Characteristics of an X-ray imaging detector with double capillary plates

A capillary gas proportional counter (CGPC) with the double capillary plates has been constructed to investigate the image of X-ray photoelectron tracks at high gas gain. The double CGPC filled with a Penning gas mixture of Ar+8%CH4+2%TMA at 1 atm can operate at a gas gain of up to 4×104. The light yield was proportional to the gas gain. The electron extraction rate is 1.2% from the upper capillary to the lower one. The energy resolutions of the double CGPC were 35% for charge signals and 40% for light signals for 5.9 keV X-rays. With the double CGPC, clear images of photoelectron tracks were obtained for 22 keV X-rays at a gas gain of 2×104. From an analysis of the energy deposit along a photoelectron track, it was indicated that the track has two bright portions and a saddle portion with low brightness. The structure of a photoelectron track image obtained at high gas gain is important for application to an imaging type X-ray polarimeter.