Dynamics of nonequilibrium quasiparticles in a double superconducting tunnel junction detector

We study a class of superconductive radiation detectors in which the absorption of energy occurs in a long superconductive strip while the readout stage is provided by superconductive tunnel junctions positioned at the two ends of the strip. Such a device is capable of both imaging and energy resolution. In the established current scheme, well studied from the theoretical and experimental point of view, a fundamental ingredient is considered, the presence of traps, or regions adjacent to the junctions made of a superconducting material of lower gap. We reconsider the problem by investigating the dynamics of the radiation induced excess quasiparticles in a simpler device, i.e. one without traps. The nonequilibrium excess quasiparticles can be seen to obey a diffusion equation whose coefficients are discontinuous functions of the position. Based on the analytical solution to this equation, we follow the dynamics of the quasiparticles in the device, predict the signal formation of the detector, and discuss the potentiality offered by this configuration.