title: Outdoor thoron and progeny in a thorium rich area with old decommissioned mines and waste rock.
abstract: Radon (222Rn), thoron (220Rn) and their decay products may reach high levels in areas of high natural background radiation, with increased risk associated with mining areas. Historically, the focus has mostly been placed upon radon and progeny (RnP), but recently there have been reports of significant contributions to dose from thoron progeny (TnP). However, few direct measurements of TnP exist under outdoor conditions. Therefore, we assessed the outdoor activity concentrations of radon, thoron and TnP in an area of igneous bedrock with extreme levels of radionuclides in the thorium decay series. The area is characterized by decommissioned mines and waste rock deposits, which provide a large surface area for radon and thoron emanation and high porosity enhancing exhalation. Extreme levels of thorium and thoron have previously been reported from this area and to improve dose rate estimates we also measured TnP using filter sampling and time-integrating alpha track detectors. We found high to extreme levels of thoron and TnP and the associated dose rates relevant for inhalation were up to 8 μSvh-1at 100 cm height. Taking gamma irradiation and RnP into account, significant combined doses may result from occupancies in this area. This applies to recreational use of the area and especially previous and planned road-works, which in the worst case could involve doses as large as 23.4 mSv y-1. However, radon and thoron levels were much more intense on a hot September day than during time-integrated measurements made the subsequent colder and wetter month, especially along the ground. This may be explained by cold air observed flowing out from inside the mines through a drainage pipe adjacent to the measurement stations. During warm periods, activity concentrations may therefore be due to both local exhalation from the ground and air ventilating from the mines. However, a substantially lower level of TnP was measured on the September day using filter sampling, as compared to what was measured with time-integrative alpha track detectors. A possible explanation could be reduced filter efficiency related to the attached progeny of some aerosol sizes, but a more likely cause is an upwards bias on TnP detectors associated with assumed deposition velocity, which may be different in outdoor conditions with wind or a larger fraction of unattached progeny. There is thus a need for better instrumentation when dealing with outdoor TnP.
authors.affiliation: Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway; Centre for Environmental Radioactivity (CERAD CoE), P.O. Box 5003, NO-1432 Ås, Norway. Electronic address: Hallvard.Haanes@nrpa.no.
authors.forename: Hallvard
authors.initials: H
authors.lastname: Haanes
authors.affiliation: Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway.
authors.forename: Ingvild E
authors.initials: IE
authors.lastname: Finne
authors.affiliation: Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway.
authors.forename: Trine
authors.initials: T
authors.lastname: Kolstad
authors.affiliation: Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway.
authors.forename: Alexander
authors.initials: A
authors.lastname: Mauring
authors.affiliation: Buskerud Telemark Vestfold County Councils, Fylkeshuset, P.O. 2163, NO-3103 Tønsberg, Norway.
authors.forename: Sven
authors.initials: S
authors.lastname: Dahlgren
authors.affiliation: Norwegian Radiation Protection Authority, P.O. Box 55, 1332 Østerås, Norway.
authors.forename: Anne Liv
authors.initials: AL
authors.lastname: Rudjord
mesh_terms: D000396:Air Pollutants, Radioactive / Q000032:analysis*; D016902:Air Pollution, Indoor / Q000032:analysis; D001417:Background Radiation; D008906:Mining; D011829:Radiation Dosage; D011834:Radiation Monitoring; D011886:Radon / Q000032:analysis*; D017687:Radon Daughters / Q000032:analysis; D013910:Thorium / Q000032:analysis*
chemical_list: D000396:Air Pollutants, Radioactive; D017687:Radon Daughters; D013910:Thorium; D011886:Radon
keywords: Alpha track detectors; Decommissioned mines; Outdoor radon; Outdoor thoron; Outdoor thoron progeny; Waste rock