Assessment of neutron shielding performance of a medical cyclotron after 10 years of operation

Abstract Purpose: Medical cyclotrons used for radiopharmaceutical production generate secondary neutron radiation during particle acceleration. This study aimed to quantify neutron radiation generated within a medical cyclotron and to compare neutron dose levels inside the cyclotron with those transmitted through the cyclotron self-shielding and surrounding structural barriers. In addition, the study evaluated the long-term effectiveness of neutron shielding materials after 10 years of cyclotron operation and assessed potential radiation risks associated with neutron leakage to verify radiation safety for occupational workers, the public, and the surrounding environment. Methods: Neutron dosimetry was conducted using passive optically stimulated luminescence dosimeters (OSLDs). A total of 33 dosimeters were deployed at 11 monitoring positions within the cyclotron facility: four positions inside the cyclotron, four positions outside the cyclotron self-shielding, one position within the high-density concrete shielding wall (80 cm thickness), one position outside the concrete wall, and one position inside the cyclotron control room. Three dosimeters were installed at each position to accumulate radiation doses over a three-month period of routine cyclotron operation. Following exposure, the dosimeters were analyzed using the OSLN readout algorithm to determine neutron dose levels and evaluate shielding performance. Results: Neutron doses inside the cyclotron ranged from 2 to 32 Sv, while doses outside the self-shielding ranged from 0.6 to 9.3 mSv. Within the concrete shielding wall, the dose was 0.8 mSv, while the dose outside the concrete wall was 0.1 mSv. Neutron dose levels in the cyclotron control room were comparable to natural background radiation. Conclusion: Neutron doses beyond the self-shielding and concrete barriers remained within safe limits despite high internal levels, confirming effective neutron attenuation after 10 years. Routine shielding verification and neutron monitoring are essential for continued radiation protection.