X-ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier

The results of an investigation into the x-ray properties of a superconducting tunnel junction (STJ) are presented. The photoabsorption of an x-ray photon by one of the thin superconducting films of the junction results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x-ray photon energy and the effective energy gap ε of the superconducting film in which the photoabsorption occurred. A charge output of 55% of the theoretical maximum has been obtained for a niobium-based STJ. Such a charge output indicates a mean energy ε of ≂4.7 meV is required to create a single charge carrier in the junction such that ε/Δ≂3, where 2Δ is the junction energy gap. This is the lowest value of ε/Δ obtained to date for x-ray photoabsorption in STJs. The energy resolution of the device is, however, still poor, with a full width half maximum of ≂200 eV for 6 keV x rays, compared with the theoretical Fano limited resolution of ≂4 eV. The principle mechanisms which are believed to degrade the resolution are discussed. These are principally quasiparticle recombination, phonon leakage out of the junction, quasiparticle diffusion into and out of the tunneling region, in addition to possible local variations in the energy gap. A Monte Carlo simulation of the nonequilibrium system has been performed. The results assist in the identification of the major charge loss mechanisms, and indicate various means by which Fano limited resolution may be obtained.