Superconducting High-Resolution X-ray Spectrometers

S. Friedrich,1,2,3 T. Niedemayr,1 O. Drury,1,3 T. Funk,2 M. F. Cunningham,1,3 S. F. Terracol, 1 S. P. Cramer,2,3 S. E. Labov1

1Lawrence Livermore National Laboratory
2Lawrence Berkeley National Laboratory
3University of California, Davis

We are developing high-resolution energy-dispersive X-ray spectrometers for synchrotron-based fluorescence-detected absorption spectroscopy. They use superconducting detector technology based on arrays of Nb-Al-AlOx-Al-Nb tunnel junctions. X-rays absorbed in the top Nb film generate excess charges in proportion to their energy, which can be measured as an increase in the tunneling current. For spectrometry at the synchrotron we have built a two-stage adiabatic demagnetization refrigerator that holds the detector array at 0.1 K at the end of a 40-cm-long cold finger within 15 mm of a room temperature sample. The spectrometer has an energy resolution between 10 and 20 eV FWHM for X-ray energies below 1 keV and can be operated at count rates above 100,000 counts/s. We show examples for fluorescence-detected absorption spectroscopy of dilute samples from biophysics and material science. We will outline directions of detector development, and discuss scientific applications that would profit both from higher sensitivity detectors and the increased brightness of third-generation synchrotron sources.

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