An Improvement of the Plateau Characteristic of a Low Background End-Window Type GM Counter by Introducing a Correcting Ring

Abstract

A low background end window type GM counter with a short anode wire of 1.1 cm length was constructed and it was filled with counting gas which was the mixture of 89.4% helium and 10.6% isobutane. The gas pressure was about 80-150 mmHg. And the plateau characteristic was measured by introducing 3-ray beam from a¹³⁷Cssource through the center of the window and parallel to the anode wire. Then it was found out that the plateau had large slope. Although there can be some reasons for the slope, considering the short anode wire and the low gas pressure which result in a few ionizing events in the counting volume, we supposed that the large slope would be due to the decreasing of the counting loss based on the increasing of the counting volume along the counter axis with increasing of applied voltage. Then in order to decrease the variation of the counting volume with applied voltage it was tried that the correcting ring was inserted at the end of the anode support to make the electric field near the anode wire uniform through the entire length of the wire. As a result the plateau characteristic was fairly improved and it was shown that the correcting ring was effective to decrease the dependence of the counting volume on applied voltage. Then in order to make sure that the electric field was improved by the ring some experiments were performed. As a result it was found that the correcting ring absorbed well electric lines of force which originated near the end of the anode support and that it fairly uniformed the electric field near the anode wire all over.