Visualization of low-level radioactive materials using an omnidirectional rotating Compton camera in an accelerator-based boron neutron capture therapy facility employing a lithium target

Abstract Objective. We report the visualization of low-level gamma radiation sources in an accelerator-based boron neutron capture therapy (AB-BNCT) facility employing a Li target. In this facility, irradiation of the Li target with accelerated protons produces neutrons and generates a large amount of radioactive 7Be with a long half-life of 53 days, while various other radioactive materials are suspected to be produced through neutron activation. Approach. To visualize gamma rays emitted from these radioactive materials across a wide energy range with high sensitivity, we prepared a high-sensitivity omnidirectional rotating Compton camera comprising six 3.5 cm × 3.5 cm × 3.5 cm CsI(Tl) scintillator cubes mounted on a motorized rotation stage. Measurements were carried out in the irradiation room 24 h after neutron irradiation. Main result. No leakage of 7Be from the Li target into the irradiation room was detected. However, some radioactive materials with long half-lives were observed in the irradiation port and on the bed. Additionally, gamma rays from 7Be remaining in a spent Li target, previously removed from the irradiation port and stored, were visualized. Significance. The Compton camera technique proposed and used in this study is shown to be effective for monitoring radioactive materials within AB-BNCT facility.