Soft x-ray (50-1800 eV) measurements of niobium superconducting tunnel junctions

Measurements are presented on the x ray response of superconducting tunnel junction (STJ) detectors, over the energy range of 50 - 1800 eV. This includes the measurement of the lowest x-ray energies published to date. Energy resolution and response linearity is measured as a function of device geometry. It is shown that self-recombination of quasi-particles leads to an energy non-linearity which depends on junction volume. The effect of count rate limitations on energy resolution is established for rates up to 10 kHz.