The spectral response of superconducting tunnel junction X-ray detectors

The Advanced Detector Group at LLNL is developing superconducting Nb–Al–AlOx–Al–Nb tunnel junction detector arrays for high-resolution soft X-ray spectrometry. We have characterized the detector response function in the energy range between 50 and 2000 eV with monochromatic X-rays, in collaboration with the Physikalisch-Technische Bundesanstalt Laboratory at the BESSY II electron storage ring. The detectors have an energy resolution between 7 and 15 eV FWHM in that energy range, and a count rate capability of ∼10,000 counts/s per detector pixel. In addition, the detector response function exhibits several spectral artifacts, and although their magnitude is only on the order of 1% of the main line, they can reduce the sensitivity for X-ray analysis of dilute specimens if they overlap with the correspondingly weak emission lines. We have characterized these artifacts by scanning a collimated synchrotron radiation beam across the detector area. We identify their origin due to X-ray absorption at locations other than the main Nb absorber film, and outline future detector designs to reduce the artifacts and thereby extend the spectrometer sensitivity.