3D absorbed dose distribution in a self-shielded portable gamma irradiator (SVHI-Co-60-T): MCNPX Monte Carlo Code...

Accurate determination of the absorbed dose distribution D_abs is essential for optimizing the efficiency and safety of industrial gamma irradiation facilities. Prior studies on the SVHI-Co-60-T portable ⁶⁰Co irradiator were limited to central axis measurements due to physical constraints, leaving the critical radial and angular dose profiles unvalidated. This work aimed to develop a three-dimensional (3D) dose map for the chamber using the MCNPX Monte Carlo code and to perform a experimental validation at different radii and selected heights and angular position. A novel mechanical dosimetry system was designed and implemented to overcome existing physical constraints, enabling experimental radial dose measurements at multiple heights and angles using Clear Perspex and RadixW dosimeters. The MCNPX Monte Carlo Code results showed excellent agreement with experimental measurements, exhibiting a maximum Relative Difference (RD) of less than 7% across all validation points. The full 3D dose mapping revealed that the maximum dose rate occurs near the wall (5.9kGy/h) in the source region, while shifting to the chamber center in the end regions (Z=±5cm). Product loading (black-eye pea) was found to reduce the absorbed dose rate by an average of 16% compared to the empty state, significantly weakening dose uniformity, particularly near the chamber center. The overall Dose Uniformity Ratio (DUR=D_max/D_min) was determined to be 1.35 (empty) and 1.37 (full), with an exceptionally uniform core region (DUR≈1.04) between R=±5cm andZ=±10cm. The validated MCNPX Monte Carlo Code and the comprehensive dose map provide crucial data for optimizing product loading strategies, maximizing efficiency, and ensuring rigorous dose control in this specific large-volume facility.