自屏效应、中子散射效应以及伽马射线自吸收作用，即便在对均匀单质样品进行测量时，图像重建结果也无法保证各位置元素响应的一致性。因此，在后续工作中，需理清PGAI物理机制，建立相应的修正模型。Abstract: Prompt gamma activation imaging (PGAI) is a new type of element distribution imaging method in a non-destructive way. At present, the PGAI measure platforms which have been completed are all applied under the ideal model, and only for analyzing small pieces of sample by using the neutron source at the reactor. To discuss the accuracy and validate the feasibility of PGAI technology based on cold and thermal neutron, the ideal model of PGAI was analyzed by using Monte Carlo software. A homogeneous sample of 5 cm×5 cm×1 cm which consisted of pure iron was measured by adopting collimated neutron beam and the high-purity Ge detector, the dimension of the isovolume was 1 cm×1 cm×1 cm. It is shown that two kinds of energy neutrons all has been successfully used in PGAI. But no matter what kind of energy neutrons, when measuring the homogeneous single element sample, the consistency of image reconstruction results can't be guaranteed because of the neutron self-absorption, neutron scattering and γ-ray self-absorption. Therefore we need to analyze the physical mechanism and establish proper correction model in our later work.
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图 2 伽马射线强度随铅块的厚度变化
Figure 2. Relationship between intensity of gamma ray and thickness of lead
图 4 每列的特征伽马射线计数随中子穿透距离的变化
Figure 4. Relationship between characteristic counts of gamma ray and penetration of neutron in each column
图 5 模拟得到的样品内部目标元素的归一化计数的图像
Figure 5. Monte Carlo simulated target-element-normalized counts maps of sample
图 6 样品内部目标元素修正后的归一化计数的图像
Figure 6. Target-element-corrected normalized counts maps of sample
表 1 高纯锗探测器结构尺寸和材料
Table 1. Structure size and material of the high-purity Ge detector
structure of the detector parameter dimension/mm material crystal diameter 54 Ge crystal length 72.6 Ge Mount Cup base thickness 3 Al Mount Cup wall thickness 0.8 Al End Up incident window 0.5 Be dead layer outside thickness 1 Ge/B dead layer inside thickness 0.007 Ge/Li 
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