Measurements of Hydrogen Peroxide Yields in Ultra-High-Dose-Rate vs. Conventional Radiation.

Hydrogen peroxide (H2O2), a key product of water radiolysis formed predominantly through hydroxyl (OH) radical recombination, plays a critical role in understanding radiation-induced oxidation processes. Differences in H2O2 yields under ultra-high dose rate (UHDR) and conventional dose rate (CDR) radiation have been proposed as a potential contributor to the FLASH healthy tissue sparing effect. Radiolytic H2O2 yields were determined under UHDR (∼120 Gy/s) and CDR (1-5 Gy/s) proton irradiation in both deaerated and oxygen-saturated systems. An escape yield of ∼0.7 molecules/100 eV was observed in deaerated solutions. In oxygen-saturated systems, yields increased to ∼1.0 molecules/100 eV due to reactions involving oxygen-derived radicals. To investigate the temporal dependence of H2O2 production, various concentrations of methanol or dimethyl sulfoxide (DMSO) were added as OH radical scavengers. In deaerated systems, H2O2 yields approached zero as scavenging capacity increased, highlighting the critical role of OH radical combination reactions. In oxygen-saturated systems, moderate scavenging initially increased H2O2 yields via interactions between OH radicals and oxygen-derived species, but at higher scavenging capacities (∼108 s-1), yields declined sharply due to the suppression of OH radical combination reactions. Methanol produced higher H2O2 yields than DMSO under oxygenated conditions, with a maximum yield approximately 40% greater than that for DMSO. Measurements using γ rays produced the same trends. UHDR irradiation did not yield statistically significant differences in H2O2 production compared to conventional radiation. Simulations using the TOPAS-nBio framework confirmed the experimental findings and provided insight into the influence of scavenger concentrations on H2O2 yields. These findings highlight the sensitivity of H2O2 yields to scavenging conditions and emphasize the need for more complex models to study fundamental radiation chemistry in biological systems. This work provides essential data for validating radiation chemistry models and advancing our understanding of radiation chemistry in different environments.