In rotating superfluid 3He-B the absorption of thermal neutrons can be used to heat locally a 10 micron-size volume into normal phase. When the heated bubble cools back in a few microsecs, vortex lines are observed to form. We record with NMR the number of lines per absorption event as a function of the superflow velocity. We find that the vortices are initially formed as rings within the bubble. Their number agrees with a simulation calculation of the escape of vortex loops from a random network, which is nucleated by the Kibble-Zurek mechanism in a very rapid phase transition.
PACS numbers: 05.70.Fh, 98.80.Cq, 67.40.Vs, 29.40.Ym
file: VortexNucl.pdf (416k)