Room: Track 3
Purpose: To develop and characterize an accelerator-based FLASH (ultra-high dose rate) electron beam that can be used to calibrate radiation dosimeters at ultra-high dose rates.
Methods: The Medical Industrial Radiation Facility (MIRF) accelerator at the National Institute of Standards and Technology (NIST) was used to deliver a pulsed, laterally uniform15 MeV electron beam to a 90x90x40 mm3 graphite phantom. The dose rate as a function of beam current was calibrated from 25 – 135 Gy/sec, using a graphite calorimeter. Next, a 7 Gy dose was delivered to GAF-Chromic EBT3 and EBT-XD films, as well as OSLD dosimeters placed at a depth of maximum dose (14 mm) in the phantom with dose rates of 25, 50, 75, and 115 Gy/sec. The accuracy and precision with which dose could be delivered was assessed for each dose rate.
Results: The dose rate, determined from the graphite calorimeter measurements, was found to be linear with the electron gun current of the accelerator across the full range of dose rates studied. The average (1-standard deviation) dose reading for the three different dosimeters was 6.97 (0.04) Gy, 7.82 (0.38) Gy, 6.97 (0.02) Gy, and 7.81 (0.048) Gy for dose rates of 25, 50, 75, and 115 Gy/sec, respectively.
Conclusion: Initial evaluations of an accelerator based FLASH electron beam showed that dose rates up to 135 Gy/sec could be delivered. Dose measurements differed from the expected 7 Gy delivery by up to 12%, most likely due to inconsistent monitor charge collection at high beam currents. Further work to improve the accuracy and precision of dose delivery with the FLASH electron beam accelerator is currently underway at NIST. These efforts include the development of a more robust beam triggering and dose monitoring system, and high-dose rate alanine dosimetry calibrations using graphite and plastic water phantoms.