Acoustic Response of Ion-irradiated Water at 4°C

The energy deposition of ions in water generates pressure waves, known as ionoacoustic signals. These are commonly described within the thermoacoustic approximation, where localised heating and subsequent volume expansion are considered the primary sources of wave generation. According to this model, no pressure wave should be observed at 4°C, a prediction confirmed in laser absorption experiments. However, when initiated by protons, the minimum of the acoustic signal shifts to significantly higher temperatures, suggesting the presence of an additional, non-thermal excitation mechanism.

We present the first experimental investigation of this effect using heavy ions (²³⁸U and ¹⁰⁰Mo), conducted at the SIS-18 accelerator at GSI Darmstadt. By analysing the polarity change of the pressure wave near the water anomaly at 4°C, we observed distinct directional dependencies in the lateral and axial components of the acoustic signal, indicating a pronounced and unexpected directionality in the excitation process.

We developed a momentum transfer model that describes the generation of non-thermal acoustic signals. The model aligns well with the experimental observations for heavy ions. These findings may enable novel diagnostic techniques and contribute to a broader understanding of mechanically induced radiation damage.