On the Statistical Analysis of Single Vortex Nucleation Events in Superfluid 4He

We investigate the statistical properties of single phase slip events observed in vortex nucleation experiments in 4He by the groups in Berkeley and Paris. From the cumulative distribution function of the events we calculate the sliprate as a function of flow velocity. The critical velocity is defned as the mean velocity and its statistical width as the standard deviation. From the slip rate and from the observed linear temperature dependence of the critical velocitywe obtain the energy barrier for vortex nucleation which is a quadratic function of the flow velocity. A comparison with the statistical properties of the laminar to turbulent transition in the flow around an oscillating sphere shows strikingly different behaviour.