补偿型长中子计数器能量拓宽方法研究

The problem existed in High-energy neutron metrology has not been well solved so far, mainly because the response of monitor for High-energy neutrons sharply declines with the increase of energy, therefore the development of monitor for this kind radiation field needs further consideration. These emerging fields have brought new needs and challenges for monitoring techniques. Long counters are well known for their flat response function over a wide neutron energy range. Their simple and reliable characteristic and excellent discrimination against gamma rays make them ideal neutron detectors. However, it is not satisfactory in high-energy neutron fields because the response drops above 5 MeV. In this work, The response of high-energy will be enhanced by ulitilizing the principle that heavy metal can generate spallation neutrons with (n,xn)reaction. The long counter is designed and optimized using the Monte Carlo codes. The interaction mechanism between neutron and material is analyzed. The configuration of long counter is investigated to obtain idea response characteristics for energy extension. It has important significance for extending the application of long counter in high-energy neutron field and then can provide theoretic supports for high-energy and broaden-spectrum neutron metrology.

高能中子辐射场计量传递至今为止没能很好解决，主要存在监测器高能中子响应随能量增加急剧下降的现象，因此该类辐射场中子注量率监测问题有待进一步深化。长中子计数器简单可靠、易甄别γ射线，可在一定能量范围使用，然而对能量在5 MeV以上中子响应显著下降，限制了其在高能中子辐射场中的应用。本项目通过在长中子计数器慢化体中嵌入重金属材料，利用重金属材料的（n,xn）或裂变材料（n,f）增殖特性改善长中子计数器的高能响应。采用蒙特卡罗模拟计算方法，分析重金属材料嵌入方案对能量响应影响规律。开展补偿型长中子计数器结构方案研究，旨在获得1 keV-400 MeV能量区间理想响应特性，拓宽其适用能量范围。本研究为扩展长中子计数器在高能、宽谱中子辐射场中的应用有重要意义，为该类辐射场的量值传递提供理论支持。

高能中子辐射场计量传递至今为止没能很好解决，主要存在监测器高能中子响应随能量增加急剧下降的现象，因此该类辐射场中子注量率监测问题有待进一步深化。长中子计数器简单可靠、易甄别γ射线，可在一定能量范围使用，然而对能量在5 MeV以上中子响应显著下降，限制了其在高能中子辐射场中的应用。本项目通过在长中子计数器慢化体中嵌入重金属材料，利用重金属材料的（n,xn）增殖特性改善长中子计数器的高能响应。首先,分析了中子与金属材料的相互作用机理，选定几种材料作为研究样本。采用蒙特卡罗模拟计算方法，分析样本金属材料嵌入方案对能量响应影响规律。经理论分析和模拟计算，表明采用钨材料补偿法优化设计长中子计数器可以改善能量响应。在内层聚乙烯圆柱中增加了1cm厚的钨层，由于钨材料对中能中子的吸收，并与高能中子发生（n,xn）散裂反应，改善了长中子计数器“平响应”特性。其次，开展了补偿型长中子计数器结构方案对长中子计数器能量响应特性影响研究。基于De Pangher长中子计数器，最终设计的长中子计数器几何结构直径30cm，长37cm，由内慢化体和外层慢化体组成。内层慢化体直径5cm，长中子计数器前端面覆盖1个镉帽以减少热中子能量响应，并在位于内直径3.8cm处有直径2cm，深度12cm空气孔以提高低能中子响应。在直径4cm处嵌入一个厚度为1cm的钨材料。可获得从1keV-20 MeV能量区间内最大偏差为7.8%的能量响应特性曲线。本研究为扩展长中子计数器在高能、宽谱中子辐射场中的应用有重要意义，为该类辐射场的量值传递提供理论支持。