Response of niobium-based superconducting tunnel junctions in the soft-x-ray region 0.15-6.5 keV

The response of niobium-based superconducting tunnel junctions to irradiation with monochromatic soft x rays in the energy range of 0.15-6.5 keV has been measured, using monochromatized synchrotron radiation and a Fe-55 radioactive source. Nonlinearities in this response have been observed which depend on the size of the device and the thickness of the electrode in which the photon absorption takes place. The nonlinearities can be explained in terms of self-recombination of quasiparticles by means of a model based on the Rothwarf-Taylor equations. No variations in the mean energy required to produce one quasiparticle have been observed near the M and L edges of niobium. The linear dependence of the energy resolution on photon energy indicates that the resolution is dominated by spatial variations in the response of the devices.