Hydrated electron yield dependence on instantaneous dose rates with electron ultra-high dose rate (UHDR) irradiation

<i>Objective.</i>The aim of this study was to quantify the characteristic transient production yield of hydrated electrons (<i>G</i>-value) in water under ultra-high dose rates (UHDR) with electron irradiation. Changes in this yield with UHDR irradiation may provide insights into the radiation chemistry origins of the normal tissue-sparing effect observed in FLASH therapy.<i>Approach.</i>A multi-pass transmission measurement technique was used to detect and quantify hydrated electrons based on its near-infrared absorption. Transient absorbance profiles of hydrated electrons were used to estimate the<i>G</i>-value as a function of variation in pulse width, source-to-surface distance and pulse frequency, to assess dependence upon instantaneous and average dose rates.<i>Main results.</i>The study confirmed a linear relationship between total radiation dose and production of hydrated electrons, giving a stable<i>G</i>-value for a fixed dose rate. However, the measured<i>G</i>-value increased with increasing instantaneous dose rates (0.18-0.33 MGy s^-1), ranging from 35 up to 48 nM Gy^-1. However, the<i>G</i>-value did not change appreciably when varying the pulse frequency and pulse width, which varies the average dose rate.<i>Significance.</i>The instantaneous dose rate of UHDR directly influenced the generation yield of hydrated electrons during UHDR water radiolysis. Further research is needed to solidify this connection and to better understand the role of hydrated electrons in the observed sparing effect of FLASH radiotherapy.