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, >40Gy/s) radiation via Cherenkov emission and radioluminescence imaging was investigated using intensified CMOS (iCMOS) cameras. MATERIAL & METHODS The cameras gated to FLASH optimized LINAC pulses, imaged radioluminescence and Cherenkov emission incited by single pulses of (UHDR, >40Gy/s) 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), and a 1 g/L quinine sulfate solution (scintillation). These optical results were compared to 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 (PMT) Cherenkov output to within 3%, after the first ~5-7 ramp-up pulses. Cherenkov emission and scintillation were linear with dose (R^2=0.987, 0.995, respectively) and dose rate independent from ~50-300 Gy/s (0.18-0.91 Gy/pulse). The surface dose distribution from film agreed more with scintillation than Cherenkov emission imaging (3%/3mm gamma pass rate 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 consistency of daily beam quality assurance parameters with generally a 2% or 2mm accuracy. Cherenkov-based surface dosimetry was affected by 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 by single UHDR beam pulses of up to 60 Hz repetition rate and 1 mm spatial resolution.