A multichannel cryogenic detector system for synchrotron-based x-ray spectroscopy

Abstract

Fluorescence-detected x-ray absorption spectroscopy probes the fine structure of electronic energy levels with sub-eV resolution by scanning a monochromatic synchrotron beam through the corresponding absorption edge and measuring the intensity of the resulting x-ray fluorescence. For dilute samples, grating spectrometers lack the detection efficiency and conventional Si(Li) or Ge detectors often lack the energy resolution to separate the weak fluorescence signal from strong nearby emission lines. We have built a high-resolution, high-efficiency cryogenic detector system for synchrotron-based soft x-ray spectroscopy. The sensor is a 3×3 array of 200 μm×200 μm superconducting Nb-Al-AlOx-Al-Nb tunnel junctions with an energy resolution of ≈15 eV below 1 keV and a total count rate capability of ≈100,000 counts/second. This sensor array is cooled to below 0.4 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into a UHV sample chamber for x-ray fluorescence measurements. We present absorption spectra of dilute compounds (≈1000 ppm) and will discuss spectrometer performance with respect to the analysis of metalloproteins.