Studies of single superconducting grains for a neutrino and dark matter detector

We have conducted various tests of the properties of single superconductinggrains, of the type that has been suggested for use in detectors for dark matter and coherent neutrino scattering. These tests included study of the variation of the critical fields of a single grain when it is rotated relative to the applied field, observations concerning the existence of the intermediate state, and the effects of irradiation by alpha and gamma particles. The materials studied were Sn, Al and Zn at ⁴He and ³He temperatures. The variation of the critical (superheating) field under rotation suggests that the uncertainty of the phase boundary observed in the past for colloids of such grains in large measure already present in the different orientations of a single grain. The irradiation experiments indicate that for certain materials a simple "global heating" picture of the grain flip process applies in that the grain shows a well defined, sharp flip threshold in an applied field, one which furthermore is in accordance with the superconducting specific heat of the grain.