Dependence of gas gain on x-ray-absorbed position in the proportional counter

Abstract

The position dependency of gas amplification in the proportional counter (PC) is investigated. We have been developing one-dimensional position sensitive PCs for MAXI/GSC and HETE/WXM and found that anomalous gas amplification occurs in a high bias voltage, even while the PC is still operated in the proportional region. This effect depends on the position where the X-ray is absorbed. Therefore it appears as a hard tail, a soft tail, or a broad peak in the traditional PC, depending on the shape of the gain curve across the cell. It degrades the apparent energy resolution. Especially, a position sensitive proportional counter (PSPC) is operated with rather high bias voltage to give higher positional resolution. We encounter the difficulty to achieve good position and energy resolutions at the same time. In this work, we have examined the anomalous gas amplification for various gas mixtures of Xe + CO<SUB>2</SUB>, Ar + CO<SUB>2</SUB> and Ar + CH<SUB>4</SUB>, for gas gain up to approximately 20000, and for energies from 6 to 17 keV to understand the phenomena.