Vorices in rotating fluid experiment schematic. . .
minibang setup

The above figure explains the experiment performed in Helsinki. Superfluid ³He-B was rotated below the critical velocity limit in a 5-mm diameter cylinder at T ≈ 1 mK, so that vortices were not created until a paraffin-moderated Am-Be neutron source was brought close to the sample (a). For thermal neutrons the mean free path is 0.1 mm in liquid ³He so that the absorption reactions occur close to the walls of the sample container. When a ³He nucleus absorbs a neutron, a “Mini Bang” occurs, n + ³He → p + ³H + 764 keV. The nucleus thus splits to a proton and a triton, which fly in opposite directions, producing 70-μm and 10-μm long ionization tracks, respectively (b). The subsequent charge recombination yields a 0.1-mm long cigar-shaped region of normal ³He (c). The heated volume cools so fast back to the superfluid state that relaxation of the order parameter lags behind and the equilibrium state cannot be established. According to the theories of Kibble and Zurek, vortices are created during this transition (d). A special feature of the Helsinki experiment is the applied bias flow caused by rotation, which forces the larger vortex loops to expand instead of shrink (e). They hit the walls and straighten to rectilinear vortex lines in the center of the cylinder (f). The formation of vortices could be verified experimentally as distinct steps.

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