Hydrated electron and hydroxyl radical end-of-track yields under proton beam in water

Abstract

This article reports on the local variations of the average yields of hydrated electron and ^•OH induced by 62-MeV proton beams within the final 10 mm of their propagation in liquid water. The dose rate was at least 30 Gy/s in the Bragg peak. The yields were determined by conducting in-situ and real-time acquisitions along the beam track using scavengers in solution, which produce either a fluorescence light or a UV–visible light absorbance. We used Resazurin and AmplexRed as scavengers, that respectively react with the hydrated electron and ^•OH to form the fluorescent Resorufin. We also performed absorption measurements with the ferro/ferricyanide system to study the combined yields involving these two radicals. Proton irradiations revealed a minimum value of 2.2 × 10⁻⁷ mol/J for ^•OH yields measured with the ferrocyanide system in the Bragg peak. Yields of the hydrated electron and ^•OH obtained by fluorescence were significantly lower, probably due to both the dose rate effect that degrades the Resorufin fluorescence and the time resolution limit of the detection. For both absorbance and fluorescence measurements, the yields partially recovered in the distal part of the Bragg peak. The origin of this behavior is difficult to explain. We suggest that it might be related to the longitudinal straggling of the beam associated with the complex track structures found in and after the Bragg peak.