Investigation of elastic waves generation in a solid target by a pulsed ion beam

The generation of elastic waves in solids during irradiation with pulsed ion beams is investigated. There are two main mechanisms of the elastic wave emission. First is a simple momentum transfer from the beam particles to the target atoms. Also the heating of the material yields an acoustic wave due to a local thermal expansion. Other effects, like a change of the density, local amorphization, charging up of the target near the surface etc. can also generate elastic waves.
The measurements are carried out using a IMSA-100 focused ion beam (FIB) system and a 3 MV-TANDETRON accelerator. The energy of the FIB is 30-35 keV at a current of 1-3nA and a spot size of about 1mm. On the accelerator a current of some mA and energies up to some MeV with Au+, Ge2+, Si2+, Si3+and Si4+ ions and beam pulse duration of some microseconds were used. The elastic waves is registered by a piezoelectric sensor on the back surface of the sample. Then the signal is amplified and digitized on a PC. The dependence of the amplitude of the acoustic signal in silicon, titanium, brass and aluminum are measured for different beams. The change of the charge state of the ions up to 6 shows no significant influence. The amplitude depends on the power of the beam linearly at low current densities. These elastic waves can be used for acoustic imaging of surface and buried structures in a scanning ion acoustic microscope.