HIGH-QUALITY SUPERCONDUCTING TUNNEL-JUNCTIONS ON NB AND TA SINGLE-CRYSTALS FOR RADIATION DETECTION

High duality Nb/Al-based tunnel junctions, fabricated with a supercond ucting interface onto thick single crystal x-ray absorbers of Nb and T a are discussed. Current-voltage characteristics, recorded at 0.5 K, s how a subgap current which is still dominated by thermally excited qua siparticles. The quality parameter R-subgap/R-normal reaches a value o f several million, which is unequalled for nonepitaxially sputtered tu nnel junctions. The fabrication process and some development steps, su ch as preparation of ultrasmooth crystal surfaces are described. Obser vations of x-ray photons absorbed in Nb and Ta single crystals detecte d by the superconducting tunnel junctions ar also presented, (C) 1997 American Institute of Physics.