Synthesis of a Round Robin about measurement of gross alpha and beta in various waters

Since 2011 the CETAMA (Analytical Methods Committee) has organized 3 Round-Robin tests successively to determine the capability of laboratories to measure gross alpha and beta in various waters. Different parameters are considered such as the origin of radionuclides (natural and artificial) and the variation of the salt content (from a low mineralized solution to highly mineralized water such as seawater). Technical specifications are defined within the working group 'Water analysis' in order to solve some problems observed in measurement of gross alpha and beta activity concentrations. The objective is to identify the causes of a discrepancy between the values obtained from the total counts and the sum of the reference values of each radionuclide alone. These Round-Robin tests allow accredited laboratories (or nearly accredited) to train in order to obtain accepted results in French regulatory proficiency tests organized by the IRSN. From the reference values of the various radionuclides (delivered by a primary metrology laboratory) contained in the samples, the averages of the results obtained by the laboratories are calculated and compared. Synthesis of the results highlighted the difficulty in obtaining reference values for gross alpha and gross beta activity. A method is proposed, established from standard and official guidelines based on robust calculation, to determine these assigned values with their associated uncertainty. Feedback from the various interlaboratory comparisons led to suggesting recommendations, particularly in terms of calibration and sample preparation, which allow a significant improvement in the accuracy (trueness and precision) of the method. (authors)