Performance of Pixel-Readout Micro-Pixel Chamber with Analog-Readout System Used as X-ray Polarimeter

Abstract

We developed an analog-readout system for a pixel-readout micro-pixel chamber (µ-PIC) to be used as an astronomical X-ray polarimeter, and demonstrated that the sensitivity of the new system reached up to that predicted by a simulation. A pixel-readout µ-PIC is a micro-pattern gaseous detector with a fine position resolution and good stability at sufficient gain operation, and is suited for astronomical X-ray polarimetry. However, as shown by Katagiri et al. (2007), the sensitivity to X-ray polarization was found to be statistically lower by a significant amount than that expected from the simulation of Ueno et al. [Nucl. Instrum. Methods Phys. Res., Sect. A 525 (2004) 28] because of the readout system and background produced by the scattering of the X-ray beam in air. We therefore developed a new readout system and carried out a beam test with aluminum tubes that reduced the background. As a result, we demonstrated that for collimated beams, the modulation factors, which are indicators of the sensitivity to X-ray polarization, were 0.24±0.08 at 8 keV and 0.18±0.07 at 15 keV in a neon-based gas mixture, and 0.18±0.04 at 15 keV in an argon-based mixture. These values are consistent with those predicted by the simulation within errors.