ABSTRACT
Poisson statistics have been shown to apply to the radioactivity counting of α-emissions from plutonium in modern, spectrometry-based α-continuous air monitors (CAMs) in the presence of an interfering background of radon progeny (Hoover and Newton, 1998). The standard deviation (SD) of the reported plutonium concentration can be determined by operating the CAM of interest in the presence of interference from radon progeny. The SD can also be used to calculate the statistical false alarm rate as a function of the alarm set point. Conversely, the SD of the reported plutonium concentration can be used to calculate the acceptable alarm set point (which can be viewed as a form of minimum detectable activity, MDA, or concentration) for a desired false alarm rate. The integration time of the counting system and the required number of positive reports for triggering an alarm can be used to determine the alarm response time for the CAM. Note that the alarm response time differs both from the resolving time of the system (normally dened as the smallest time interval which must elapse between the occurrence of two consecutive pulses or ionizing events and still be recognized as separate pulses or events) and from the response time of a measuring system (which is normally dened as the time required after a step variation in the measured quantity for the output signal variation to reach a given percentage, usually 90%, of its nal value) for the rst time (IEC, 1993).
