A 3D scan-driven method for activity estimation under complex source-detector geometries.

In gamma-ray spectrometry, efficiency calibration using geometry-matched certified reference materials is often impractical for irregularly shaped radioactive sources and off-axis setups. Semi-empirical efficiency calibration software packages have been developed to address this issue; however, because these tools rely on simplified assumptions and user-defined parameters, they have limitations in covering complex source-detector geometries, leading to significant deviations in activity estimation. This study presents a three-dimensional scanner-based method that directly models the complete source-detector geometry and incorporates their relative positions into Monte Carlo simulations for efficiency calibration. This framework enables precise activity estimation for complex geometries without relying on geometric simplifications. Experimental validations were performed using fabricated showerhead- and turbine-shaped reference materials, and the results were compared with those obtained using commercial efficiency transfer software. The proposed method reproduced certified activity values with deviations of up to ±15%, whereas the commercial software exhibited larger deviations depending on the assumed model dimensions. This result highlights the potential of the proposed method for reliable in-situ gamma spectrometry of irregularly shaped materials and nonstandardized measurement conditions.