Induced radioactivity analysis of HIMM accelerator hall and treatment room under different operation modes

The operation of the Heavy Ion Medical Machine (HIMM) induces radioactivity primarily through neutron activation and secondary nuclear reactions. Investigating induced radioactivity within the accelerator hall and treatment room is crucial for guiding radiation protection designs for staffs and the public. The accelerator operates in two modes: commission and treatment, with continuous operation times of 12 h and 9 h respectively, followed by cooling periods of 12 h and 15 h. This study examines the induced radioactivity in both areas under these operational modes. The findings indicate that after one year of commission, the total absorbed dose for occupational workers in the accelerator hall remains below the regulated safety limit. Additionally, the residual dose rate returns to background levels after 30 days of cooling. The operational conditions of the preceding week influence the ultimate irradiation dose rate in treatment mode. The study also demonstrates that the induced dose rate in the treatment room depends solely on the last day of irradiation. Furthermore, this study evaluates the dual functionality of the water beam dump, which effectively absorbs the ion beam while minimizing activation of the forward wall. Since the radionuclides produced in water have short half-lives, regulatory exemption requirements can be met after a brief decay period. These findings provide valuable insights for radiation safety planning and operational protocols in heavy ion therapy facilities.