The Defect of Plateau Curve due to Electric Charge on the Source Backing Film in a Gas Flow Counter

Abstract

An effect of the electric charges on a radiation source in 4πβ and 27πβ gas flow counter was examined. This effect gave serious damage on counter characteristics such as plateau curve. The lower side anode of the box type 4πβ proportional counter was replaced by a source holder. A vibrating reed electrometer with resistor of 10⁹-10¹³ ohms or a constant voltage power supply were connected between the source backing film and the cathode, and current on the backing film was measured or some potential was supplied on the backing film. At the same time, the plateau curves of upper half 2πβ proportional counter were taken. ⁶⁰Co beta-ray source on an aluminum foil was placed on the source holder. The results were as follows: a) Plateau curves were shifted to higher supply voltage linearly with source potential. Ratio of the shift of starting voltage to source potential was 0.26-0.64, and it varied with the diameter of source backing film and anode shape. b) Higher electric resistance between the source backing film and the cathode of 22πβ counter gave higher plateau threshold voltage and longer plateau length, but the starting voltage was not changed. c) Finally, when the counting rate of 27πβ counter was about 10⁴ cpm, electric resistance between the source backing film and the cathode should be maintained lower than 10¹⁰-10¹¹ ohms.