Superconducting High‐Resolution X‐Ray Spectrometers For Chemical State Analysis Of Dilute Samples

Cryogenic X‐ray spectrometers operating at temperatures below 1 K combine high energy resolution with broadband efficiency for X‐ray energies up to 10 keV. They offer advantages for chemical state analysis of dilute samples by fluorescence‐detected X‐ray absorption spectroscopy (XAS) in cases where conventional Ge or Si(Li) detectors lack energy resolution and grating spectrometers lack detection efficiency. We are developing soft X‐ray spectrometers based on superconducting Nb‐Al‐AlOx‐Al‐Nb tunnel junction (STJ) technology. X‐rays absorbed in one of the superconducting electrodes generate excess charge carriers in proportion to their energy, thereby producing a measurable temporary increase in tunneling current. For STJ operation at the synchrotron, we have designed a two‐stage adiabatic demagnetization refrigerator (ADR) with a cold finger that holds a 3 × 3 array of STJs inside the UHV sample chamber at a temperature of ∼0.1 K within ∼15 mm of a room temperature sample. Our STJ spectrometer can have an...