前言在用快中子治疗恶性肿瘤时,为了使中子束流集中轰击人体的肿瘤部位而尽可能不伤害健康组织,一般需要使用中子束准直器~([1])。准直器由准直孔引出照射肿瘤部位的有用中子束,由屏蔽层减弱和吸收其他非准直孔方向来的中子。在一定源强下,为提高有用中子束强度,就要求准直孔不能太长,即限制了屏蔽层沿准直孔方向的厚度;为达到屏蔽效果,就需选用宏观分出截面尽可能大的屏蔽材料。另外,还希望从准直孔方向引出的有用中子束中的低能成分尽可能地小,即具有较好的单色 Introduction In the treatment of neutrophils with fast neutrons, neutron beam collimators ([1]) are generally required in order to concentrate the neutron beams on the body’s tumor site without harming the healthy tissues as much as possible. The collimator leads the useful neutron beam that illuminates the tumor site by the collimation hole, and the shielding layer weakens and absorbs neutrons from other non-collimation holes. In a certain source intensity, in order to improve the useful neutron beam intensity, it is required that the collimation hole can not be too long, which limits the thickness of the shielding layer along the direction of the collimation hole. In order to achieve the shielding effect, it is necessary to select the macroscopic separation section as much as possible Large shielding material. In addition, it is also desirable that the low-energy component of the useful neutron beam exiting from the collimation aperture be as small as possible, i.e., have a relatively good monochromatic