The first experimental measurements of the rate nu at which vortex rings are nucleated by negative ions in isotopically pure superfiuid 4He 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 vv for vortex nucleation and of a characteristic nucleation rate R0 as functions of pressureP in 16 < P < 25 bar are deduced.