Spatial inhomogeneities in the energy response of a tunnel junction detector due to penetration of Abrikosov vortices

For the application of superconductive tunnel junctions (STJs) as high resolution X-ray detectors the homogeneity of the detector response is of utmost importance. In this article it is shown how this homogeneity is degraded by the penetration of Abrikosov vortices (AVs) into the junction electrodes. These AVs may exist because of small misalignments in the parallel magnetic field, which is applied to suppress the Josephson current and Fiske steps. By means of Low Temperature Scanning Electron Microscopy (LTSEM) the response of the tunnel junction to local energy deposition is mapped and found to be reduced in areas where AVs are present. On the basis of these experiments the threshold perpendicular field at which vortices start to penetrate the junction is derived.