Measurement-based characterization of spatial and depth-dependent activation in medical cyclotron vault concrete for decommissioning: The case study of the Severance Hospital

This study evaluated the spatial and depth-dependent activation characteristics of the concrete vault of a non-self-shielded IBA 18/9 Cyclone after a cooling period of approximately 3.5 years. A total of 520 concrete samples were systematically collected using a 1 m × 1 m grid-based sampling strategy with depth-resolved characterization up to 45 cm. Activation-related elemental compositions were analysed using ICP-MS and radionuclides were quantified using HPGe gamma spectrometry. The radionuclide inventory was dominated by 152 Eu and 60 Co, accounting for more than 85% of the total activity in most samples. Spatial distributions exhibited heterogeneous behaviour on wall surfaces, whereas the floor showed comparatively homogeneous activation characteristics. The highest activity levels were observed on the right wall where Targets 1 and 3 were intensively utilized during operation. Elevated activity values were identified above and below the target elevation. Most radioactivity was concentrated within the first 10–15 cm from the concrete surface. Exponential regression analyses showed attenuation coefficients of 0.047–0.055 cm −1 . The average activity decreased below the clearance level within the 25–30 cm depth interval. The present study provides a comprehensive measurement-based dataset and a quantitative framework that are directly applicable to decommissioning planning for regulatory release of activated cyclotron facilities.