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 similar to 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. (c) 2005 Published by Elsevier B.V.