How Induced Activated Accelerator Parts have an Impact on the Radiation Safety of a Proton Therapy Facility

Residual activity induced in particle accelerators is a radiation safety concern as the long-lived radionuclides produced by fast or moderated neutrons and impact protons can cause problems of radiation exposure. Staff involved in maintenance of the proton therapy installation or in the quality control measurements done for evaluation of the beam output need to be aware of this. This study presents dose rate and energy spectrum-based measurements done on 1) phantoms used during quality control measurements; 2) on the accelerator components that become activated due to the backscattered neutrons from the target and also due to the direct proton interactions and their secondaries. As a part of this study, a correlation is made between de continuously monitored neutron and gamma dose rate measurements in the cyclotron vault and the registered beam-on time. In this way we can evaluate how quickly the dose rate, measured at contact of these activated parts, goes down to a safe level after irradiation. These measurements are used to assess operator exposure and to evaluate the need for specific radiation safety measures during maintenance and regular quality control.