ESCAPE OF PHOTOELECTRONS: A MAJOR ENERGY RESOLUTION DEGRADING MECHANISM IN THIN SUPERCONDUCTING TUNNEL JUNCTION X-RAY DETECTORS

Previous European and Japanese work using granular tin tunnel junctions has established that there is a detectable excursion in the subgap current caused by the non-equilibrium quasiparticles excited by the energy of single x-ray photons. Energy resolution better than 1% for 1 to 10 keV incident photons is predicted by simple models. This would be sufficient for space based astrophysical applications. Because of their advanced fabrication technology and high operating temperature, we are experimentally testing devices with Nb electrodes ( absorbers). In general the theoretical limit is proving difficult to approach. The calculation presented herein demonstrates that Nb electrodes should be about 0.8 mu m thick to reduce the escape probability of photoelectrons of energy >= 5 eV during the first 1 ps of an event. Improved energy resolution and quantum efficiency should also result.