How does a coincidence counter operate?
In addition to legitimate pulse output when stimulated by light, photomultipliers tend to produce many false output pulses (“noise”) derived from the effects of ambient heat upon their sensitive surfaces. The size of these noise pulses and the size of legitimate pulses derived from the scintillations produced by all of the 3H and the less energetic 14C decay events are quite similar and with a single tube it is difficult to distinguish between photomultiplier noise and pulses from real events. However, in a coincidence counter, the sample is examined simultaneously by the two tubes which are each selected for inherently low noise. Events are only recorded when both pulse within a very short time (“coincidence resolving time”), typically 20-100 nanoseconds. The likelihood of two photomultipliers producing simultaneous noise pulses (“accidentals”) is small and individual pulses from either tube are rejected. This technique eliminates well over 99.9% of noise pulses.
