Acoustic signal-based precise positioning of Bragg peak for laser-accelerated monoenergetic proton pulses

Background: With the advancement of ultra-short pulse technology and the rapid progress of FLASH radiotherapy, it is clinically desirable and technically possible to utilize the radiation acoustic effect of radiotherapy pulses for noninvasive real-time in vivo dose monitoring. Purpose: As a crucial foundation of in vivo dose monitoring using laser-accelerated proton acoustics, this study focuses on measuring, analyzing, and processing the acoustic signals to precisely position the Bragg peak of laser-accelerated monoenergetic proton pulses. Materials and methods: Nanosecond-scale high-energy broadband proton bunches were produced from the interaction of ultra-intense femtosecond laser pulses with thin film targets. After energy selection and focusing through an electromagnetic beamline, approximately 10⁷ quasi-monoenergetic protons per shot were delivered into a water gel or tank. Ultrasonic transducers with different center frequencies detected acoustic signals across various frequency bands. Time and frequency domain analyses were conducted to achieve precise positioning of Bragg peaks. Results: This study successfully achieved measurement of acoustic signals of laser-driven ultra-short monoenergetic protons for the first time. Subsequent analysis and processing of signals enabled the precise positioning of the Bragg peak with a deviation of 45 µm, demonstrating the potential of this method for dose monitoring. Conclusions: Our findings indicate that this method can be applied to single-shot in vivo dose monitoring in radiotherapy equipment based on laser proton accelerators. It can potentially promote the precise and effective dose delivery of radiotherapy.