Abstract

Efficiency of parallel spark chambers having different gap widths has been measured as a function of applied voltage and gas pressure for cosmic ray muons of mean energy 4 GeV in the pressure range 5–750 torr. A linear relationship analogous to Paschen's Law for the quantities threshold voltage, argon pressure and gap width has been found to hold with a gradient K = 141.4 dyne V −1 cm −1. With a fixed plate voltage 10 kV, the efficiency remained substantially flat at nearly 100% in the pressure range 100–550 torr. Beyond these limits it was observed to fall steeply. The efficiency characteristics with argon fillings below 100 torr for the same plate voltage have shown that the threshold pressure down to which only defined sparks are possible was 30 torr; with an alcohol contamination of 20% with argon this pressure could be further lowered to about 22 torr. Below this pressure undefined sparks in addition to defined ones appeared across the spark gaps along the path of the ionizing radiation. The behaviour towards the net efficiency of defined and undefined sparks was analysed.

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