Hydrated-Electron Dosimetry for Proton Flash Beams: Feasibility and Response Characterization
Abstract
Purpose
To evaluate the feasibility of a hydrated-electron dosimeter prototype for proton FLASH beams and to characterize its sensitivity and response.
Methods
Absorbed dose can be quantified by monitoring hydrated-electron concentration using absorption spectroscopy. We previously demonstrated the feasibility of a prototype hydrated-electron dosimeter in conventional photon (6–15 MV) and electron (6 MeV) beams (up to 2 mGy/pulse) from a Varian TrueBeam linac, indicating tissue-equivalent potential. However, its performance under ultra-high dose-rate conditions remains unexamined. In this study, a glass cavity (5×4×2 cm³) was filled with N₂-purged NaOH solution (pH 12) with four dielectric mirrors mounted on the cavity walls. A 660-nm laser beam was passed five times through the cavity using the mirrors, increasing effective optical path length. Transmitted light was recorded with a silicon photodetector and picoscope. The cavity was irradiated with proton FLASH beams at a fixed dose rate of 125 Gy/s (3.125 mGy-12.5 Gy/pulse, corresponding to 0.025-100 ms pulse length) at the research proton beamline at OncoRay (Center for Radiation Research in Oncology, Dresden, Germany). Time-resolved absorption transients were recorded to evaluate the response.
Results
The hydrated-electron dosimeter yielded reproducible optical responses under proton FLASH beams. The dose-normalized response was beam-parameter dependent: it was notably higher in the low dose-per-pulse (DPP) regime (<0.5 Gy/pulse) but remained stable at higher DPP values. A significant increase in response was observed for short pulse lengths (<1 ms), reflecting an increased influence of beam rise-time. The response was also affected by accumulated dose and solution aging. An initial response increase with accumulated dose (pre-irradiation effect) was likely attributed to residual oxygen scavenging. Over subsequent hours, the response became time-dependent rather than dose-dependent, likely due to pH drift, solution contamination, or oxygen re-diffusion.
Conclusion
Hydrated-electron dosimetry demonstrated real-time feasibility in proton FLASH beams, with stable response achievable through controlled pre-irradiation.