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

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 cm3 cm for beam port), 40 cm length graphite covered by 0.5 cm thickness boral (5 cm3 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 1.05108#/(cm2s), 2.52107 #/(cm2s), 6.08107 #/(cm2s), and 5.10107 #/(cm2s) respectively. Although the configuration with graphite had a relatively higher neutron source on beam port, however, the neutron flux outside the beam port didnt 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.