Investigation of epitaxial niobium based superconducting tunnel junction detectors

Superconducting Nb/Al-AlO sub(x)/Nb tunnel junctions (STJs) with epitaxial niobium base electrode are investigated as weak x-ray detectors. The recorded x-ray spectra show a two peak structure that is attributed to the absorption of 6 keV x-rays in the two junction electrodes. Low Temperature Scanning Electron Microscopy (LTSEM) proves, that the high charge output peak of the x-ray spectra is associated with x-rays absorbed in the epitaxial niobium film. The significantly reduced charge output for events in the polycrystalline niobium electrode is found to be bias voltage dependent.