We have measured the drift velocity v of negative ions in He I1 near 1 K, under a pressure of 25 bar, for electric fields E up to 17 kV/cm. Beyond Ec1, the critical field for creation of stable charged vortex rings, we have observed a second critical field Ec2 at which v passes through a minimum and then starts to increase again. Numerical values of the rate nu at which rings are nucleated, and of the rate P at which the ions escape from the rings again when E > Ec2, are calculated from the experimental data.