Mass estimation equation for active neutron multiplicity counting considering the source to object distance
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摘要: 主动中子多重性计数测量方法是常用的核材料质量无损测量方法，已广泛应用于核材料衡算、核安保测量与军控核查等领域。我们通过对JMCT中子-光子输运程序的二次开发，实现了对经典点模型铀样品质量估算实验的数值模拟，并提出了改进的铀样品质量计算公式。该算法可以显著降低本实验中源-样品耦合与源中子反照等作用对铀样品质量估算精度的影响。建立了主动中子多重性计数测量探测系统模型和32个铀样品半球壳模型，模拟得到了与铀样品距离不同的DT源和AmLi源主动中子多重性计数，利用数值模拟手段检验了质量估算算法的有效性。数值模拟结果表明，改进的铀质量估算算法可以使质量估算的平均偏差率降低到10%以下。Abstract: Active neutron multiplicity counting method could provide the multiplicity distribution of neutrons multiplied from one fission event induced by an external neutron source in nuclear components. The equivalent mass of material in uranium component can be obtained by substituting the neutron multiplicity counting into the mass inversion equation with the known characteristic parameters. Therefore, this method is widely used in nuclear material accounting, nuclear security measurement and arms control verification. In this paper, Mont Carlo neutron transport calculation was used to simulate the experiment of the uranium mass estimation by classical point model equation. Based on this, an improved equation is proposed, which could significantly reduce the effect of source-sample coupling and neutron back irradiation on the accuracy of uranium mass estimation for the experiment. The validity of the mass equation is tested by numerical method, and the deviation between real mass and estimated mass obtained by the original point model equation and the improved mass equation is compared. The active neutron multiplicity counting simulation calculation is realized based on the original neutron transport simulation software. A model of active neutron multiplicity counting detection system and 32 models of uranium metal hemispherical shell are established. The active neutron multiplicity counts of DT source and AmLi source with different distances from uranium components are simulated and the estimated mass of the object are obtained with the simulated results. The numerical simulation results show that the average deviation of the improved uranium component mass inversion is reduced to less than 10%.
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Key words:
- neutron multiplicity counting /
- mass estimation /
- uranium metal component /
- arms control
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图 1 利用中子多重性计数方法估算铀样品质量的流程
Figure 1. Process of uranium mass estimation by neutron multiplicity counting method
图 2 裂变中子多重性分布的阶乘矩
Figure 2. Factorial moment of neutron multiplicity distribution for fission neutrons
图 4 经典点模型公式的质量估算结果
Figure 4. Mass estimation results by classical point model function
图 5 DT源的质量修正因子及其拟合结果
Figure 5. Mass correction factors and its fitted results for DT source
表 1 两种中子源的C-M关系方程参数
Table 1. Parameters for C-M relation functions for two kinds of neutron sources
source type a b c DT $ -1.00\times {10}^{4} $ $ 6.75\times {10}^{4} $ $ -7.54 $ AmLi $ -6.90\times {10}^{3} $ $ 8.44\times {10}^{4} $ $ 13.0 $ 
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表 2 两种中子源的质量校正因子
Table 2. Mass correction factors for two kinds of neutron sources
neutron source $ \alpha $ β DT 7.80 $ 1.64\times {10}^{-3} $ AmLi 8.20 $ 5.00\times {10}^{-5} $
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表 3 两种中子源的质量估算平均偏差率
Table 3. Average mass divergence rate for two kinds of neutron sources
neutron source average mass divergence rate by point model/% average mass divergence rate by our model/% DT 21 4 AmLi 19 9
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