用于BNCT-SPECT的帕维亚TRIGA MARK Ⅱ反应堆热柱优化设计

Optimization of thermal column of Pavia TRIGA Mark II reactor for BNCT-SPECT experiment

针对坐落于意大利帕维亚大学的TRIGA Mark Ⅱ反应堆热柱结构进行优化设计,从而满足面向硼中子俘获治疗(BNCT)的单光子发射计算机断层成像(SPECT)研究要求。为提高计算效率并减小统计误差,对比分析使用SSW/SSR方法与直接使用反应堆为源项时热柱内照射位置处中子能谱,其结果基本一致,从而验证了SSW/SSR方法的可靠性。为在该反应堆开展BNCT中SPECT实验,热柱中子束需准直为笔形束。对比分析四种热柱优化方案下束流口处及探测器处热中子和光子通量:40cm长石墨(射束口5cm×3cm);0.5cm厚硼包裹40cm长石墨(射束口5cm×3cm);30cm长天然锂聚乙烯(射束口直径4cm);30cm长天然锂聚乙烯(20cm长射束口直径5cm,5cm长射束口直径4cm,5cm长射束口直径2cm)。结果显示,射束口处热中子通量分别为1.05×108,2.52×107,6.08×107和5.10×107#/(cm2·s)。综合考虑中子准直效果及光子污染,方案三具有最优性能。为后续进行BNCT-SPECT理论和实验研究提供了基础,从而有效促进BNCT剂量准确评估方法的研究进程。

The structure of thermal column of the 250 kW TRIGA Mark II reactor,installed at the University of Pavia,was modified to perform BNCT on the explanted liver of patients affected by multiple metastases,which is not suitable for Single Emission Computed Tomography(SPECT)in BNCT.The purpose of this work is to optimize the thermal column of the reactor particularly for SPECT imaging experiment for BNCT.To reduce the computational cost,surface source writing(SSW)capability,which scores the particle information at certain position and can be further used repeatedly,was employed at the thermal column in MCNP6.The neutron energy distribution of the irradiation position in thermal column was investigated with standard method and the SSW approach to validate the feasibility of the SSW approach.To perform the SPECT experiment for BNCT,the thermal column of the reactor was modified to collimate the neutron source to a pencil beam.Here,three materials with four configurations were compared through thermal neutron fluxes and gamma fluxes on beam port and at detector position,which included 40 cm length graphite(5 cm×3 cm for beam port),40 cm length graphite covered by 0.5 cm thickness boral(5 cm×3 cm for beam port),30 cm length natural lithium polyethylene(4 cm in diameter for beam port),and 30 cm length natural lithium polyethylene(5 cm diameter in 20 cm length+4 cm diameter in 5 cm length+2 cm diameter in 5 cm length).The results show that the neutron energy distribution obtained using SSW source were comparable with that produced by the original source,which proves the validity and reliability of the SSW approach.With the above four configurations as the beam collimator,the thermal neutron fluxes on beam port were 1.05×108#/(cm2·s),2.52×107#/(cm2·s),6.08×107#/(cm2·s),and 5.10×107#/(cm2·s)respectively.Although the configuration with graphite had a relatively higher neutron source on beam port,however,the neutron flux outside the beam port didn't reduce,representing a higher background for detector.Therefore,lithium polyethylene with a 4 cm diameter has the best performance in terms of the collimation of neutron source and gamma contamination.In conclusion,the SSW approach in MNCP6 showed good performance to reduce the computational resource with good agreement for BNCT-SPECT study.The configuration with 4 cm diameter and 30 cm length lithium polyethylene was selected as the collimator for neutron source.