nd - m at . s of t ] 1 8 Fe b 20 04 Vortex formation in neutron - irradiated rotating superfluid 3 He - B

A convenient method to create vortices in meta-stable vortex-free superflow of 3 He-B is to irradiate with thermal neutrons. The vortices are then formed in a rapid non-equilibrium process with very distinctive characteristics. Two models were suggested to explain the phenomenon. One is based on the Kibble-Zurek mechanism of defect formation in a quench-cooled second order phase transition. The second model builds on the instability of the moving front between superfluid and normal 3 He, which is created by the heating from the neutron absorption event. The most detailed measurements with single-vortex resolution have been performed at temperatures close to T c. We present an overview of the main experimental features and demonstrate that the measurements are consistent with the Kibble-Zurek picture. New data, collected at low temperatures, support this conclusion, but display superfluid turbulence as a new phenomenon. Below 0 .6 T c the damping of vortex motion from the normal component is reduced sufficiently so that turbulent vortex dynamics become possible. Here a single absorbed neutron may transfer the sample from the meta-stable vortex-free to the equilibrium vortex state. We find that the probability for a neutron to initiate such a turbulent transition grows with increasing superflow velocity and decreasing temperature. Exposure to radiation causes changes in the structure of matter – radiation damage – which depends on the type of radiation and the absorbing material. Defects are produced, which may range from nuclear reactions to ionization and other non-elastic scattering effects. The interaction products