Escape of photoelectrons: a major energy resolution degrading mechanism in thin superconducting tunnel junction X-ray detectors

The authors consider the consequences of fabricating integral X-ray detectors with thin electrodes, 0.17 mu m for Nb and 0.25 mu m for Sn, with 6-keV photons incident. A simple geometric argument demonstrates that energetic electron loss during the first 1 ps of each event will cause less than the full energy of the incident photon to be captured in a large fraction of the events. This reduces the ultimate energy resolution the devices can achieve. Use of thicker absorbers (electrodes) is thus highly desirable, both from this point of view and to increase the quantum efficiencies of the devices. Nb electrodes should be about 0.8- mu m thick to reduce the escape probability of photoelectrons of energy >or=5 eV during the first 1 ps of an event.<>