Quasiparticle lifetimes and tunneltimes in SIS junctions for X-ray spectroscopy

The quasiparticle lifetimes and tunneltimes in SIS tunnel junctions are essential parameters in the development of these devices for high resolution X-ray spectroscopy. We present a simple analytical model which allows us to calculate both the risetime and the total charge of the integrated tunnel current following an X-ray event. Simultaneous measurement of both the total charge and the risetime thus becomes a useful diagnostic tool for quantitative analysis. We have applied the model to recent X-ray measurements on a Nb/AlOx SIS junction. We find that the energy resolution achieved in this device is mainly limited by variations in the thickness of the counter electrode