MODELING THE SPECTRA OF SUPERCONDUCTING TUNNEL JUNCTION X-RAY-DETECTORS

SIS tunnel junctions (STJD) are predicted to have exceptionally good e nergy resolution as x-ray detectors. This paper interprets spectra ill ustrative of the current state of the art. They were obtained when Fe- 55 and Cd-109 x-ray sources bombarded an epitaxial base layer Nb/Ta/Al /AlOx/Al/Nb junction having a diamond shape and sides 15 microns long. Our model of energy loss vial the escape of photoelectrons early in t he energy cascade is demonstrated to be qualitatively correct. Earlier results based on fine-grained junctions suggested that such small are a devices will have a large characteristic ''foot'' (asymmetric broade ning of low energy side) on the peaks in the spectra that correspond t o complete capture of the incident energy in the device. Our device is small, yet its full energy peaks lack significant feet. We conclude t hat this device is in the fast diffusion limit. This result contradict s the suggestion that Andreev reflection should severely restrict late ral diffusion in clean superconducting bilayers.