Gas Scintillation Proportional Counters for High-Energy X-ray Astronomy

A focal plane array of high-pressure gas scintillation proportional counters (GSPC) for a balloon-borne hard-x-ray telescope is under development at the Marshall Space Flight Center. These detectors each have an active area of ~ 20 cm, and are filled with a high pressure (10 Pa) xenon-helium mixture. Imaging is via crossed-grid position-sensitive phototubes sensitive in the UV region. The performance of the GSPC is well matched to that of the telescope’s x-ray optics which have response to 75 keV and a focal spot size of ~ 500 μm. The detector’s energy resolution, 4% FWHM at 60 keV, is adequate for resolving the broad spectral lines of astrophysical importance and for accurate continuum measurements. Full details of the instrument and its performance will be provided.