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Ruutu, Eltsov, Gill, Kibble, Krusius, Makhlin and colleagues observe vortices forming when a thermal neutron is absorbed in a rotating container of superfluid helium-3 B phase. The reaction releases 0.76 MeV and heats a cigar-shaped region into the normal phase. As it cools back through the superfluid transition, quantised vortices nucleate. Large enough rings grow under the Magnus force and are detected one by one with nuclear magnetic resonance, and varying the superflow velocity reveals the shape of the initial defect network. They propose this rapid cool-down mirrors defect formation during cosmological phase transitions in the early universe.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation