Radon in air - methods of measurement

Methods of measuring radon in air, have been reviewed by the US National Council on Radiation Protection Measurement (1988). The most commonly used methods are as follows. 

The air containing the radon is passed into a chamber. After sufficient time for the decay products down to 214Po to reach equilibrium with radon, the activity is assessed from the ionisation current. To allow for radioactive contamination in the materials of the chamber, two identical chambers have been used, one filled with the radon-bearing air, the other with aged air, and with the ionisation currents opposed. Using steel chambers of 6.31 capacity, Hultqvist (1956) measured 4 Bq m-3 of 222Rn with 10% accuracy. 

An alternative, for low activities of 222Rn, is to count individual alpha pulses in the ionisation chamber. Kraner et al. (1964) used this method to measure exhalation of 222Rn from the soil. To obtain maximum sensitivity, radon from a large volume of air is adsorbed in activated charcoal, and transferred to an alpha-pulse ionisation chamber in a flow of inert gas. 

Air with radon is passed into a vessel coated internally with zinc sulphide. Alpha particles from radon in the chamber, and from decay products deposited on the walls, give scintillations which are counted by photomultiplier tubes viewing the chamber through windows (Lucas, 1957). With a chamber of volume 0.11, and a counting time of 1 h the detection limit of 222Rn in air was about 10 Bq m-3, but by concentrat- 

To meet the need to monitor levels of 222Rn in houses, passive samplers have been developed which measure average concentrations over long periods and do not need power suplies. In the Karlsruhe dosimeter (Urban Piesch, 1981), a polycarbonate nuclear track detector foil is mounted inside a plastic cup. The mouth of the cup is closed with a filter to allow radon to enter but to exclude decay products. After exposure, the detector foil is etched and the tracks counted optically. This is a 

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