Charge Cloud Diffusion in a Sealed Position Sensitive Proportional Counter Filled with Xenon-Methane and Xenon-Carbon Dioxide

The influence of diffusion in a xenon filled imaging proportional counter has been studied in two gas mixtures, xenon-methane and xenon-carbon dioxide. As expected in xenon-methane, by increasing the charge cloud diameter from 280 to 1130 microns, interpolation between the 2mm pitch anode wires is greatly improved. The use of a long diffusion region also delays secondary avalanches and consequently improves the resolution. In xenon-carbon dioxide the charge clouds are smaller, and the improvement in interpolation is less. The resolution in this case is not limited by secondary avalanches and diffusion causes a very small degradation.