Measurement of X-ray polarization using optical imaging detector with capillary plate

The performance of the optical imaging capillary gas proportional counter (CGPC) was investigated using polarized X-rays with an energy of 15 keV at SPring-8. The CGPC with a fill gas of Ar+8% CH4+2% TMA at 1 atm was operated at a gas gain of 15 000. The CGPC was irradiated by a polarized X-ray beam in the center region using a slit of 300 μm×300 μm. Images of single photoelectron tracks were obtained by an image-intensified CCD camera combined with the CGPC through a lens system. The average distance between the starting point and the barycenter for single photoelectron tracks was 1.1 mm. The angular distribution is obtained from the directions of the lines connected between the starting point and the barycenter for each image of the photoelectron tracks. From a least-squares fitting, the modulation factor was 25.1±2.0% for the X-rays with a polarization degree of 99%.