Author: Fred B. Bateman, Ilaria M. Luna, Ronald E. Tosh 👨🔬
Affiliation: Sidwell Friends School, NIST, Dosimetry Group, Radiation Physics Division, National Institute of Standards and Technology 🌍
Purpose: To exploit the rapid response time and dose sensitivity of ultrasonic detection methods for resolving pulse structure in FLASH-level beams.
Methods: Two ultrasonic transducers positioned 15 cm apart on opposite sides of a 10 L water tank were used to transmit and receive a continuous 5 MHz, ultrasonic beam that traversed the water in the space between the transducers. This system was set up 40 cm downstream of the vacuum window of a medical linac that produced 11.5 MeV electron pulses (6 ms duration, rep rate 120 Hz, ~1-10 Gy/pulse) propagating into the tank and intersecting the ultrasonic beam at approximately 2.5 cm depth in the water. Electrical signals from the receiving transducer were sampled at 210 MS/s using a lock-in amplifier with phase analyzer. The phase difference between the transmitter and receiver waveforms was used to monitor changes to the speed of sound in the water caused by heating from dose delivered by the electron beam. A photodiode Cerenkov detector positioned outside of the water tank served as a beam intensity monitor.
Results: Phase output showed the usual step response attributable to temperature rise in the volume sampled by the ultrasonic beam, but closer inspection revealed much smaller, pulsed increments of phase whose magnitude and timing comport with the expected dose-per-pulse and rep rate.
Conclusion: This work demonstrates proof-of-concept for use of ultrasound in absolute dosimetry suitable for studying pulse structure and instantaneous dose rate in FLASH-level beams.