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

<i>Objective</i>. We report the visualisation 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 radioactive7Be with a long half-life of 53 d, while various other radioactive materials are suspected to be produced through neutron activation.<i>Approach</i>. To visualise 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 motorised rotation stage. Measurements were carried out in the irradiation room 24 h after neutron irradiation.<i>Main result</i>. No leakage of7Be 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 from7Be remaining in a spent Li target, previously removed from the irradiation port and stored, were visualised.<i>Significance</i>. The Compton camera technique proposed and used in this study is shown to be effective for monitoring radioactive materials within AB-BNCT facility.