Primary standards of radon

This paper describes the present status of radon ( Rn-222) reference standards developed by national metrology institutes. The main contributions to this field are reviewed with emphasis both on the traditional methods ( based on Ra-226 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.