Radioactive scintillators have been used to calibrate photodetectors for many years. When these photodetectors are photon counters, an unusual calibration error occurs. Differences in the throughput or quantum efficiency of detection systems normally produce corresponding differences in the amount of light measured by the detection systems. But scintillators can produce pulses composed of several photons. When these sources are used to calibrate photon counting detection systems, differences in throughput can cause differences in pulse intensity rather than differences in pulse rate. Since photon counters are actually pulse rate detectors, differences in the pulse rate produced by photon counters may not accurately reflect differences in the throughput of these detection systems. This paper will present data and a model illustrating this problem, as well as a solution for the problem. The paper will also show how this effect can be used for absolute measurement of the efficiency of a detection system.
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