Hard X-ray spectroscopy using small format GaAs arrays

We present preliminary results of X-ray measurements on a prototype 5×5 GaAs array. The device was fabricated from 40 μm thick epitaxial material and has a pixel size of 200 μm×200 μm and pitch 250 μm. Measurements from 5.9 to 100 keV were carried out both in our laboratory and at the HASYLAB synchrotron research facility in Hamburg, Germany. The average non-linearity in the spectral response is <1% across the energy range 5.9-98 keV. Using a 12 keV, 20 μm×20 μm pencil beam, the uniformity in a pixels spatial response was found to be better than 98%. The energy resolution at -40°C is 400 eV, FWHM at 5.9 keV rising to 700 eV at 98 keV. By best-fitting the expected resolution function to the entire data set, we derive a Fano factor of 0.1440±0.005, assuming trapping noise is insignificant. Further improvement in the energy resolution has been achieved by replacing the conventional resistive feedback preamplifiers with a new resistor-less design. In this case, an energy resolution of 266 eV FWHM at 5.9 keV has been achieved at room temperature (23°C) and 219 eV FWHM with only modest cooling (-31°C).