Experimental activation assessment of open-type yoke in a PET cyclotron by non-destructive/destructive γ-ray...

During decommissioning of positron emission tomography (PET) cyclotron facilities, metallic components-including the iron yoke-are typically classified as radioactive waste, incurring substantial disposal costs. In contrast to concrete, standardized decommissioning procedures for metallic components have not yet been established.

This study aimed to experimentally assess activation in a PET cyclotron and provide fundamental data for establishing decommissioning guidelines for metallic components. The investigation focused on residual activity in the yoke and center pole, which are among the heaviest structural parts.

A PET cyclotron that operated for 23 years at the Tokyo Metropolitan Institute for Geriatrics and Gerontology was examined 6 years after the cyclotron's complete shutdown using two complementary approaches: non-destructive γ-ray measurements and destructive core-boring analysis.

⁶⁰Co was the predominant residual radionuclide. On the outer surfaces, ⁶⁰Co specific activity typically ranged from 0.02 to 0.10 Bq/g, with many locations below the 0.1 Bq/g clearance level. Cross-sectional mapping showed exponential attenuation behavior from the interior outward. Higher activation in the center pole (up to ∼1.0 Bq/g) was observed. ⁵⁴Mn was one to two orders of magnitude lower than ⁶⁰Co at 6 years post-shutdown. Non-destructive and destructive results were in close agreement, validating the quantitative use of γ-detectors for near-surface activity. A hypothetical time-dependent decay estimation, based on the non-destructive/destructive findings, was performed to project a required cooling period to meet clearance.

Activation levels in PET cyclotron components can be effectively assessed using combined non-destructive and destructive approaches. The complex distribution of residual activity was observed in the yoke and center poles. Decay projections indicated that approximately half of the yoke would fall below the clearance level within 15 years and nearly all components within 23 years after shutdown. These findings provide essential benchmarks for improving simulation-based activation modeling and can be broadly applied to similar PET cyclotron systems.