High resolution x-ray spectroscopy using GaAs arrays

We have produced a number of small format gallium arsenide (GaAs) arrays to address the material, electronic, and technological problems that need to be solved in order to develop mega pixel, Fano-limited spectroscopic x-ray imagers. Results will be presented of a series of x-ray measurements carrie d out on a prototype 5x5 array, fabricated from 40 mum thick epitaxial GaAs . The device has pixel sizes of 200x200 mum(2) and pitch 250 mum. As a prel iminary investigation of performance, two pixels have been instrumented. Me asurements from 5.9 to 98 keV were carried out both in our laboratory and a t the Hamburger Synchrotronstrahlungslabor research facility in Hamburg, Ge rmany. Both pixels were found to be remarkably uniform, both in their spect ral and spatial response to x-rays. The average nonlinearity in the spectra l response is <1% across the energy range 5.9-98 keV. Using a 12 keV, 20x20 mum(2) pencil beam, the spatial uniformity was found to be better than 98% over the entire pixel surfaces, consistent with the statistical precision of the measurement. The energy resolution at -40 degreesC is 400 eV full wi dth at half maximum (FWHM) at 5.9 keV rising to 700 eV FWHM at 98 keV. No d ifference in energy resolution was found between full area and pencil beam illumination. An analysis of the resolution function has shown that the det ector is dominated by electronic noise at low energies and Fano noise at en ergies above 30 keV. By best-fitting the expected resolution function to th e entire data set, we derive a Fano factor of 0.140 +/-0.05, together with a charge transport factor as low as 1.4x10(-3). Further improvement in the resolution function has been achieved by replacing the conventional resisti ve feedback preamplifiers with a new resistorless design, which provides a lower component of electronic noise. In this case, a resolution of 266 eV F WHM at 5.9 keV has been achieved at room temperature (23 degreesC) and 219 eV FWHM with only modest cooling (-31 degreesC). The expected Fano noise at this energy is similar to 140 eV. (C) 2001 American Institute of Physics.