EPITAXIAL BASE LAYER NB SUPERCONDUCTING TUNNEL-JUNCTIONS WITH TA ABSORBERS

Superconducting tunnel junction detectors have shown theoretical promi se in high resolution X-ray spectrometry. Here we present results for epitaxial Nb tunnel junctions with relatively thick Ta absorbers. In t hese devices the epitaxial Ta absorber also forms the base electrode f or the junction. We report on several junction geometries where both t he total size of the junction and the size of the absorber is varied. Current results show a maximum resolution of 65 eV at 5.89 keV, limite d primarily by external noise in the junction signal. Significant diff erences in junction performance are observed for the variation in abso rber size, although variation in total junction size from 15 x 15 mu m to 40 X 40 mu m shows no consistent effect on the resolution. The res ults indicate a significant loss of recombination phonons into the sub strate.