Design optimization and validation of a DOI-based directional radiation monitoring system for real-time nuclide identification and source localization

The increasing use of radioactive materials requires real-time source localization and nuclide identification for potential radiation accidents. The initial depth-of-interaction (DOI)-based directional radiation monitoring system (DRMS) was designed to achieve 45° angular resolution to partition 360° into eight sectors, but penumbra degraded the resolution to 63°, requiring optimization. This study optimized and experimentally validated the DOI-based DRMS. The system consists of a CsI:Tl crystal, two PMTs, and a multi-slot collimator, with slots at different heights and directions. Gamma rays passing through a slot aligned with the source interact at a specific crystal height, and the two-PMT amplitude ratio estimates the interaction height. Simulations optimized slot shape (diverging vs parallel) and the collimator-crystal air gap as key variables. Comparing diverging and parallel types over a 0-30 mm gap showed that increasing the gap reduced penumbra and improved overall angular resolution, enlarged the entrance area and improved overall sensitivity, and that a diverging type with a 15 mm gap provided optimal performance. The optimized simulation achieved 48° angular resolution and 5.09 cps/MBq sensitivity; a prototype with the same design showed 48.33 ± 1.69° and 4.87 ± 0.08 cps/MBq, indicating quantitative similarity. Design guidance supports a field-deployable DRMS for emergency response.