To understand detectors, we must first have a basic understanding of the statistics associated with measuring purely random events. The radioactive decay of a nucleus is a purely random event. There is no way to determine when a particular nucleus will decay. If two identical counts are taken in sequence on a sample of a long-lived radioisotope, the counts will almost always differ even though the activity of the sample is essentially unchanged. If a large number of these individual counts are taken, then the average Χe of these counts can be determined.
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