The properties of niobium superconducting tunneling junctions as X-ray detectors

The properties of superconducting tunnel junctions based on niobium are investigated. The limiting resolution of such junctions should be ≅4 eV for 6 keV X-rays. Currently only between 2 to 25% of the theoretical charge is detected. The principal loss mechanisms, which not only reduce charge but seriously degrade resolution, are found to be phonon loss to the substrate, and recombination of the excess quasi-particle population in both films. The phonon loss is probably due to relaxation phonons from quasi-particles relaxing towards the bandgap. The quasi-particle self recombination is a direct result of the very large excursion from equilibrium produced during the X-ray photoabsorption process. Finally 6 keV X-rays have been detected directly in sapphire crystals by using the niobium junction only as a detector of beamed ballistic phonons. The use of a suitable crystal as the X-ray absorber and phonon source opens up interesting possibilities for position sensitive spectrometers based on high quality niobium junctions. (orig.)