Superconducting tunnel junction detectors: the quasiparticle diffusion in the nonlinear regime

X-ray STJ detector response is analyzed in the framework of the kinetic transport equation for the non-equilibrium quasi-particles. Numerical solutions of the resulting equation which describes the diffusion of excitations in the plane of the junction are presented and discussed. A particular attention is paid to the limits of “small” and “large” junctions, with respect to the effective diffusion length of quasi-particles. An almost linear behaviour is expected, under suitable conditions, for “large” Nb-based devices, in agreement with experiments reported in the current literature. Deviations from linearity are however possible in “small” devices or in case of short electronic mean free path.