THE USE OF AVALANCHE PHOTODIODES FOR THE DETECTION OF SOFT X-RAYS

Avalanche photodiodes (APDs) offer an attractive alternative to the gas flow proportional counter (GFPC) for the counting of individual soft x-ray photons. They are reasonably efficient detectors and have the capability of handling higher count rates than a GFPC. With the advent of intense monochromatic x-ray beams from synchrotron radiation sources, the ability to handle high count rates is becoming more important than the need for any energy resolution. Another attraction of using solid-state detectors is the possibility of multielement fabrication of devices: configured detector arrays can then be considered for a number of applications. For example, in the scanning transmission x-ray microscope such a detector system allows much more sophisticated imaging techniques, such as differential phase contrast imaging with a quadrant APD array. In this paper we compare results for several commercially available APDs used as photon counters with a GFPC over the energy range 200-700 eV. APDs operated in the Geiger mode with a simple passive circuit to quench the avalanche current pulses can achieve count rates of the order of 10(6) s-1, and this can be greatly improved upon with the use of an active quenching circuit.