We are developing a high-pressure Gas Scintillation Proportional Counter (GSPC) for the focus of a balloon-borne hard-xray telescope. The device has a total active diameter of 50 mm, of which the central 20 mm only is used, and is filled with xenon + 4% helium at a total pressure of 10(6) Pa giving a quantum efficiency of greater than 85% up to 60 keV. The detector entrance is sealed with a beryllium window, 3-mm thick, which provides useful transmission down to 6 keV, well below the atmospheric cut-off at balloon float altitudes. Scintillation: light exits the detector via a UV transmitting window in its base and is registered by a Hamamatsu position-sensitive crossed-grid-readout photomultiplier tube. Initial testing is underway and preliminary measurements of light yield, energy resolution and spatial resolution will be reported. Simulations show that a spatial resolution of 0.5 mm FWHM or better should be achievable up to 60 keV, and this is well matched to the angular resolution and plate scale of the mirror system. The energy resolution has been measured to be around 5% at 22 keV. Full details of the instrument design and its performance will be presented. A first flight is scheduled for the Fall of 99, on a stratospheric balloon to be launched from Fort Sumner, New Mexico.
