Vortex Nucleation in Isotopically Pure Superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$

The first experimental measurements of the rate $\nu$ at which vortex rings are nucleated by negative ions in isotopically pure superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are presented. It is found that $\nu$ is considerably smaller than in the ordinary well helium employed in earlier work. By interpreting the data in terms of a generalized Landau argument values of the critical velocity $v_v$ for vortex nucleation and of a characteristic nucleation rate $R_0$ as functions of pressure $P$ in $16<~P<~25$ bar are deduced.