Visualization of heavy ion particle Bragg peaks in water by ultrasound imaging

Abstract Methods for visualizing charged particles in water have been extensively studied, whether for observing subatomic particles originating from space or monitoring charged particle beams used in radiation therapy. At the Heavy Ion Medical Accelerator in Chiba (HIMAC) facility of the National Institute of Radiological Sciences, we conducted ultrasound echo imaging of a water volume irradiated with 500 MeV n −1 Fe ions using an ultrasound imaging device. The resulting ultrasound echo images revealed microbubble-like signals near the Fe ion Bragg peak. Although the intensity of the Fe ion beam pulses was insufficient to boil the water, and the ultrasound device did not generate sound pressure levels high enough to induce cavitation, microbubbles were nonetheless observed in the ultrasound echo images. These microbubbles likely formed through a mechanism in which microscopic cavities (bubble nuclei) created by individual Fe ions aggregated to form larger cavities, leading to the appearance of microbubbles.