Proportional counters for the Stellar X-Ray Polarimeter with a wedge and strip cathode pattern readout system

We describe the design and performances of the four imaging proportional counters for the flight model of the Stellar X-ray Polarimeter (SXRP) to be flown on the Spectrum X-Gamma Satellite at the focus of the SODART X-ray telescope. The SXRP will measure the linear polarization from celestial sources in an energy band between 2 and 15 keV. We describe the design and the performance of the X-ray detectors with particular care to the design philosophy for a space-borne focal plane polarimeter. We demonstrate the use of a wedge and strip pattern as a cathode which provides the desired position resolution in the whole energy band with moderate power consumption and only three preamplifiers per detector. The design, manufacturing techniques, and the performance of the wedge and strip position readout are described. Measurement of the background produced by impact of high-energy protons on different species of the material used to build the detector are presented. Also, the effect of high ionization charge density produced by end-of-range high-Z particles has been measured and the results are discussed.