DIFFUSION OF QUASI-PARTICLES IN SIS TUNNEL-JUNCTIONS FOR X-RAY SPECTROSCOPY

X-ray detectors based on Superconductor-Insulator-Superconductor (SIS) tunnel junctions have the potential of a very high energy resolution. We investigate limitations on the attainable energy resolution due to the combined effect of lateral diffusion of the excess quasiparticles , generated after absorption of an X-ray photon, and loss of the quasi particles at the edges. We present an exactly solvable model, which we apply to recent X-ray measurements on a set of tunnel junctions which only differ in size. This clearly shows the detrimental influence of loss at tile edges. The model gives quantitative criteria which have t o be met to reach the desired 10 eV energy resolution for 6 keV photon s.