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Validation of a Novel Device for Characterizing Ultra High Dose Rate (FLASH) Electron Beams

S Jain*, J Woollard, A Cetnar, N Gupta, A Ayan, Ohio State Univ, Dublin, OH

Presentations

SU-E-BRC-8 (Sunday, 7/10/2022) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

Ballroom C

Purpose: Ion chambers are unsuitable for use at FLASH dose rates due to greater ion recombination at high dose per pulse (DPP). Radiochromic film provides a DPP-independent alternate for acquiring beam data. We devised a custom 3D-printed vertical film holder (VFH) apparatus to acquire beam data in water (PDDs, profiles, output factors) in a more efficient manner compared to the traditional method of placing multiple films at different depths between solid water slabs placed perpendicular to the beam (perpendicular method).

Methods: The VFH latches on to vendor supplied collimators for the IntraOp Mobetron and can be lowered in a water tank. It holds the film at a 1˚-angle with respect to beam central axis so that PDDs are not affected by self-attenuation in the film. We validated this method by comparing the 9MeV (conventional dose rate) VFH PDD with the PDD measured with an ion chamber in a water tank. We also spot checked the FLASH beams PDDs (6H and 9H) with film and alanine measurements at various depths in solid water. Profiles extracted from VFH measurements were compared with those acquired using the perpendicular method. Cone factors obtained for various collimators were also compared for the two methods.

Results: The ratio of 9MeV VFH PDD and ion chamber PDD was within ±1% up to R50 and up to ±5% beyond. The VFH PDDs for FLASH dose-rate beams compared to alanine and perpendicular film measurements were within ±5% up to R50. The difference between profiles acquired with the VFH and perpendicular methods were within ±3% in the central 80% region. Cone factors were also obtained using both methods and were within uncertainties of output reproducibility.

Conclusion: The VFH method was validated with conventional methods. Through this method, beam data was acquired for the FLASH capable Mobetron with greater efficiency.

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