Purpose: To accurately calculate absorbed dose to water in hydrated electron (e⁻(aq)) dosimetry, the radiation chemical yield (i.e. G-value) of hydrated electrons, G(e⁻(aq)), must be precisely determined. The relationship between the G-value, linear energy transfer (LET), and ionization cluster size were also investigated for better understanding of factors affecting the G-value using Geant4-DNA.
Methods: Different scenarios were investigated using Geant4-DNA track structure Monte Carlo toolkit. In scenario A, a 10 x 4 x 2 cm³ phantom of liquid water was placed at SSD 70 cm and irradiated with monoenergetic electron beams of different incoming energies from 1 keV to 1 MeV. G(e⁻(aq)) and the G-value of all generated species with respect to incoming energy were scored. In scenario B, the relation between G(e⁻(aq)) and cluster size was investigated by irradiating a 0.1 x 0.1 x 0.1 cm³ water phantom with electron point sources of energies from 0.01 keV to 1 MeV, starting at the center of the volume.
Results: The time-evolution of the G-value showed good agreement with previous simulations and experiments. It was observed that as the incoming electron energy decreases (i.e. higher LET), G(e⁻(aq)) generally decreases, and its rate of change over time becomes steeper. For 1 MeV electron beams, the initial G(e⁻(aq)) at 1 picosecond was calculated to be 4.1±0.1. Moreover, we observed a negative correlation between G(e⁻(aq)) and cluster size as a function of incoming electron energy. The results indicated that cluster size had a pattern similar to that of LET, which has been shown to remain relatively constant as the incoming electron energy increases.
Conclusion: As the incoming electron energy increases to tens of MeV, G(e⁻(aq)) will remain relatively constant, since it decreases with increasing cluster size and LET. This is beneficial, as Geant4-DNA can only simulate electron interactions up to 1 MeV.
Funding Support, Disclosures, and Conflict of Interest: Funding support: Research Institute - McGill University Health Centre (RI-MUHC). No disclosures and conflicts of interest.