  The nucleation of vortex rings accompanies the collapse of ultrasound bubbles
in superfluids. Using the Gross-Pitaevskii equation for a uniform condensate we
elucidate the various stages of the collapse of a stationary spherically
symmetric bubble and establish conditions necessary for vortex nucleation. The
minimum radius of the stationary bubble, whose collapse leads to vortex
nucleation, was found to be about 28 healing lengths. The time after which the
nucleation becomes possible is determined as a function of bubble's radius. We
show that vortex nucleation takes place in moving bubbles of even smaller
radius if the motion made them sufficiently oblate.
