Performance of optical imaging capillary plate gas detector with synchrotron radiation

We have been developing an X-ray photoelectron tracking imager based on a glass capillary plate (CP) gas detector for cosmic X-ray polarimetry applications. The optical imaging system consists of a CP gas detector filled with a noble-gas mixture and a cooled CCD camera coupled to an optical lens system. The performance of the optical imaging CP gas detector as an X-ray polarimeter was investigated with highly polarized, well-monochromized, and well-collimated X-ray beams at the synchrotron radiation facilities in SPring-8 and KEK-PF. Owing to the fine imaging capability of the optical imaging CP gas detector, images of X-ray photoelectron tracks were clearly observed for the polarized X-rays. A modulation factor of 26% was obtained for a 15 keV polarized X-ray beam, for which the energy resolution was 13%. We report on the properties of the optical imaging CP gas detector as an X-ray polarimeter and describe the outlook on future developments on the basis of the measured characteristics and the results of Monte Carlo simulations.