Minimum detectable activity in gamma-ray spectrometry--statistical properties and limits of applicability

Some statistical properties of the essential physical quantity "Minimum Detectable Activity" were analyzed, as well as the limits of its applicability in gamma-ray spectrometry. It was shown that the Minimum Detectable Activity is not linear with respect to the quantity of the measured sample and it is advisable not to calculate the Minimum Detectable Activity per unit mass or volume (in Bq kg-1 or Bq cm-3). The MDA has a clear decisive relevance to samples that have an identical geometry and chemical composition with that of the sample that was used to derive the MDA. The relative uncertainty of the Minimum Detectable Activity is approximately +/- 32%. It is recommended not to exclude from evaluations those results that are below minimum detectable activities.