Radiologic assessment of a self-shield with boron-containing water for a compact medical cyclotron

The cyclotron at our hospital has a self-shield of boron-containing water. The amount of induced radioactivity in the boron-containing water shield of a compact medical cyclotron has not yet been reported. In this study, we measured the photon and neutron dose rates outside the self-shield during cyclotron operation. We estimated the induced radioactivities of the boron-containing water used for the self-shield and then measured them. We estimated the activation of concrete outside the self-shield in the cyclotron laboratory. The thermal neutron flux during cyclotron operation was estimated to be 4.72 × 102 cm−2 s−1, and the activation of concrete in a cyclotron laboratory was about three orders of magnitude lower than the clearance level of RS-G-1.7 (IAEA). The activity concentration of the boron-containing water did not exceed the concentration limit for radioactive isotopes in drainage in Japan and the exemption level for Basic Safety Standards. Consequently, the boron-containing water is treatable as non-radioactive waste. Neutrons were effectively shielded by the self-shield during cyclotron operation.