Coincidence Counters

Author: J.B. Hoag

The principle of a coincidence-counter circuit may be understood from Fig. 30 J.

Fig. 30 J. A coincidence counter circuit. (From E. & N. P.)

At the left there are two G-M counter tubes, but any other sources of pulses can be used. It is seen that, by means of resistance-capacitance coupled amplifiers, the impulses are applied to the grids of two tubes (labeled 1 and 2). Of major importance, note that the output plate circuits of these two tubes are connected in parallel. Voltage changes across R are coupled to a thyratron tube in whose output plate circuit some kind of mechanical counting device has been installed. At the start, the grid of the thyratron T is highly biased so that its plate current is zero.

When impulses are applied to just one of the two joint circuits, say the upper one, the plate current in tube 1 is momentarily interrupted. The voltage drop in R is such that its upper end becomes less negative by a certain amount. However, with tube T adjusted well below its starting voltage, the small impulse across R proves insufficient to start its plate current.

On the other hand, if impulses occur simultaneously in the two input circuits, due perhaps to the passage of a single particle through the two G-M tubes or perhaps to the incidence of light simultaneously upon two photoelectric cells located at these points, then the plate currents of both tubes are reduced at the same time and the voltage drop across R becomes sufficiently great to start a current in the plate circuit of T, operate a recording mechanism and give a coincidence count.

Due to the necessary time for the various condensers in these circuits to discharge through their various resistors, the exact simultaneity with which the events occur at the input cannot be established closer than about 1/10,000 of a second.