Acoustic signals of nonthermal origin from high energy protons in water

Acoustic pulses generated by the passage of 30‐GeV protons through water show evidence for the presence of a nonthermal source. In addition to the bipolar leading‐compression signal expected from thermal shock, a tripolaf leading‐rarefaction component is observed. This component is dominant at 4°C, where the thermal signal should vanish. A space‐charge effect may be the source of this component, since other proposed mechanisms leading to tripolar pulses should give a leading‐compression signal.