An All-Sky Survey for tens of keV

The HEXIS (High Energy X-Ray Imaging Spectrometer) is a MIDEX-class mission concept to perform a deep survey and continuous monitoring of nearly the entire sky in the 5-200 keV band. It uses arrays of position-sensitive Cadmium-Zinc Telluride (CZT) detectors and coded masks to achieve 26/ resolution and a 5 sigma sensitivity (> 20 keV) of 4 milliCrab in one day and 0.2 milliCrab in one year. With these capabilities it is estimated that similar to 5000 sources can be discovered and localized and have their spectra and variability characterized. Hundreds of gamma-ray bursts would be detected each year and localized to < 20/. HEXIS also contains a narrow field, 5 degrees, coded mask imager for detailed studies of selected regions. This has three are minute resolution and is seven times more sensitive than the all-sky system. The HEXIS detector concept uses crossed strip readout to achieve 0.5 mm resolution pixels for large area arrays, similar to 400 cm(2). This technique is under development at UCSD and Washington University. Detectors have been studied with tuneable monochromatic x-ray beams and mapped with finely collimated 30 micron beams. These results show that the crossed strip readout has the necessary spatial and spectral characteristics. The HEXIS concept is described and results are presented on the detectors' spatial and spectral properties.