A Superconducting Tunnel Junction X-Ray Spectrometer Without Liquid Cryogens

Superconducting tunnel junctions (STJs) are being developed as X-ray detectors because they combine the high energy resolution of cryogenic detector technologies with the high count rate capabilities of athermal devices. We have built STJ spectrometers for chemical analysis of dilute samples by high-resolution soft X-ray spectroscopy at the synchrotron. The instruments use 36 pixels of 200 mum times 200 mum Nb-Al-AlOx-Al-Nb STJs with 165 nm thick Nb absorber films. They have achieved an energy resolution of ~ 10-20 eV FWHM for X-ray energies below 1 keV, and can be operated at a total count rate of ~ 10⁶ counts/s. For increased user-friendliness, we have built a liquid-cryogen-free refrigerator based on a two-stage pulse tube cryocooler in combination with a two-stage adiabatic demagnetization stage. It holds the STJ detector at the end of a 40-cm-long cold finger, and attains the required operating temperature of ~ 0.3 K at the push of a button. We describe the instrument performance and present speciation measurements on Eu dopant activators in the novel scintillator material SrI₂ to illustrate the potential for STJ spectrometers at the synchrotron.