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 sup erconducting tunnel junctions, which could lead in the next future to an ad vanced instrumentation with an interesting fall out for applications. So fa r, this area is restricted to LTc superconductors. Among them, niobium has the highest critical temperature and is therefore less demanding in terms o f operational temperature. The best performances achieved so far in pure Nb -based junction are far away from the expected theoretical limit. The reaso n of this discrepancy could be attributed to various problems. Quasiparticl e diffusion and edge losses seem to be responsible for the broadening of th e signal amplitude spectra so often observed and reported in literature. In this paper we report on efforts to investigate the response to X-Ray of Nb -based superconducting tunnel junctions in terms of a, model which includes the quasiparticle diffusion and the edge losses. Our model takes into acco unt the contribution from both the electrodes of the junction and well repr oduces the experimental data.