Superconductive tunnel junctions for X-ray spectroscopy

In order to investigate the influence of quasi-particle trapping on the performance of superconductive tunnel junctions as X-ray detectors, a series of Nb/Al/Al/sub 2/O/sub 3//Al/Nb junctions have been produced with different Al-layer thicknesses in the bottom electrode. The proximity effect between the Nb absorber and the Al trapping layer plays a dominant role, because it greatly influences the trapping time of excess quasi-particles from the Nb electrode to the Al trapping layer. A study of the influence of Al layer thickness on the operation of tunnel junctions as X-ray detectors is presented. Quasi-particle trapping is shown to be quite an efficient process in Nb junctions with Al-layers of 10 nm and thicker. The proximity model of A.A. Golubov and E.P. Houwman can explain the present data quite well. Nb junctions suffer from the bandgap reduction at oxidized and anodized surfaces. Without any special attention, extremely fast loss processes, about 30 ns, are present.<>