Superconducting tunnel junctions (STJs) can be used as fast high-resolution energy-dispersive X-ray detectors. STJs consist of two superconducting electrodes separated by a thin insulating tunnel barrier. Their operation as detectors is based on measuring the temporary increase in tunneling current after X-ray absorption in one of the electrodes produces excess charge carriers in proportion to the X-ray energy. STJ detectors offer an order of magnitude improvement in energy resolution over conventional Si(Li) or Ge detectors, because the superconducting gap is three orders of magnitude smaller than the energy gap in semiconductors. We have developed a Nb-Al-AlOx-Al-Nb STJ detector arrays and characterized them in synchrotron experiments. They have an energy resolution between 1.7 and 8.9 eV FWHM for photon energies between 50 and 1000 eV, and they can be operated at count rates exceeding 10,000 counts/sec per pixel. For STJ operation in synchrotron-based high-resolution X-ray spectroscopy, we have designed a two-stage adiabatic demagnetization refrigerator (ADR) with a cold finger to hold the STJ inside the UHV sample chamber at a temperature of ~0.1 K within ~15 mm of a room temperature sample. We will discuss the performance of the spectrometer with special emphasis on its response function and on recent upgrades to improve its efficiency. We will also present different representative applications in material science, such as the characterization of novel semiconductors and superconductors.

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