Ra activity distribution of rocks in the Sopron Mts. (West-Hungary)

Radon gas is the largest natural source of human exposure to ionizing radiation and most of that exposure occurs in indoor air. Bedrock geology is an important factor in radon hazard evaluation of an area. The presence of rock types usually rich in uranium can be considered an indication of a potential radon hazard. In this study the average Ra activity concentration of the main rock types (orthogneiss, micaschist, leucophyllite) in the Sopron Mountains was measured by gamma-spectroscopy, to reveal the uranium rich areas. This work is focusing on the distribution of Ra among the different rock types of the Sopron Mountains with similar geological origin. The effect of different retrograde processes such as mylonitisation, fluid migration and argillitic–limonitic alteration on Ra activity concentration was investigated. A few anomalies occurred in these metamorphic rocks. One explanation of the high uranium concentration is the high radioactive level of the rocks before the metamorphosis, but we demonstrated the significance of the above mentioned secondary processes as well. At Nándormagaslat quarry the presence of radium anomaly we found in the limonitic alteration of weathered gneiss (range: 131–726 Bq kg) in fractures explains the high air concentration nearby in houses (96–2,051 Bq m) and in a corresponding tunnel (maximum 600 kBq m).