Superconducting high-resolution X-ray detectors for metalloprotein L-edge spectroscopy

Abstract Superconducting tunnel junctions (STJs) can be used as high-resolution energy-dispersive X-ray detectors. STJ detectors are based on the measurement of an increased tunneling current from excess charge carriers that are excited above the superconducting energy gap by the absorption of an X-ray. Nb-based STJ detectors have a theoretical energy resolution limit below 5 eV FWHM for X-ray energies below 1 keV at count rates up to 10 000 counts/s. We have developed Nb–Al–AlOx–Al–Nb X-ray detectors and operated them in a cryostat below 0.4 K in synchrotron experiments. Their resolution varies between 4.6 and 8.9 eV FWHM for X-ray energies between 0.2 and 1 keV. We present fluorescence spectra of metalloproteins and discuss the potential of STJ detectors for fluorescence-detected L-edge absorption spectroscopy of dilute samples.