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When the sample activity is measured for various reasons several times, then with each measurement can be associated an individual decision threshold and limit of detection. Each measurement can be analyzed through its own decision threshold. The whole measurements can sometimes present contradictory results, some measurements being lower than the decision threshold and other higher. The problem then arises to build a decision threshold and a detection limit taking into account all the individual results, and to decide if the radioactivity is finally detected or not. It is interesting to note that it is possible sometimes that the decision threshold taking account all results makes it possible to decide that the radioactivity is present whereas the totality of the individual results are negative in terms of individual decision threshold. The purpose of this article is to show how these thresholds and these coherent limits cumulated can be determined in way according to the experimental conditions. In a general way a rigorous method of cumulating makes it possible to systematically decrease the decision threshold and limit of detection in terms of activity. This approach has interesting applications in gamma spectrometry with multi-emitters, discharge or periodical environmental measurements. On the basis of measurements realized by the IRSN within the framework of the national monitoring of the environment, we will see the potential impact of these methods on the final assessments.

RÉSUMÉThe use of decision threshold and detection limit concepts often poses many problems for metrologists in biological analysis laboratories in charge of the management of nuclear plant workers. These problems are frequently related to the understanding of a normalized formula or to the possible choice between several formulas whose numerical results are exceptionally significantly different. The true problems are in fact firstly the significance and the interpretation of these statistical tests, and secondly the numerical values used in these tests. Among those, the background estimate remains the essential and the most delicate value to define. The aim of this first article is to go back to the initial bases of the concepts of the decision threshold and detection limit. In the first step, a distinction will be made between the methods of decision threshold determination based upon definition of a priori first species error risks (i.e. before sample measurement) and the a posteriori interpretation of the sample Article publié par EDP Sciences A. VIVIER et al. 322 RADIOPROTECTION -VOL. 45 -N° 3 (2010) measurement result. In the second step, some examples of decision threshold and detection limit optimization in gamma spectrometry will be described. These examples will show that the essential parameter is the optimization of the estimation of background values, which requires the control of measurement rather than the management of statistical tools.

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