The rate nu at which negative ions create quantized vortex rings in superfluid He-3/He-4 solutions has been measured for temperatures T within 90 < T < 500 mK, for He-3/He-4 isotopic ratios x3 < 10-7, pressures P within 17 < P < 25 bar, and electric fields E within 102 < E < 106 Vm-1. For fixed P, x3 and E, it has been found that nu(T) exhibits unexpected sharp local maxima, together with associated small velocity anomalies.