Nucleation of Bubbles by Electrons in Liquid Helium-4*

We report on experiments in which we study cavitation resulting from electrons in liquid helium. Electrons are introduced into the liquid by a radioactive source. After electron comes to rest, it forms an electron bubble. To study cavitation a sound pulse is generated by a hemispherical piezoelectric transducer producing a large pressure oscillation at the acoustic focus. If an electron is near the focus and the negative going pressure swing exceeds a critical value, a cavitation bubble is produced which can be detected by light scattering. Three distinct critical pressures have been measured as a function of temperature. The first, also the largest, corresponds to cavitation resulting from the application of a reduced pressure to liquid where an electron bubble is already present. The second is the critical pressure needed to lead to cavitation when an electron enters the liquid at a time and place where there is already a reduced pressure. The third, also the smallest, is surprisingly close to SVP, which indicates that the object resulting in cavitation must have high energy concentration. We have no explanation for its origin. It is only detected when the radioactive source is present.