TA-BASED TUNNEL-JUNCTIONS FOR ENERGY RESOLVING X-RAY-DETECTION

We report the results of a programme to develop an energy-resolving su perconducting tunnel junction based on a Ta X-ray absorber. The design is based on a vertical heterostructure consisting of a Nb underlayer to act as a quasiparticle reflector, a thick Ta absorber layer and an Al trap layer. The upper surface of this trap layer is oxidised to for m the tunnel barrier. The counterelectrode consists of an Al and thin Nb layer with thicknesses controlled to give approximately equal energ y gaps in both electrodes and minimal absorption in the Nb counterelec trode. The effect of electro-magnetic barrier resonances on detection performance has been evaluated. Very high quality junctions have been fabricated, tested and evaluated for X-ray energy resolution.