In recent years, considerable effort has been expended in producing semiconductor based x-ray detectors for x-ray astrophysics with high spectral and high spatial resolution. In practical terms, this means producing pixelated detectors, comprising over 103 pixels each less than 100 microns in size, with spectral resolving powers, E/(Delta) E > 20 at 10 keV. While progress at soft x-rays wavelengths has been spectacular, largely due to the introduction of x- ray sensitive CCD's, progress at higher energies has been slow. This is because traditional high resolution detectors either suffer from poor detection efficiencies above 10 keV, as in the case of Si based technology, or are very constrained by cryogenic and fabrication problems as in the case of Ge based detectors. Recent developments in the material science of wide-gap semiconductors, and in particular GaAs and CdZnTe, have shown that it may now be possible to construct efficient hard x-ray detector with near Fano limited energy resolution. In this paper, we report on hard x-ray measurements with two prototype deep depletion epitaxial GaAs detectors of active areas 2.22 mm2 and thicknesses 40 and 400 microns at the ESRF and HASYLAB synchrotron research facilities. The results show that charge collection efficiencies must be in excess of 98 percent and that the material used to produce them is of extremely high purity, with impurity concentrations < 1013 cm-3.