Emanation of radon and thoron from the soil

In a passive detector developed by the National Radiological Protection Board (Wrixon et al., 1988), the etched pits in the detectors are filled with scintillator fluid. After exposure to radon, the detector is irradiated with an alpha source, and the resulting scintillations counted with a photo-multiplier tube. In this way, track density over 1 cm2 of detector can be measured in a few seconds. Passive detectors used in the UK National Survey were sensitive down to 20 kBq m-3 h of accumulated exposure, equivalent to a radon concentration of 5 Bq m-3 measured over 4000 h exposure. 

Typical activities of building materials, such as bricks and concrete, in the UK and USA are in a similar range (20 to 50 Bq kg-1) (Nero, 1983), but granite used in older houses in Cornwall has about 100 Bq kg -1 of radium (O Riordan et al., 1982). In Sweden, Hultqvist (1956) found about 1000 Bq kg-1 radium equivalent gamma activity in lightweight concrete made from alum shale. 

To measure exhalation of radon a vessel called an accumulator is placed over the soil and sealed to the surface (Wilkening et al., 1972 Keller et al., 1982). To equalise pressures, the vessel is connected to atmosphere or to a bladder by a small vent. The build-up of radon in the accumulator is measured periodically or, in a flow-through arrangement, measurements are made by continuous withdrawal of air to a radon monitor (Schery etal., 1984). 

Numerous measurements of 222Rn exhalation have been reported. Global continental means of 15, 16 and 25 mBq m-2 s-1 have been calculated by Israel (1951), Wilkening et al. (1972) and Turekian et al. (1977), respectively. Higher rates of emission are found in regions of recent tectonic activity, and where the uranium content of the topmost rocks or soil is enhanced. 

The concentration of radium per kg in seawater is only about 10-4 of that in rocks. Exhalation of radon from the ocean has been studied to elucidate gas exchange at the surface (Broecker Peng, 1974). The emission is typically about 0.04 mBq m 2 s-1, very small compared with land. 

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