Fiske modes in superconducting tunnel junction detectors

We are developing superconducting tunnel junction (STJ) X-ray spectrometers with high-energy resolution (<10 eV for keV X-rays) and high count rate capabilities (>10 000 counts/s). We have examined the influence of Fiske mode resonances on energy resolution, maximum count rate and area of Nb–Al–AlOx–Al–Nb STJ detectors. At low count rate, these detectors have an energy resolution of 2.3 eV FWHM at 70 eV. At 20 000 counts/s, the resolution is 8.2 eV FWHM at 277 eV. We observe the optimum detector performance when the influence of Fiske modes is minimized.