PCM-2 Manual.pdf - Voss Associates

A TYPE II false alann is defined as an alarm produced by activity (in addition to background) that is
below RDA. Illustrated in Figure 9 is the distribution of probabilities for varying activity levels
under two different count times -- all other parameters being held constant. In both instances, the
90% confidence level exists for the selected RDA of 110 Bq. Also, in both cases, diminishing
probabilities of an alann occurring are associated with diminishing levels of activity. However, the
longer count time causes the perfonnance of the instrument to more closely approximate the "Step
Function" that an ideal instrument would exhibit wherein zero probability of an alarm exists below
RDA= 110 Bq. and a probability of 1.00 exists above that value. What is suggested by Figure 10 is
that Type II alanns are minimized by operating a radiation monitor to alarm at RA(MAX) (fixed RDA)
with the maximum reasonable count time.
PCM2.MAN