Time-resolved measurement of acoustic pulses generated by MeV protons stopping in aluminum

A wide-band capacitive detector has been constructed to make time-resolved measurements of acoustic pulses generated in aluminum and in other solids by ns pulses of MeV protons. The near-field signal observed traveling in the beam direction is a measure of the initial pressure distribution along the path of the stopping protons. The measured amplitudes and shapes of such signals are consistent with the thermoelastic model of sound generation. From the signal rise time (<2 ns), it can be concluded that the conversion into heat of the energy loss of a slowing proton is localized within a radius of several micrometers. For the data presented, the fraction of beam-pulse energy absorbed from acoustic radiation by the detector is about 5×${10}^{\mathrm{-}14}$. Acoustic signals which generate ${10}^{\mathrm{-}2}$ eV of electrical energy in the detector (∼4 μV into 50 Ω for 5 ns) are easily discernible after signal averaging. Acoustic signals were used to probe the formation of microbubbles in hydrogen-implanted aluminum. From bubble growths inferred from acoustic measurements, the diffusivity of hydrogen in aluminum is estimated at a temperature where the diffusivity is 8 orders of magnitude smaller than in previous measurements.