  The maximum energy loss (Bragg peak) located near the end of range is a
characteristic feature of ion stopping in matter, which generates an acoustic
pulse, if ions are deposited into a medium in adequately short bunches. This
so-called ionoacoustic effect has been studied for decades, mainly for
astrophysical applications, and it has recently found renewed interest in
proton therapy for precise range measurements in tissue. After detailed
preparatory studies with 20 MeV protons at the MLL tandem accelerator,
ionoacoustic range measurements were performed in water at the upgraded SIS18
synchrotron of GSI with 238U and 124Xe ion beams of energy about 300 MeV/u, and
12C ions of energy about 200 MeV/u using fast beam extraction to get 1
microsecond pulse lengths. Acoustic signals were recorded in axial geometry by
standard piezo-based transducers at a 500 kHz mean frequency and evaluated in
both the time and frequency domains. The resulting ranges for the different
ions and energies were found to agree with Geant4 simulations as well as
previous measurements to better than 1%. Given the high accuracy provided by
ionoacoustic range measurements in water and their relative simplicity, we
propose this new method for stopping power measurements for heavy ions at GeV
energies and above. Our experimental results clearly demonstrate the potential
of an ionoacoustic particle monitor especially for very intense heavy ion beams
foreseen at future accelerator facilities.
