Alpha Induced Upsets in Josephson Tunnel Junctions

The bias current dependence of the alpha induced upset rate has been investigated for a number of Nb-amorphous Si-Nb Josephson junctions. An upset occurs when an alpha particle is able to normalize a sufficiently large part of a junction\´s area so that after the current has redistributed, the critical current density is exceeded in the remainder of the junction. A one-parameter model has been developed, and it has been found to give reasonably good fits to the bias dependence of the upset rate for a number of devices. The model takes into account the angle of incidence of the alpha particles and the rate at which the particles deposit energy in the device. The one "free parameter" is the constant of proportionality between the area of the normalized region and the rate of energy loss experienced by the alpha particle. When calculating the upset rate, it is necessary to count those alpha particles which pass near enough to the junction to normalize a sufficiently large portion of the junction\´ s area as well as those which pass directly through the junction\´s physical area.