为了减少Am-Li中子本底对高浓铀部件质量主动多重性测量的影响,对大空腔探测系统(NPL-NMC)Am-Li中子本底的优化屏蔽进行了模拟研究,提出了一个基于高密度聚乙烯为中子屏蔽体的优化方案。通过对比模拟结果与屏蔽前实验测量结果,发现屏蔽使Am-Li中子本底探测效率明显降低,从原来的15.77%降为屏蔽后的1.94%,大约降低了87.7%;而屏蔽对裂变中子计数的影响却相对较小,只比屏蔽前降低约2.4%。本底中子计数的降低明显提高了系统对铀部件质量测量的灵敏度,在3000 s测量时间内,其质量测量下限从原来的大约6.4 kg下降到屏蔽后的2.6 kg;同时,屏蔽后的NPL-NMC系统在相同测量条件下,铀部件质量测量准确性提高50%以上。

In order to reduce Am-Li neutron background that had affected active neutron multiplicity assay for highly enriched uranium, optimization shielding of Am-Li was simulated for New Pile Laboratory-Neutron Multiplicity Counter (NPL-NMC). A cylindrical high density polyethylene was chosen as neutron shield with the radius of 15 cm. By comparing simulation results with the shielding to experimental results without the shielding, the Am-Li neutron background detection efficiency was significantly reduced, from 15.77% to 1.94%, reducing by about 87.7%. But shielding effect on fission neutron count was relatively small, only about 2.4% lower than the original. Reducing of the background neutron counts significantly improved the mass measurement sensitivity of uranium components. Mass measurement limit of uranium components dropped to 2.6 kg from the original about 6.4 kg with 3000 s counting time. Meanwhile, the measurement accuracy of uranium components was significantly improved in NPL-NMC, which could be increased by 50% under the same measurement conditions.