HPGe-based radiological baseline and uncertainty-propagated external exposure screening of beach sands from Rio de Janeiro State, Brazil

Beach sands may contain elevated concentrations of naturally occurring radionuclides because of sediment provenance, hydraulic sorting and local accumulation of heavy minerals. This study establishes an HPGe-based radiological baseline for beach sands from Rio de Janeiro State, Brazil, and evaluates external gamma exposure through an uncertainty-propagated screening approach. Forty-seven surface sand samples were collected from recreational beach sectors and analysed by high-purity germanium gamma-ray spectrometry. Activity concentrations of ²²⁶Ra, ²²⁸Ra, ²²⁸Th and ⁴⁰K were determined, and their reported analytical uncertainties were propagated to derived exposure quantities. To avoid double counting when applying a conventional whole-series thorium dose coefficient, a central ²³²Th-series screening equivalent was calculated as the arithmetic mean of the ²²⁸Ra and ²²⁸Th indicators. Sensitivity calculations using ²²⁸Ra alone and ²²⁸Th alone were also performed. Activity concentrations ranged from 1.10 to 1408.10 Bq kg⁻¹ for ²²⁶Ra, 0.03 to 5449.93 Bq kg⁻¹ for ²²⁸Ra, 0.42 to 6957.21 Bq kg⁻¹ for ²²⁸Th and 2.07 to 1008.36 Bq kg⁻¹ for quantified ⁴⁰K results. Canto was the dominant point-specific anomaly, with a central calculated absorbed dose-rate estimate of 4420 ± 169 nGy h⁻¹. Excluding Canto, the mean absorbed dose rate was 38.6 nGy h⁻¹, whereas the complete-dataset median was 16.8 nGy h⁻¹. Under all three thorium-series assumptions, the same eight sampling points exceeded the comparative outdoor value of 55 nGy h⁻¹ and only Canto exceeded 1 mSv y⁻¹ under the frequent local-use scenario. All short-term tourist-use estimates remained below 0.3 mSv y⁻¹. The 55 nGy h⁻¹ value is used only for comparison and not as a regulatory or action level. Most sampling points therefore represent lower-priority external gamma exposure conditions, whereas a limited number of local anomalies require confirmatory sampling, in situ dose-rate mapping and mineralogical characterisation before any beach-wide radiological protection conclusion is drawn.