The operation of proportional counters at low pressures for microdensity
P.J. Campion · Physics in Medicine and Biology · 1971
The characteristic peak in the gain versus pressure curve, observed experimentally by some workers using low pressure proportional counters, is shown to be due to the Townsend avalanche extending from anode to cathode. Theoretical curves relating the variation in gain (due to the variation in position of the initial ionization) with operating pressure indicate that the limiting pressure for a typical Rossi type counter is about 1.5 torr for a gain variation of 10%. A method for reducing this limit by an order of magnitude is suggested.