Time Response to Irradiations of Tin Superheated Superconducting Grain Dispersions

We report a study of the time response to particle irradiations of superheated superconducting granule dispersions. This work, which complements a previous study of the magnetic field response to energy depositions, has been performed at 70mK with dispersions having volume filling factors up to 8.3%. In order to observe irradiation induced transitions, the applied magnetic field was ramped up and stabilized at different pause field values. During pauses, the measured transition rates decay in time and are found not to be affected by magnetostatic inter-grains interactions. On this basis, we elaborate a model of time response for a set of isolated grains which assumes that normal state nucleation results from adiabatic and homogeneous heating of full granule volumes (calorimetric response). The analysis of experimental results allows the identification of two different types of time response which we correlate to the way the initial energy deposition nucleates the normal state. Most of the time, transitions result from calorimetric responses: as described by the proposed model, the proximity between the pause field and the transition field of a granule uniquely defines its probability of transition. Its value is shown to be determined by the spectroscopic characteristics of the radioactive source. Less often, nucleation of the normal state occurs before the heat is homogeneously distributed within the granule volume. In this case, the transition probability also depends on the interval separating the pause field and the superheating limit.