Problems of Radon Screening in the Upper Layer of the Soil

Soil is known to be the main source of high radon concentrations indoors which contribute about half of the mean natural annual radiation exposure of the German population. It is an aim of precautionary radiation protection to collect information about regions with elevated radon concentrations. To screen the radon potential of the regions of interest, a simple measurement of radon at the surface is desirable. However, the upper layer of the soil is mostly influenced by metrological factors such as solar heating and rainfall, e.g. in the upper layer of the soil, as a consequence of a sudden rainfall, there occur short-term increases in the radon concentration by a factor of 3 and more. Conversely, a slow and long-term increase of the water content of the soil may reduce the diffusion coefficient of radon by several orders of magnitude. The daily heating of the atmosphere results in an enhanced gas exchange between the upper layer of porous and dry soils and the air above. Consequently, variations of the radon concentration by a factor of 5 arise according to the intensity of the heating and the porosity of the soil. Test measurements on radon rich soils show both diurnal and seasonal variations of the radon concentration in the upper layers. This result excludes the use of radon measuring devices with integration times less than one day for the radon screening of any area. A combination of alpha spectroscopy and solid-state nuclear track detectors is suggested to scan the radon potential of a soil. This method is neither expensive in equipment, time or costs, and nevertheless sufficiently accurate.