Modeling the spectra of superconducting tunnel junction x-ray detectors

SIS tunnel junctions (STJD) are predicted to have exceptionally good energy resolution as x-ray detectors. This paper spectra illustrative of the current state of the art. They were obtained when /sup 55/Fe and /sup 109/Cd x-ray sources bombarded an epitaxial base layer Nb/Ta/Al/AlO/sub x//Al/Nb junction having a diamond shape and sides 15 microns long. Our model of energy loss via the escape of photoelectrons early in the energy cascade is demonstrated to lie qualitatively correct. Earlier results based on fine-grained junctions suggested that such small area devices will have a large characteristic "foot" (asymmetric broadening of low energy side) on the peaks in the spectra that correspond to complete capture of the incident energy in the device. Our device is small, yet its full energy peaks lack significant feet. We conclude that this device is in the fast diffusion limit. This result contradicts the suggestion that Andreev reflection should severely restrict lateral diffusion in clean superconducting bilayers.<>