Airborne gamma-ray spectrometry mapping for relating radon concentrations to geological parameters in the Fen region, southeast Norway

Extremely high thorium and considerable uranium concentrations are observed in carbonatite rocks of the Fen Complex—an alkaline intrusive complex in southern Norway. Since uranium-bearing bedrock and its weathering products are responsible for increased radon-222 concentrations in nearby dwellings, knowledge about the uranium concentrations of the individual rock types is important for evaluating the associated health risk. Earlier core-sample and ground-based scintillator measurements were limited in relating geological setting to indoor concentrations of radon-222 in such a region with very small-scale geological variations. We have performed airborne radiometric measurements over the entire Fen Complex and the nearby town of Ulefoss. The processed airborne data show that regions dominated by different carbonatite types vary significantly in mean thorium concentrations, but have similar uranium concentrations. Despite the complexity of the region, the obtained thorium/uranium ratios have proven to be a well-suited measure to distinguish regions that are dominated by specific carbonatite types. Furthermore, derived ground-concentration maps enable us to compare uranium ground concentrations directly with indoor radon concentrations of 139 individual dwellings in the Fen region. A positive correlation between local uranium concentrations and percentage of dwellings with indoor radon concentrations > 200 Bq m -3 was observed in regions where bedrock or its weathering material crops out. Similarly, high radon concentrations were observed for all carbonatites, indicating that the associated health hazard is largely independent of the dominant carbonatite type. In regions covered by clayey marine sediments, gamma radiation from bedrock is strongly attenuated. Also, indoor radon concentrations are predominantly low because radon transport is strongly limited by the low permeability of the marine sediments.