  By focusing a high intensity acoustic wave in liquid helium, we have observed
the nucleation of solid helium inside the wave above a certain threshold in
amplitude. The nucleation is a stochastic phenomenon. Its probability increases
continuously from 0 to 1 in a narrow pressure interval around Pm + 4.7 bars (Pm
is the melting pressure where liquid and solid helium are in equilibrium). This
overpressure is larger by two to three orders of magnitude than what had been
previously observed. Our result strongly supports a recent suggestion by
Balibar, Mizusaki and Sasaki that, in all previous experiments, solid helium
nucleated on impurities.
