Cold season dose rate contributions from gamma, radon, thoron or progeny in legacy mines with high natural background radiation

In areas with high natural background radiation, underground cavities tend to have high levels of airborne radionuclides. Within mines, occupancy may involve significant exposure to airborne radionuclides like radon (Rn-222), thoron (Rn-220) and progeny. The Fen carbonatite complex in Norway has legacy mines going through bedrock with significantly elevated levels of uranium (U-238) and especially thorium (Th-232), and significant levels of their progeny Rn-222 and Rn-220. There are also significantly elevated levels of gamma radiation in these mines. These mines are naturally chimney ventilated and release large volumes of air to the outdoors giving a large local outdoor impact. We placed alpha track detectors at several localities within these mines to measure airborne radionuclides and measured gamma radiation of bedrock at each locality. The bedrock within the mines shows levels up to 1900 Bq kg(-1) for U-238, 12 000 Bq kg(-1) for Th-232 and gamma dose rates up to 11 mu Sv h(-1). Maximum levels of airborne radionuclides were 45 000 Bq m(-3) for Rn-220 and 6900 Bq m(-3) for Rn-222. In addition, we measured levels of thoron progeny (TnP). In order to estimate radiation dose contribution, TnP should be assessed rather than Rn-220, but deposition-based detectors may be biased by the airflow of mine-draft. We present dose rate contributions using UNSCEAR dose conversion factors, and correcting for airflow bias, finding a combined cold season dose rate within these mines of 17-24 mu Sv h(-1). Interestingly, fractional dose rate contributions vary from 0.02 to 0.6 for gamma, 0.33 to 0.95 for radon and 0.1 to 0.25 for TnP.