Answer to Question #2018 Submitted to "Ask the Experts"

First, I would direct you to the references noted in ATE Q&A #807 by Tom Gesell.

Secondly, you should also review Section 4 of DOE's EML handbook HASL 300. In the discussion and Figures 1 and 2 you will note radioactive decay is a random event, following certain statistical principles. Additionally, a "background" count rate (for example, from cosmic ray and terrestrial gamma ray interactions) will be observed by your detector in a sample measurement. This too will be random in nature and exhibit a distribution about a mean. I believe what you are asking about in your question relates to what most health physicists refer to as the minimum detectable activity (MDA) and low limit of detection (LLD).

Consider the scenario where one has such a low count distribution that the background and sample count distributions begin to overlap (for example, shown in Figure 2). The LLD accounts for the background distribution and statistical "confidence level" and provides a point where one can say that the sample contains detectable net counts, given an acceptable risk of a false positive. The LLD is calculated by multiplying 4.65 times the standard deviation of the background count rate. The MDA is obtained by dividing the LLD by the detector efficiency (E in counts per disintegration), that is, to obtain activity. Other factors can be applied too, such as detector area or sample size to obtain other minimum detectable quantities.

David J. Allard, CHP