Abstract
An international intercomparison of activity measurements of a solution of selenium-75 was organized by the Bureau International des Poids et Mesures, France in May 1992. The solution was standardized by a 4π beta-gamma coincidence counting system using the efficiency extrapolation technique. The beta efficiency was varied by changing the high voltage of the proportional counter below the knee of the plateau, the absorber thickness and the solid content of the sources. The measurements were taken for two window settings of the γ-channel, namely, from 15 to 340 keV and 340 to 640 keV energies. The extrapolation was done by a linear least squares fit. It was found that the extrapolated values obtained by different methods differ by as much as 4%. The results of the measurements are discussed in detail. A correction factor for the metastable state was calculated using the most recent decay scheme parameters.
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