A STUDY OF THE RISE-TIME IN A MICROSTRIP GAS PROPORTIONAL COUNTER FORTHE DEVELOPMENT OF AN X-RAY POLARIMETER

The signal rise times in a microstrip gas proportional counter were in vestigated 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 invest igated as a function of fill-gas pressure, energy, gas gain, and reduc ed field. For lower pressure and X-rays with higher energy, it was obs erved that the rise time of signals from the MSGC is slower. We confir med from these results that the size of electron clouds produced by ph otoabsorption affects the rise time from the MSGC. Because electron cl ouds produced by photoelectric absorption extend in the direction of t he electric vector of incident polarized X-rays, we also investigated the dependence of rise time on the direction of X-ray polarization. Wh en the prototype counter was irradiated with polarized X-rays with ele ctric vector parallel or perpendicular to the MS-plate, a difference o f 21 nsec between the peak channels of the rise time distributions was observed.