Simulation study of neutron source for bimodal imaging target system based on low energy hgh current cyclotron
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摘要: 无损检测方法在各领域都上发挥着重要的作用，伽马射线、热中子成像均是重要的无损检测方法，各有优劣，在许多方面上具有互补性。热中子-伽马射线双模成像将两者融合，在兼具这两类射线检测方法优点的同时，与单一射线检测相比，还具有物质识别的能力。以原子能院正在研发的18 MeV回旋加速器为设计基础，利用质子加速器驱动的中子源可同时产生中子和伽马射线这一特性，通过模拟对双模成像中子源进行研究。其中选用具有高（p, n）反应截面的铍做中子靶产生中子，为得到热中子，用聚乙烯做中子慢化体和反射体。利用热中子和伽马束流在空间上的分布不同，通过设计在不同空间取向上分别引出这两种射线，实现一靶同时得到两种射线。此外，通过在聚乙烯上对中子引出口和伽马射线引出口的设计，进一步提高热中子束流和伽马射线束流的引出效率。Abstract:
Background Gamma and thermal neutron imaging are important non-destructive testing methods, which are complementary in many aspects. The thermal neutron and Gamma bimodal imaging can combine the advantages of both. Compares with single beam imaging, the bimodal imaging has the ability to identify different substances and the sensitivity to both nuclides and elements simultaneous.Purpose Utilizing the reaction between protons and target material producing neutrons and Gamma together, based on the 18 MeV cyclotron accelerator being developed by the Institute of Atomic Energy, this paper designs a bimodal imaging neutron source by simulation.Methods Beryllium with a high （p, n） reaction cross-section is selected as the neutron target to generate neutrons. To obtain thermal neutrons with higher flux, polyethylene is used as the neutron moderator and reflector. By the different spatial distributions of thermal neutrons and Gamma, these two types of radiation are separately extracted from different directions. Besides, by designing the neutron and Gamma exits on polyethylene, high neutron flux and Gamma beams are simultaneously obtained.Results After simulation optimization, the thermal neutron flux at the thermal neutron outlet can reach 1.78×1010 n/（cm2·s） , and the gamma dose at the gamma outlet can reach 2.23×104 rad/h.Conclusions This paper design a neutron source for thermal-neutron-gamma imaging based on the 18 MeV/1 mA cyclotron accelerator. The design efficiently extracts thermal neutron flux and gamma flux from a single target, implementing a single-target-dual-source configuration.-
Key words:
- bimodal imaging /
- neutron source /
- cyclotron /
- thermal neutron /
- gamma
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图 1 不同能量质子与锂和铍的（p, n）反应截面及模拟得到的不同能量质子分别与锂和铍反应得到的中子产额
Figure 1. The （p, n） cross section of the reaction between different energy neutrons and Li or Be and neutron yields produced by different energy proton with Li or Be by simulation
图 3 模拟得到的中子靶产生的中子角度分布及中子能谱
Figure 3. The neutron angle distribution and the neutron energy distribution produced by the target by simulation
图 4 模拟得到的中子靶产生的伽马射线角度分布及伽马射线能谱
Figure 4. The Gamma angle distribution and the Gamma energy distribution produced by the target by simulation
图 5 18 MeV质子与中子靶反应产生的中子在聚乙烯中的碰撞位置分布
Figure 5. the distribution of neutron collision positions in PE
图 7 热中子引出口开设在不同位置处引出的热中子通量的模拟模型及模拟结果
Figure 7. The simulation model and simulation result of the thermal neutron flux extracted when the thermal neutron outlet is in different positions
图 9 当伽马射线引出口凹槽底面直径不同时， 伽马射线引出口处伽马射线剂量以及热中子引出口处的热中子通量
Figure 9. The Gamma dose at the Gamma outlet and the thermal neutron flux at the thermal neutron outlet when the diameter of the Gamma outlet underside is different
图 11 热中子引出口处的中子能谱
Figure 11. The neutron energy spectrum at the thermal neutron outlet
表 1 不同材料的慢化本领和减速比[22]
Table 1. The moderation ability and deceleration ratio of different materials
moderator material slowing-down power/cm−1 gear ratio (thermal neutron) water 1.53 72 polyethylene (PE) 1.84 64 heavy water 0.37 12000 graphite 0.064 170 
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表 2 模拟得到的18 MeV质子与铍靶反应产生的中子经过不同慢化材料后慢化体内最大热中子通量
Table 2. The max thermal neutron flux in the moderator when the neutrons produced by 18 MeV proton and Be are moderated by different moderator materials by simulation
moderator material the most thermal neutron flux in the moderator/(n/(p·cm2)) water 1.27×10−5 polyethylene (PE) 7.42×10−5 heavy water 3.87×10−5 graphite 3.19×10−5
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