Technical Details of the Sigma Factor Alarm Method within Alpha CAMs

Some test must be applied to the low-level count data from alpha continuous air monitors (CAMs) to determine if the count is statistically significant (i.e., significantly above background). The method should be as sensitive as possible, automatically desensitizing when required due to radon progeny, yet in such a manner as to not exceed an acceptable false-alarm rate. The test that has been historically used within many alpha CAMs to accomplish these goals is the so-called sigma factor approach. In this paper, this approach is discussed with respect to four common CAM algorithms. These algorithms are referred to as the theoretical best possible algorithm, the 4-ROI algorithm, the tail-fitting algorithm, and the peak-fitting algorithm. Relative sensitivities are estimated for each algorithm.