High quality superconducting tunnel junctions on Nb and Ta single crystals for radiation detection

High quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto thick single crystalx-ray absorbers of Nb and Ta are discussed. Current-voltage characteristics, recorded at 0.5 K, show a subgap current which is still dominated by thermally excited quasiparticles. The quality parameter Rsubgap/Rnormal reaches a value of several million, which is unequalled for nonepitaxially sputtered tunnel junctions. The fabrication process and some development steps, such as preparation of ultrasmooth crystal surfaces are described. Observations of x-ray photons absorbed in Nb and Tasingle crystals detected by the superconducting tunnel junctions are also presented

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