Influence of bound nuclear effects on thermal neutron activation
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摘要: 在地表核泄漏场景下，辐射中子与物质中原子核经过多次散射后，能量很快降低到只有几个电子伏的热中子能区，热中子的活化将对核反应过程产生强烈影响。在固体、液体材料中，原子核通常以束缚态核的形式存在，在与物质相互作用反应方面，束缚核与气态的自由核不同。为更准确地评估核辐射效应，本文研究了束缚核效应对热中子活化过程的影响。使用蒙特卡罗方法模拟粒子输运，基于地表核辐射的场景建立了相应的空地界面模型，模拟了核辐射中子束入射土壤、海水以及混凝土产生的一系列核反应。以热中子的活化反应为重点，通过替换入射中子在介质中的反应截面引入束缚核效应，计算并对比了考虑束缚核效应影响前后活化产物次级γ的注量变化。研究结果表明，在活化过程中考虑束缚核效应的影响，能够使固液介质的热中子活化强度出现明显增强，从而在一定程度上增强地表次级γ场的辐射强度。由于元素组成、粒子屏蔽能力等因素的综合作用，三种介质场景下次级γ注量的最高涨幅分别为18%，8%和11%，涨幅随探测距离的变化规律也有所差异。Abstract: In a surface nuclear leakage scenario, radiation neutrons undergo multiple scatterings with atomic nuclei in the material, rapidly reducing their energy to the thermal neutron range (a few eV). The activation of thermal neutrons significantly impacts the nuclear reaction process. In solid and liquid materials, nuclei typically exist in bound states, differing from free nuclei in gaseous form regarding their interaction with matter. To accurately assess nuclear radiation effects, this study investigates the impact of bound-nucleus effects on thermal neutron activation. Using the Monte Carlo method for particle transport simulation, an air-ground interface model was developed based on surface nuclear radiation scenarios. The study modeled neutron beam interactions with soil, seawater, and concrete, focusing on thermal neutron activation reactions. By incorporating bound-nucleus effects through adjusted reaction cross-sections, the study calculated and compared changes in secondary gamma flux before and after considering these effects. The results show that accounting for bound-nucleus effects enhances thermal neutron activation in solid and liquid media, thereby increasing surface secondary gamma field intensity. Due to factors such as elemental composition and particle shielding, the maximum increases in secondary gamma flux were 18%, 8%, and 11% for the three media, with varying patterns of flux increase over detection distances.
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图 2 截断组与对照组的γ通量对比
Figure 2. Comparison of γ-flux between the truncated group and the control group
图 4 地表核辐射场景模型竖切面示意图
Figure 4. Vertical cross-sectional diagram of surface nuclear radiation scene model
图 6 探测环俯视结构(左)及横截面(右)示意图
Figure 6. Schematic diagram of the top view structure (left) and cross-section (right) of the annular detector
图 7 地表次级γ注量比较
Figure 7. Comparison of secondary γ-ray dose rates at the Earth’s surface
图 9 混凝土表面次级γ注量比较
Figure 9. Comparison of secondary γ-ray dose rates at the surface of concrete
表 1 土壤元素构成
Table 1. Element composition of soil
element mass fractions atom fractions H 0.023834 0.316843 O 0.598899 0.501587 Al 0.080446 0.039952 Si 0.296821 0.141618 
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表 2 海水元素构成
Table 2. Element composition of sea water
element mass fractions atom fractions H 0.107974 0.661583 O 0.858766 0.331501 Na 0.010785 0.002897 Mg 0.001284 0.000326 S 0.000906 0.000174 Cl 0.019471 0.003392 K 0.000399 0.000064 Ca 0.000415 0.000064
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表 3 混凝土元素构成
Table 3. Element composition of concrete
element mass fractions atom fractions H 0.004000 0.078437 O 0.482102 0.595591 Na 0.002168 0.001864 Mg 0.014094 0.011462 Al 0.069387 0.050830 Si 0.277549 0.195334 K 0.013010 0.006577 Ca 0.080229 0.039567 Fe 0.057461 0.020338
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