Evaluation of secondary neutron distribution around the synchrotron in an ion therapy centre.

Secondary neutrons produced in ion therapy facilities pose unwanted radiation risks to both patients and occupational operators. In this study, dual-channel neutron detectors based on CR-39 were employed to measure the intensity distribution of secondary fast and thermal neutrons around an ion therapy accelerator. The capability of the detectors to detect fast and thermal neutrons was validated through calculations. The detectors were placed at 44 different locations in the accelerator room. After continuous irradiation for 39 d, the neutron distribution map was obtained using inverse distance weighting interpolation algorithm based on the discrete measured results. The findings revealed distinct differences in the distribution patterns of fast and thermal neutrons. An analysis of the neutron distribution and main beam loss locations showed that the beam dump was the dominant source of secondary neutrons. Additionally, a comparison with the results of prior research indicated a strong correlation between thermal neutron distribution and induced radioactivity production. This study highlights the importance of considering the distinct characteristics of fast and thermal neutrons in shielding design and offers practical insights for the safe operation of ion therapy facilities.