A Multichannel Superconducting Tunnel Junction Detector for High-Resolution X-Ray Spectroscopy of Magnesium Diboride Films

Abstract—We have built a high-resolution soft x-ray spectrom-eter using a 3 3 array of superconducting Nb-Al-AlOx-Al-Nb tunnel junctions. The detector is cooled to 0.1 K by a two-stage adiabatic demagnetization refrigerator while being held at the end of a 40-cm-long detector snout that can be inserted into an experi-mental chamber. The 0.6 mm 0.6 mm array has an energy reso-lution of 10–15 eV FWHM for x-ray energies below 1 keV and total count rate capabilities above 100 000 counts/s. We demonstrate the capabilities of the instrument in a study of anisotropy in-axis ori-entedMgB films by near edge x-ray absorption spectroscopy. The absorption fine structure at the BK-edge differs for excitation with x-rays polarized along the-axis and in the-plane of the MgB film. We discuss the implications of the anisotropy for the density of states in MgB at the Fermi surface. Index Terms—Josephson junctions, magnesium diboride, super-conducting devices, X-ray spectrometers. I