Tests of gases in a mini-TPC with pixel chip readout

Gases for potential use as targets for directional dark matter detection were tested in a prototype detector using two sequential Gas Electron Multipliers, or GEMs. The sensitive volume consists of a mini-TPC of 12 cm length and 7.5 cm diameter. An FEI3 pixel chip, developed for the ATLAS experiment, was used to produce spatial measurements with high resolution. An Fe55 source produced photoelectrons by X-ray conversions in the sensitive volume, and images of these were recorded by the chip. Spatial resolution plots are shown for the gases, which include the practical electron range of the photoelectrons and the effects of diffusion in the mini-TPC. Avalanche gain and gain resolution measurements were made for the four gases tested, at atmospheric and sub-atmospheric pressures: Ar(70)/CO2(30), CF4, He(80)/CF4(20) and He(80)/isobutane(20).