The Systeme International de Reference and its application in key comparisons

The paper begins with the reasons which led to the installation of the Systeme International de Reference ( SIR) more than 30 years ago, describes its main features and presents briefly the way it works. Then more detail is given on the treatment of impurities and on the determination of the efficiency curve. The improvements, which have been made recently to update the SIR and make it more efficient, are also discussed. A brief survey of the status of the present SIR is then given before focusing on the new developments which are in progress to enlarge the scope of the SIR towards measurements of radiopharmaceuticals with short half-lives and of alpha- and beta-particle emitters.