Nucleation of negatively charged vortex rings in superfluid helium-4 near its solidification pressure

The rate ν at which negative ions nucleate vortex rings in He II has been measured at 25 bar for electric fields E up to 12 k V/cm and temperatures T down to 0.4 K. A strong temperature dependence of ν observed for T< or =0.6 K and E<5 kV/cm is attributed to the influence of isotopic impurities. Although the temperature-independent behavior found for T< or =0.6K and 5%12 kV/cm is consistent with a theoretical prediction by Bowley, the relatively very much larger values of ν measured near 1 K indicate a serious deficiency in the model on which the theory is based