Superconducting tunnel junctions as detectors for high‐resolution x‐ray spectroscopy

Abstract

Superconducting tunnel junctions (STJs) are a type of cryogenic detector with a working temperature of about 100 mK. They allow the combination of low energy threshold, high quantum efficiency and good count rate capability with an excellent energy resolution; at an x-ray energy of 5.9 keV an energy resolution of 10.8 eV (FWHM) has been achieved. The detector system described is based on STJs which consist of two superconducting Al electrodes separated by a thin dielectric tunnel barrier. The tunneling process of quasi-particles created by deposition of energy in the electrodes leads to a detectable current signal. The STJ is equipped with a superconducting Pb absorber which is read out via phonons. The Pb absorber increases absorption efficiency (∼50% at 6 keV) and suppresses detector artefacts. The degeneration of Pb, most probably due to oxidation, is overcome by the introduction of a protective SiO layer on top of the absorber. This layer leads to a slight reduction of energy resolution. Nevertheless, a resolution of 9.7 eV at 1.7 keV and of ∼20 eV at 5.9 keV could be realized with a prototype detector. Currently this STJ-based detector system is being incorporated into a prototype cryogenic spectrometer for XRF analysis.

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“…Going to lower T c , and hence lower operating temperature, improves the theoretical R. Operating at 100 mK yields a theoretical spectral resolution of R ≈ 100 at 400 nm. Previous research with superconducting tunnel junctions (STJs) with superconducting absorbers has shown that superconducting absorbers can approach the Fano limit (Li et al 2001;Huber et al 2004;le Grand et al 1998).…”

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“…The review year has seen a welcome recovery in the number of publications on ultra-high energy resolution cryogenic detectors. Huber and colleagues 18 described a new superconducting tunnel junction (STJ) comprising two aluminium electrodes and a novel isolated lead (Pb) absorber, which improved absorption efficiency at 6 keV from o1% to around 50% and significantly reduced detector artefacts. A thin (1.3 mm) layer of silicon monoxide was used to protect the lead absorber from oxidation, and although this additional layer slightly degraded the energy resolution, it eliminated serious perfor-mance degradation previously observed with lead alone.…”