Residual radioactivity determination of brass aperture irradiated with wobbling proton beam by considering volume-source geometries

The purpose of this study is to determine the residual radioactivity of the patient-specific brass aperture induced by the proton beam of a single-ring wobbling system and the consequent radiation exposure to patients and staff. The gamma-ray spectra of the activated brass block were obtained using a high-purity germanium detector, following which the radionuclides were analyzed. Residual activity within the brass block was measured through the efficiency transfer method, and volume-source models were developed according to the distribution of the residual activity. The fitting equations for efficiency transfer factors were established with consideration of the activity distribution along the proton-beam direction. The ambient dose rate contributed by the activated brass was reduced to the level of the background radiation within approximately 3 h. Nevertheless, the residual activity of induced radionuclides with long half-lives, such as cobalt, which dominate the residual activity, should be monitored to ensure radiation safety.