A geology-based predictive model for outdoor background gamma radiation in Bulgaria: framework and preliminary spatial assessment

A developed model translates lithology into expected concentrations of ⁴⁰K, ²³⁸U-series, and ²³²Th-series progeny using literature-based transfer functions, and subsequently converts these concentrations into ambient dose equivalent rates. The outdoor radiation background across the territory of Bulgaria was assessed theoretically and mapped in terms of expected gamma dose rates and previously evaluated geogenic radon potential. The results highlight regional-scale variations associated with granite and metamorphic units (elevated values), consolidated and semi-consolidated sedimentary rock (suggested low values), and localized anomalies related to uranium-bearing formations. The model outputs will be validated against data from a planned national survey conducted in the framework of a fundamental scientific project. This integrated, geology- driven approach provides an effective baseline for future evaluation of outdoor radiation background, using integrated systems radiation surveys in Bulgaria.