A new technique has been developed for measuring the nucleation rate nu of negatively charged vortex rings in HeII when the temperature T, pressure P and electric field E are such that the bare-ion drift velocity is limited principally by roton emission. For P = 25 bar, E = 7.9 x l0^5 V/m and 0.7 < T < 0.9 K, the relation nu(T) = 8.0 x l0^7 exp( - 6.2/T) was obtained, which suggests that the magnitude of nu may be closely related to the thermal roton density