A study of the rise time in a microstrip gas proportional counter for the development of an X-ray polarimeter

The signal rise times in a microstrip gas proportional counter were investigated as part of the development of an X-ray polarimeter. With a prototype counter, the characteristics of the rise time distributions of pulses from a MSGC (microstrip gas proportional counter) were investigated as a function of fill-gas pressure, energy, gas gain, and reduced field. For lower pressure and X-rays with higher energy, it was observed that the rise time of signals from the MSGC is slower. We confirmed from these results that the size of electron clouds produced by photoabsorption affects the rise time from the MSGC. Because electron clouds produced by photoelectric absorption extend in the direction of the electric vector of incident polarized X-rays, we also investigated the dependence of rise time on the direction of X-ray polarization. When the prototype counter was irradiated with polarized X-rays with electric vector parallel or perpendicular to the MS-plate, a difference of 21 nsec between the peak channels of the rise time distributions was observed.