Decommissioning procedure and induced activation levels in the vault walls of a 11 MeV self-shielded Medical Cyclotron: simulations and measurements

Abstract

This study outlines the decommissioning process of an 11 MeV self-shielded medical cyclotron operated for over 12 years at the Ospedale San Raffaele, Milano, Italy. A comprehensive analysis of neutron-induced activation in the vault walls was performed to guide safe demolition procedures. Using FLUKA Monte Carlo simulations and laboratory-based gamma and beta spectrometry, the activity of long-lived radionuclides, particularly ⁵⁵Fe, was evaluated across multiple wall depths. Results demonstrated that residual activity was primarily confined to the top 5 cm of the wall, with some samples exceeding the Italian clearance level of 1 Bq g^-1 for ⁵⁵Fe. Occupational exposure to demolition staff was assessed via external dosimeters and internal contamination checks, with all values reported below regulatory limits. These findings confirm that while personnel are not at radiological risk, certain components and wall materials must be classified as low-level radioactive waste (LLRW) and require long-term storage.

Keywords: Cyclotron; Dosimetry; Exposure; Occupational; Waste management.