Author: Xu Cao, Wesley S. Culberson, Aubrey Parks, Brian W Pogue, Matthew Reed, William Scott Thomas 👨🔬
Affiliation: University of Wisconsin: Madison, Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, University of Wisconsin-Madison, University of Wisconsin - Madison, UW-Madison 🌍
Purpose: Under Ultra-High Dose Rate (UHDR) irradiation, radical-radical interactions are likely enhanced due higher density production in time and space and this alters the cascade of reactions that stem from the primary hydrolysis radicals. Direct measurements of the products of these reactions, such as hydrated electrons, hydroxyl radicals, and reactive oxygen species provide critical background information as to what may cause the FLASH effect. This study developed a measurement system to quantify the transient production yields of hydrated electrons in water under UHDR electron irradiation. The changes in hydrated electrons with dose rate parameters can help provide a complete model of the cascade that leads to biological damage change.
Methods: A multi-pass transmission cell measurement technique was employed to detect and quantify production of hydrated electrons through their near-infrared absorption at 670nm wavelength from a diode laser beam. Transient absorbance profiles of hydrated electrons were analyzed to estimate the G-value as a function of variations in pulse width, source-to-surface distance, and pulse frequency, enabling an assessment of its dependence on both instantaneous and average dose rates.
Results: The study confirmed a linear relationship between the total radiation dose and the production of hydrated electrons, yielding a stable G-value at a fixed dose rate. However, the G-value increased with rising instantaneous dose rates (0.18–0.33 MGy/s), ranging from 35 to 48 nM/Gy, as shown in Fig. 1. The G-value did not appear to depend upon average dose rate within the range tested.
Conclusion: This study provides significant new data into the production rate of hydrated electrons changes from water hydrolysis with UHDR changes. The observed increase in the G-value of hydrated electrons with rising instantaneous dose rates should be expected given the expected shift towards hydroxyl ion quenching that might occur. This is relevant to the potential mechanisms underlying FLASH radiotherapy.