(n,γ)反应实验研究中的中子屏蔽设计

用射线全吸收型装置(Gamma-ray Total Absorption Facility，GTAF)，可以对中子俘获反应截面进行高精度测量。为了降低实验本底，实验中需要对源中子进行准直和屏蔽，还要对被样品散射的中子进行吸收以减少它们进入探测器后所形成的干扰。采用MCNP对中子的准直器、屏蔽体和中子吸收体进行了模拟设计，中子准直屏蔽体材料选用含硼聚乙烯(BC4 的质量分数为3%) 和铅。准直孔直径为13 mm，长度为500mm，经准直后样品处中子束斑坪顶直径为21 mm。中子吸收体材料选用聚乙烯和碳化硼，吸收体球壳内腔半径30 mm，聚乙烯壳层厚度60 mm，碳化硼壳层厚度10 mm，被样品散射的中子经吸收体后衰减93.7%。Neutron capture cross section can be measured by Gamma-ray Total Absorption Facility (GTAF) with high precision. To reduce the background of experiments, the neutron source must be collimated and shielded, and the neutrons scattered from the sample must be absorbed to minimise interference after they go into the detector. The shield, collimator and absorber were simulated and designed with MCNP code. Boron-ontainingpolyethylene with 3% BC4 and lead are used as the materials for the neutron collimator and shield. The diameter of the collimating aperture is 13 mm, and the length of the collimator is 500 mm. After being collimated, the diameter of neutron beam plateau at the sample position is 21 mm. The neutron absorber is made of polyethylene and BC4, and the thickness of polyethylene shell and BC4 shell are 60 and 10 mm, respectively. The simulated result shows that neutrons scattered from the sample can decay 93.7% through the neutron absorber.