基于加速器7Li(p,n)反应的BNCT中子源及慢化体的优化设计

Optimization design of BNCT neutron source and moderating body based on accelerator 7Li(p,n)reaction

硼中子俘获治疗(Boron Neutron Capture Therapy,BNCT)作为一种有效的治疗癌症的方法,在全世界范围内已被广泛认可。基于^(7)Li(p,n)反应的加速器中子源具有反应阈值低、中子产额高、平均中子能量低、易慢化等优点,是最具潜力实现BNCT的加速器中子源。首先采用蒙特卡罗程序Geant4(GEometry And Tracking)程序模拟了不同能量的质子轰击不同厚度锂靶的中子源特性,提出用3 MeV的质子轰击0.2 mm厚度锂靶的加速器参数设计方案。随后开展了中子慢化体的模拟和优化设计研究,提出了一种“26 cm MgF_(2)+10 cm TiF_(3)”的组合作为慢化层结构,采用0.08 cm的^(6)Li作为热中子吸收层、1.5 cm的Bi作为γ吸收层。在加速器质子束流15 mA的条件下,模拟获得的慢化体出口超热中子注量率达到1.16×10^(9)n·cm^(−2)·s^(−1),快中子与超热中子比率(Φ_(epi)/Φ_(fast))为21.64,热中子与超热中子比率(Φ_(epi)/Φ_(th))为125.23,γ成分(D_(γ)/Φ_(epi))达到1.82×10^(−13)Gy·cm^(2)·n^(−1)(epi),可以满足BNCT的研究需求。

[Background]Boron Neutron Capture Therapy(BNCT)has been widely recognized around the world as an effective cancer treatment.The accelerator neutron source based on 7Li(p,n)reaction has the advantages of low reaction threshold,high neutron yield,low average neutron energy and easy slowing down.It is the most promising accelerator neutron source for BNCT.[Purpose]This study aims to design an accelerator-based neutron source for BNCT,and design a reasonable beam shaping model.[Methods]The Geant4(GEometry And Tracking)program was used to simulate the neutron source characteristics of protons bombarding lithium targets with different thicknesses at different energies.The accelerator parameter design scheme was proposed by using a 0.2 mm thick lithium target bombarded with 3 MeV protons.Then,the neutron beam shaping assembly(BSA)model was designed and optimized.A combination of"26 cm MgF_(2)+10 cm TiF_(3)"was taken as the moderating layer structure with 6Li of 0.08 cm taken as thermal neutron absorption layer and Bi of 1.5 cm taken asγabsorption layer.Finally,simulation of accelerator proton beam at 15 mA was carried out to calculate the optimal material combination and thickness.[Results]Simulation results show that the rate of epithermal neutron flux at the outlet of the moderator reaches 1.16×10^(9)n·cm^(−2)·s^(−1),the ratio of fast neutron to epithermal neutronΦ_(epi)/Φ_(fast) is 21.64,the ratio of thermal neutron to epithermal neutronΦ_(epi)/Φ_(th)is 125.23,and theγcomponent D_(γ)/Φ_(epi)reaches 1.82×10^(−13)Gy·cm^(2)·n^(−1)(epi).[Conclusions]The parameters at the BSA exit designed in this study meet BNCT requirements.