QUANTUM CAVITATION IN SUPERFLUID-HELIUM-4

We have observed the nucleation of bubbles in superfluid helium-4 at n egative pressure and at temperatures down to 65 mK. Cavitation is foun d to be a random process. Above 1 K, its probability varies with tempe rature in a manner consistent with classical activation by thermal flu ctuations. Below 0.4 K cavitation is still random but temperature inde pendent, in agreement with a recent theory of quantum tunneling. We al so consider another possible interpretation involving turbulence in th e focal region.