Giaever-Type Superconducting Tunnelling Junctions as High-Resolution X-Ray Detectors

Giaever-type superconducting tunnelling junctions are shown to be sensitive to the ionization energy of low-energy X-ray photons. The Mn K α and Mn K β photons from a 55 Fe source were detected with an energy resolution of 90 eV (FWHM) at an energy of 5.89 keV. The operating temperature of the detector (Sn/Sn-oxide/Sn junctions on a quartz substrate) was 0.32 K. At a fixed bias voltage, pulses from both film of the junction were observed to have the same sign. With a silicon substrate, most of the pulses originate, via phonon back scattering, from X-ray photons interacting in the substrate. Quasi-particle diffusion is essential in explaining the observed data. The main features of this detector are well described by dynamical Cooper-pair breaking and the relaxation mechanisms of nonequilibrium superconductivity.