Time-Resolved Acoustic Pulses Generated by Mev Protons Stopping in Solids.

Aluminum and other solids were bombarded with proton pulses of 1 nsec duration and with energies up to 5 MeV/proton. Acoustic waves generated by the stopping protons were measured with nsec time resolution by using a wide-band capacitive detector. The detector output voltage was nearly proportional to the target surface velocity. The displacement amplitude of the acoustic signal due to a typical beam-pulse density (10('8) protons/cm('2)) was about 10('-9) cm in aluminum and was smaller in other materials. For such a small acoustic pulse, the resulting voltage signal at the output of a fast preamplifier was somewhat smaller than the average voltage fluctuations due to broad-band thermal noise. Signal averaging was accomplished by electronically digitizing the signals and storing the results in a computer. By this means signal-to-noise ratios were increased more than 100 fold. The near-field signal observed traveling in the beam direction is a measure of the initial pressure distribution along the path of the stopping beam. The measured amplitudes and shapes of such signals are consistent with a simple thermoelastic model, according to which stopping ions produce nearly instantaneous localized heating, followed by adiabatic expansion. Measurements of acoustic signals were also made on aluminum targets that had been implanted with hydrogen. The recorded signals show acoustic reflections from the implanted layer, which indicates the presence of hydrogen microbubbles. The fractional volume occupied by microbubbles and their spatial distribution have been estimated by fitting model waveforms to the acoustic data. Finally, measurements made on an implanted target that was being heated in place produced a record of the blistering process in real time.