Acoustic Detection of High Energy Particles.

The acoustic signals generated by charged particles traversing a medium offer a new technique for particle detection. Rapid thermal expansion of the medium produces an impulsive signal with a shape similar to one cycle of a sine wave. In the case of charged particles, the energy deposition results from thermalization of the particles ionization energy loss in the medium. A thermodynamic model which derives an acoustic potential for the signal produced by a given energy deposition is presented. The relationship between the acoustic potential and the signal period and amplitude is illustrated. Three non-thermal mechanisms (1) microbubble production; (2) molecular dissociation and (3) electrostriction are discussed. The acoustic potential and pulse shapes for these mechanisms are also shown. The results of two experiments are presented. In the first, acoustic signals generated by absorption of pulsed laser beams in water are observed to be similar to the signals generated by charged particle beams. Evidence of a non-thermal acoustic mechanism, possibly microbubbles or molecular dissociation, was found in this experiment. The second experiment used the BNL proton beam to check a previously observed discrepancy with the thermodynamic theory. The earlier experiment reported that the amplitude of the acoustic signals generated by particle beams traversing water vanished at 6(DEGREES)C rather than at 4(DEGREES)C where the thermal expansion of water is zero. The new experiment attributes this feature to the superposition of the thermal signal and a non-thermal pulse, due to electrostriction of the water. Calculations are presented which indicate that the source of this electrostriction is the electric field of the large number of charges freed by the ionization of the water molecules