Size measurement of positive impurity ions in liquid helium

We have measured the drift velocity of several positive impurity ions in He II in electric fields beyond the critical field for vortex-ring nucleation. The large differences in observed velocities for the various ions are satisfactorily accounted for by assuming that the ions differ in radius and hence in thermally activated escape rates from the vortex rings to which they are bound. These size differences are consistent with the low-field mobilities and are not consistent with the usual simple model for the ion structure.