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

Area of Science:

• Radiochemistry
• Physical Chemistry
• Radiation Biology

Background:

• Understanding hydrated electron (e^-_aq) kinetics is vital for biological and chemical applications, particularly FLASH radiation therapy.
• Traditional scavenging methods for radical yield measurement have limitations, especially for microsecond time scales relevant to homogeneous chemistry.
• Direct measurement of hydrated electrons using pulsed ion beams, especially protons, is challenging due to instrumental complexity.

Purpose of the Study:

• To address the gap in direct measurement of hydrated electron yields and kinetics using pulsed ion beams.
• To measure transient hydrated electron radiolytic yield (G-value) in pure water under varying oxygen concentrations.
• To provide data for understanding oxygen's dependence on G_TS(e^-_aq) and validating computational models for FLASH radiotherapy.

Main Methods:

• Employed a time-resolved UV-visible absorption spectrometer coupled to a pulsed 68 MeV H+ ion beam at the Arronax cyclotron facility.
• Conducted pulsed radiolysis experiments under ultra-high-dose rate (UHDR) conditions.
• Measured hydrated electron yields and decay times at 2 microseconds post-irradiation in pure water with varying oxygen levels (0.04% to 21%).

Main Results:

• Measured track segment (TS) yields G_TS2microS(e^-_aq) = (1.00 ± 0.17) × 10^-7 mol·J^-1 in deaerated water (≤0.04% O₂) with a half-life of 8.8 ± 1.5 µs.
• Determined G_TS2microS(e^-_aq) values under increasing oxygen concentrations: (0.90 ± 0.20) × 10^-7 (0.4% O₂), (0.62 ± 0.24) × 10^-7 (1% O₂), and (0.13 ± 0.04) × 10^-7 mol·J^-1 (21% O₂).
• These direct measurements at the homogeneous chemistry stage reveal the impact of oxygen on hydrated electron yields without scavengers.

Conclusions:

• This study provides the first direct measurements of hydrated electron yields at the microsecond timescale using pulsed proton beams.
• The data elucidate the dose-dependent behavior of hydrated electrons and their significant quenching by oxygen.
• These findings are crucial for advancing UHDR chemistry in FLASH radiation therapy and validating simulation tools like Geant4-DNA and Topas-nBio.