Oxygen Dependence on Hydrated Electron Yields: Pulsed Radiolysis Studies Using Proton Beams

Understanding the yield and kinetics of the hydrated electron (e^-_aq) is critical for applications in both biological and chemical systems, especially in the context of innovative FLASH radiation therapy. Most studies have employed chemical scavenging methods to measure radical yields. Although these techniques have undeniable advantages, such as probing early processes down to nanosecond time scale, and are easy to implement, they also have significant limitations. The main one is that they are not appropriate to longer time scales, especially the microsecond one, which is a key part of the homogeneous chemistry stage. Direct measurements of hydrated electron using pulsed ion beams are rare, especially with lighter particles such as protons used in radiotherapy, mainly due to instrumental complexity. This study addresses this gap by employing a dedicated time-resolved UV-visible absorption spectrometer coupled to a pulsed ion beam to measure transient hydrated electron radiolytic yield (<i>G</i>-value) in pure water, under both aerated and deaerated conditions. Pulsed radiolysis experiments were conducted at the Arronax cyclotron facility (Nantes, France) with a 68 MeV extracted H⁺ ion beam under ultra-high-dose rate (UHDR) conditions. The pulsed proton beam provides a radiolytic yield at <i>t</i> = 2 μs as well as e^-_aq formation and decay times. The results present the following track segment (TS) yields <b><i>G</i></b>_{<b>TS_2μ<i>s</i></b>}(e^-_aq) = (1.00 ± 0.17) × 10^-7 mol·J^-1 under deaerated conditions (≤0.04% O₂) with a half-life of 8.8 ± 1.5 μs, and <b><i>G</i></b>_{<b>TS_2μ<i>s</i></b>}(e^-_aq) = (0.90 ± 0.20) × 10^-7 mol·J^-1, <b><i>G</i></b>_{<b>TS_2μ<i>s</i></b>}(e^-_aq) = (0.62 ± 0.24)×10^-7 mol·J^-1, and <b><i>G</i></b>_{<b>TS_2μ<i>s</i></b>}(e^-_aq) = (0.13 ± 0.04) × 10^-7 mol·J^-1 under oxygen concentrations of 0.4, 1, and 21% O₂, respectively. These results represent the yields measured directly at the homogeneous chemistry stage, without the use of scavengers, and offer new data within several oxygen contents, allowing a better understanding of the dependence of oxygen on <i>G</i>_TS(e^-_aq). They are essential to explore UHDR chemistry dedicated to FLASH radiation therapy and to produce reliable data to validate Monte Carlo codes, such as Geant4-DNA and Topas-nBio.