Superconducting tunnel junctions

Superconducting tunnel junctions (STJ) are a class of cryogenic detectors that rely on the generation of free charge carriers by breaking Cooper pairs in a superconducting material with the use of absorbed photon energy. In an STJ, consisting of two superconducting films separated by a thin insulating barrier, the charge carriers can be detected through the tunnel-current pulse they produce if the STJ is under a finite voltage bias. The number of charge carriers generated is proportional to the energy of the absorbed photon, and, depending on the material of choice, ranges from several hundreds to a few thousand per electronvolt of photon energy. This allows STJs to be used as photon-counting detectors with intrinsic energy resolution over a wide energy band from the near infrared to well into the X-ray band. The operating temperature is typically at 10 % of the critical temperature Tc of the superconducting material and may range from 0.1 K to 1 K. Although they have not been deployed in space applications yet, they could well be envisaged as spectrometers with an energy-resolving power of several hundreds in the soft X-ray range, or as highly efficient order-sorting detectors in UV grating spectrographs. STJs can be used simultaneously as absorbers and read-out elements, or alternatively, if a larger sensitive area is required, two or more can be attached as read-out elements to a separate absorber. The state of the art performance comprises energy resolutions of 2 eV to 11 eV in the soft X-ray energy range from 0.4 keV to 5.9 keV, and of 0.1 eV to 0.2 eV in the near-infrared and visible range from 0.5 eV to 5 eV. Imaging arrays of >100 pixels of close-packed STJs have been made and operated in ground-based astronomical applications.