Spatial and temporal dosimetry of individual electron FLASH beam pulses using radioluminescence imaging

Purpose.In this study, spatio-temporal beam profiling for electron ultra-high dose rate (UHDR; >40 Gy s^-1) radiation via Cherenkov emission and radioluminescence imaging was investigated using intensified complementary metal-oxide-semiconductor cameras.Methods.The cameras, gated to FLASH optimized linear accelerator pulses, imaged radioluminescence and Cherenkov emission incited by single pulses of a UHDR (>40 Gy s^-1) 10 MeV electron beam delivered to the isocenter. Surface dosimetry was investigated via imaging Cherenkov emission or scintillation from a solid water phantom or Gd₂O₂S:Tb screen positioned on top of the phantom, respectively. Projected depth-dose profiles were imaged from a tank filled with water (Cherenkov emission) and a 1 g l^-1quinine sulfate solution (scintillation). These optical results were compared with projected lateral dose profiles measured by Gafchromic film at different depths, including the surface.Results.The per-pulse beam output from Cherenkov imaging agreed with the photomultiplier tube Cherenkov output to within 3% after about the first five to seven ramp-up pulses. Cherenkov emission and scintillation were linear with dose (R² = 0.987 and 0.995, respectively) and independent of dose rate from ∼50 to 300 Gy s^-1(0.18-0.91 Gy/pulse). The surface dose distribution from film agreed better with scintillation than with Cherenkov emission imaging (3%/3 mm gamma pass rates of 98.9% and 88.8%, respectively). Using a 450 nm bandpass filter, the quinine sulfate-based water imaging of the projected depth optical profiles agreed with the projected film dose to within 5%.Conclusion.The agreement of surface dosimetry using scintillation screen imaging and Gafchromic film suggests it can verify the consistency of daily beam quality assurance parameters with an accuracy of around 2% or 2 mm. Cherenkov-based surface dosimetry was affected by the target's optical properties, prompting additional calibration. In projected depth-dose profiling, scintillation imaging via spectral suppression of Cherenkov emission provided the best match to film. Both camera-based imaging modalities resolved dose from single UHDR beam pulses of up to 60 Hz repetition rate and 1 mm spatial resolution.