The Parallel Plate Imaging Proporticnal Counter and Its Performance with Different Gas Mixtures

Measurements of the position resolution of a parallel plate imaging proportional counter are presented. Diffusion of the electron cloud in the drift region is shown to be an important factor, which suggests the use of small drift depths and gases which give low average electron energies. Using pure propane and a drift depth of 2 mm, we have achieved a FWHM resolution of 150 ¿m. We also present a simulation of the effect of converging rays from an X-ray mirror, showing that for a typical mirror the resolution for sharp features is comparable with other components, but that with deep transparent counters, a large fraction of the absorbed events appears in the wings of the distribution.