Ultrasonic Measurement of Flash Dose Profiles for 13 Mev Electrons In Water
Abstract
Purpose
To use ultrasonic methods to measure FLASH-beam dose distributions within irradiated water, thereby to validate ultrasonic phase-shift methods for FLASH dosimetry and, ultimately, to differentiate regions where FLASH thresholds are exceeded.
Methods
A 10-L rectangular water tank (15 cm x 15 cm x 45 cm) was irradiated with a narrow beam of 13 MeV electron pulses from a medical linac (6 ms duration, rep rate 120 Hz, ~1-10 Gy/pulse, beam diameter ~0.5 cm). The tank was positioned 40 cm downstream from the linac vacuum window and oriented so that the electron-beam axis coincided with its long axis. Radiation-induced temperature rise was determined at a reference depth of 2.5 cm and at variable depths between 1.9 cm and 7.9 cm by measuring phase-shifts in narrow (~0.5 cm) ultrasonic probe beams oriented along independent transverse cords through the tank. A relative depth-dose profile was obtained from the ratio of phase shift at each depth to that at reference depth and compared with the results of a Monte Carlo simulation.
Results
The relative depth-dose profile obtained by ultrasonic methods shows good agreement with the results of Monte Carlo convolved with a blurring function representing the finite width of the ultrasonic probe beams. Experimental noise in this preliminary data set is elevated by ~100x due to pick up from vibrating machinery and stepper motor scanning system affecting one of the two ultrasonic sensors, for which isolation measures are being developed.
Conclusion
This work demonstrates proof-of-concept for use of ultrasound for calorimetric measurements of dose (and dose-rate distributions) in water for FLASH beams covering an order-of-magnitude variation in dose rate. An expected reduction of vibration noise of 100x will enable much-greater measurement precision and, thereby, a much-enhanced ability to differentiate where within an irradiated volume FLASH conditions are achieved.