Superconducting Sn/Sn-oxide/Sn tunneling junctions as high-resolution x-ray detectors

In a superconducting detector, the energy resolution is expected to be superior than in a semiconductor detector, owing to the thousand times smaller energy gap Δ. The performance of Sn/Sn-oxide/Sn tunneling junctions (operated in the Giaever mode at T=0.32 K) exposed to 6-keV x-ray photons of a 55Fe source is presented. The best energy resolution observed is 67 eV (FWHM). Extrapolating to vanishing electronic noise, an intrinsic resolution of 0.7% (FWHM) is obtained. This is an order of magnitude worse than expected from the statistics of the 2.5×106 primary electron charges produced, and is believed to be due to geometrical effects in the films. The energy response of the detector is nonlinear, owing to the self-recombination of the free charge carriers. At a fixed bias voltage, the pulses generated in either film of the junction were observed to have the same sign. If the junctions are evaporated onto a silicon substrate, most of the pulses originate, via phonon backscattering, from x-ray photons interacting in the substrate. Quasiparticle diffusion is essential in explaining the observed data.