用3He球形4π中子探测器, 通过直接反应法可以对核素的（n,2n）反应截面进行测量。在此类实验中, 要求对中子束进行很好的准直。好的准直器设计要求能够提高样品处的中子注量率的均匀性, 同时保证中子束流边缘处的下降幅度, 能够降低低能中子所占份额。本工作采用FLUKA和MCNPX对圆柱形准直器、单锥形准直器及三种不同斜率的双锥形准直器对中子注量率的均匀性和低能中子所占份额进行了对比研究, 结果表明, 低斜率的双锥形准直器可以满足实验对这两方面的要求。同时, 在有样品和无样品两种条件下, 对探测系统经由这五种不同准直器准直后的中子束流的响应做了对比, 结果显示, 经过低斜率的双锥形准直器准直的中子束流, 探测系统在有样品时表现为较高的计数率, 在无样品时表现为较低的本底。除此之外, 对准直器出口处的斜率对准直效果的影响也做了比较。最终中子准直器选用为这种低斜率的双锥形准直器, 材料选用为紫铜、不锈钢、聚乙烯和铅。准直孔开口处直径为2.64 cm, 长度为137 cm, 经准直后样品处的中子束斑直径为3.2 cm。

(n,2n) cross section can be measured through the direct measurement method with a 4π spherical 3He neutron detector. The neutron beam must be collimated in such experiments. A satisfactory design of the collimator is required to: (1) improve the uniformity of neutron flux at sample position and guarantee enough decreasing amplitude of the border of neutron beam at the same time; (2) reduce the scattered neutron background at low energy region. In this work, using FLUKA and MCNPX codes, the comparison between cylindrical collimator, conical collimator, and three double-truncated conical collimators with different slope on the uniformity of neutron flux and the proportion of low energy neutron was studied. The result shows that the double-truncated conical collimator with low slope can satisfy the experiment requirement. At the same time, the response of the detection system was compared after neutrons collimated by five collimators respectively with and without sample. The result shows that when neutrons are collimated by the double-truncated conical collimator with low slope, the detection system has the higher count rate with sample, and the lower background without sample. In addition, the collimation efficiency affected by the slope of collimator outlet was also compared. The double-truncated conical collimator with low slope was finally chosen as the neutron collimator and the materials of the collimator are red copper, stainless steel, polyethylene and lead. The diameter at the beginning of the collimator aperture is 2.64 cm and its length is 137 cm. The beam diameter after collimation at the sample position is 3.2 cm.