ABSTRACT
So far, we have dealt with the emission and attenuation of radiation as if they were entirely predictable processes, and unfortunately, they are not. For example, suppose the detection system records the number of photons from a radiation source striking a detector in a specied time interval. If this counting experiment is repeated multiple times, the number of counts recorded will vary. In fact, the number of counts would be quite random, and even if you knew exactly the number of counts recorded in one trial, you could not predict the number of counts in the next or in any subsequent one. To paraphrase Einstein, God apparently does play dice with the universe, which does not behave chaotically. However, the measurements of counts in this example are randomly distributed about some average value. If a large number of measurements are made, you can determine an average (or mean) number of counts. You can also estimate the standard deviation, which indicates the spread in the number of counts about the mean value. You can determine the “probability” that a certain range in the number of counts would be recorded for any time interval, but you cannot predict the exact count. erefore, while you can determine statistical descriptors, such as the mean value and standard deviation, using a large number of measurements, you cannot predict individual measurements. is is the nature of measurements associated with any random process.
