LOW-TEMPERATURE SCANNING ELECTRON-MICROSCOPE MEASUREMENTS ON A NB TA JUNCTION/

Until now X-ray detectors based on superconducting tunnel junctions do not meet the theoretical expectations with regard to their energy res olution. To investigate a possible dependence of the signal on the abs orption position of the X-ray photon, measurements on a Nb/Ta-junction with a low temperature scanning electron microscope are performed. Th e results indicate a large lifetime of the quasiparticles as long as t hey are in the central part of the counter electrode. Near the edges o f the counter electrode the superconducting gap is reduced. For interm ediate and large bias voltages, this causes a strong reduction of the response at the edges, For small bias voltages the tunnel current of e lectron-like quasiparticles in the central part of the junction is can celed by the tunnel current of hole-like quasiparticles. For these sma ll bias voltages the signal is mainly created by tunneling from areas with a reduced superconducting gap which results in a flat response ov er the junction area, For very large amounts of energy dumped in the j unction self-recombination losses show up, especially at the corners a nd edges. (C) 1997 Elsevier Science B.V.