Effects of quasiparticle diffusion in Nb-based superconducting tunnel junctions under x-rays irradiation

A novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in the future to an advanced instrumentation with interesting applications. So far, this area is restricted to LTc superconductors. Among them, niobium has the highest critical temperature and is therefore less demanding in terms of operational temperature. The best performances achieved so far in pure Nb-based junction are far from the expected theoretical limit. The reason for this discrepancy could be attributed to various problems. Quasiparticle diffusion and edge losses seem to be responsible for the broadening of the signal amplitude spectra so often observed and reported in literature. In this paper the authors report on efforts to investigate the response to x-ray of Nb-based superconducting tunnel unctions in terms of a model which includes the quasiparticle diffusion and the edge losses. The model takes into account the contribution from both the electrodes of the junction and well reproduces the experimental data