quote from Andy3055:
Some 30+ years back I worked for a company that supplied Gamma counters among other laboratory instruments to universities, research institutes and hospitals. One brand we dealt with was LKB based out of Sweden, if I remember correctly. Those machines always had 2 PMTs and the noise in one of them (not visible to the sample) was used to cancel out the noise signals from the "counting" PMT, leaving the "required" signals usable. The 2 PMTs had to be factory matched and replaced together if required, for obvious reasons. Does your design take this sort of thing into consideration?
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LKB still makes radiation counters. They were based in Sweden, but are now in Australia. The concept you were referring to is called coincidence counting. AFAIK, it has always been used in liquid scintillation counting. In crystal scintillation counting, it is used in high-end counters, but not low-end. The most massive example of coincidence counting is the LIGO system of gravitational wave detection.
Mike in California