Research on Coincidence Detection Efficiency Based on Positron Decay Nuclides
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摘要: 为了降低一回路压力边界冷却剂泄漏监测的测量下限，本文基于正电子衰变核素开展了符合探测效率研究。为了提高符合探测效率，采用蒙特卡罗程序Geant4对符合探测装置中的粒子输运过程进行了仿真模拟分析，研究了符合探测装置结构和探测器属性对符合探测效率的影响。研究结果表明：通过在滤纸源两侧添加β+吸收层可明显提升符合探测效率，采用0.5 mm厚度的铝或0.2 mm厚度的铁作为β+吸收层时符合探测效率最佳；由于不同类型探测器的能量分辨率存在差异，因此不同类型探测器达到最佳符合探测效率时的能量窗系数不同，NaI(Tl)最佳能量窗系数为14%，BGO最佳能量窗系数为26%，LaBr3(Ce)最佳能量窗系数为7%。本研究成果可为以18F核素为放射性示踪剂的一回路压力边界泄漏监测系统探测装置结构设计及符合判断逻辑设计提供参考。
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关键词:
- 一回路压力边界泄漏监测 /
- 符合探测效率 /
- 闪烁体探测器 /
- 正电子源 /
- Geant4
Abstract: In order to reduce the measurement lower limit of the primary pressure boundary leakage monitoring, the coincidence detection efficiency based on positron decay nuclides has been studied. In order to improve the coincidence detection efficiency, the particle transport process in the coincidence detection device has been simulated and analyzed by Monte Carlo code Geant4, and the effects of coincidence detection device structure and detector properties on the coincidence detection efficiency have been studied. The research results show that: adding β+ absorbing layers on both sides of the filter paper can significantly improve the coincidence detection efficiency, and the optimal coincidence detection efficiency is obtained when 0.5mm Al or 0.2mm Fe is used as β+ absorbing layer; due to the difference in energy resolution of different types of detectors, different types of detectors have different energy window coefficients for reaching the optimal detection efficiency. The optimal energy window coefficients of NaI(Tl), BGO and LaBr3(Ce) are 14%, 26% and 7% respectively. The results of this research can provide a reference for the structural design of the detection device and the coincidence judgment logical design of the primary pressure boundary leakage monitoring system with 18F nuclide as the radioactive tracer. -
图 2 四种探测器结构设计方案的Geant4计算模型
Figure 2. Geant4 Calculation Model of Four Kinds of Detector Structure Design Schemes
图 4 添加β+吸收层前后的粒子径迹对比（以方案三为例）
蓝线——β+射线；绿线——γ射线
Figure 4. Comparison of Particle Tracks before and after Adding β+ Absorption Layer (Taking Scheme 3 as an Example)
图 5 符合探测效率随β+吸收层材料和厚度的变化趋势
Figure 5. Variation Trend of Coincidence Detection Efficiency with the Material and Thickness of β+ Absorption Layer
图 6 3种晶体符合效率随能量窗系数的变化趋势
Figure 6. Variation Trend of the Three Crystal Coincidence Efficiency with Energy Window Coefficient
图 7 3种晶体k值随能量窗系数的变化趋势
Figure 7. Variation Trend of the Three Crystal k Values with Energy Window Coefficient
表 1 添加铝吸收层后四种探测器结构设计方案符合计数情况
Table 1. Coincidence Counting of Four Detector Structure Design Schemes after Adding Aluminum Absorbing Layer
探测器结构设计方案 方案一 方案二 方案三 方案四 符合计数类型 (1-2)符合计数 46850 46659 47053 46726 (1-3)符合计数 15 15 11 (1-4)符合计数 14 13 (2-3)符合计数 17 14 21 (2-4)符合计数 17 18 (3-4)符合计数 29985 30083 (5-6)符合计数 31273 总符合计数 46850 46691 77098 108145 总符合探测效率 0.004685 0.004669 0.007710 0.010815 探测器数量/个 2 3 4 6 单探头符合利用率 0.002343 0.001556 0.001927 0.001802 
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