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.