Direct measurement of radon

Either this measurement can be made directly at the source or the time and date of sampling must be recorded so that, following measurement in the laboratory, it is possible to calculate back to the time at which the sample was taken. 

If the presence of radon 222 in a water sample can be verified in this manner, and if an airtight water sample is examined again after, for example, 5 half-lives and radon 222 radioactivity (residual activity) is 

Radon 222 is shaken out of the water and the discharge of an electroscope is measured in the gas phase following the establishment of equilibrium distribution between the aqueous and the gas phase. This simple, long-practised method is still proving its worth today. 

Measure the amount of water, e.g. 1 litre, with a graduated cylinder and pour into the zinc or brass vessel. Seal the vessel with a rubber bung and then shake it for 1 minute. This produces the vapour-space equilibrium between the liquid and gas phases. Carefully discharge any gauge pressure and then fit the dispersion cylinder and measuring head in position in the vessel. 

Commence measurement roughly 5 minutes after charging, so as not to record the short-lived thoron (radon 220). Measure the discharge of the electroscope once a minute for example at the start and then every 5 minutes. 

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Dalu, G. and G. A. Dalu, An Automatic Counter for Direct Measurements of Radon Concentration, Aerosol Science, 2 247-255 (1971). 

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