Proton damage on Nb-based Josephson junctions

The effects of proton irradiation on Nb-based Josephson junctions have been investigated. The fabrication technology employed is essentially the same of Josephson digital circuit designs, the main difference being a much lower Josephson critical current density. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunnelling cur rent, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found. The profile of the energy lost by protons within the Nb-targets and the dose is calculated by means of Monte Carlo simulations. A schematic description of the vacuum scattering chamber used for proton irradiation is also reported.