This paper describes the present status of radon (222Rn) reference standards developed by national metrology institutes. The main contributions to this field are reviewed with emphasis both on the traditional methods (based on 226Ra standards) and also on the new absolute methods that have become available since 1996, especially those more recently developed. The topics covered include a description of the various methodologies adopted, instrumentation used, uncertainties achieved and the internal and international comparisons performed to demonstrate the validity of the developed standards. © 2007 BIPM and IOP Publishing Ltd.
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