Environmental radiological baseline characterization of a proposed medical cyclotron facility in Indonesia using dual-detector measurements and geostatistical analysis.

Establishing a robust pre-operational radiological baseline is essential for distinguishing pre-existing environmental variability from changes associated with future nuclear medicine operations. This study characterizes the ambient gamma-radiation field at a proposed medical cyclotron facility in South Cikarang, West Java, Indonesia. Nineteen monitoring locations were designated across heterogeneous terrain, with direct measurements obtained at 18 locations because Point B was unavailable. Ambient dose equivalent rates, H*(10), were measured at the ground surface and 1 m above ground level using complementary Geiger-Müller (GM) and NaI(Tl) detectors, with ten replicate measurements per configuration. Across all detector systems and measurement heights, H*(10) ranged from 0.030 to 0.120 µSv h-1, and median surface-to-1 m reductions were 11.8% for GM and 14.2% for NaI(Tl). Inter-detector agreement was strong, with Pearson r = 0.958 at the surface and 0.919 at 1 m. Ordinary Kriging identified spatial dependence over approximately 100 m, with higher values associated with engineered sand and crushed-stone fill and lower values generally associated with natural clay-dominated and vegetated ground. Screening-level annual external dose estimates ranged from 0.052 to 0.172 mSv y-1 and excess lifetime cancer risk from 1.76 × 10-4 to 5.86 × 10-4. The findings establish a site-specific pre-operational radiological baseline and demonstrate a transferable framework integrating measurement validation, uncertainty analysis, and spatial modelling for environmental surveillance around planned radiological facilities.