GEANT4-DNA simulation of temperature-dependent and pH-dependent yields of chemical radiolytic species

Abstract

Objective.GEANT4-DNA can simulate radiation chemical yield (G-value) for radiolytic species such as the hydrated electron (eaq-) with the independent reaction times (IRT) method, however, only at room temperature and neutral pH. This work aims to modify the GEANT4-DNA source code to enable the calculation ofG-values for radiolytic species at different temperatures and pH values.Approach.In the GEANT4-DNA source code, values of chemical parameters such as reaction rate constant, diffusion coefficient, Onsager radius, and water density were replaced by corresponding temperature-dependent polynomials. The initial concentration of hydrogen ion (H⁺)/hydronium ion (H₃O⁺) was scaled for a desired pH using the relationship pH = -log₁₀[H⁺]. To validate our modifications, two sets of simulations were performed. (A) A water cube with 1.0 km sides and a pH of 7 was irradiated with an isotropic electron source of 1 MeV. The end time was 1μs. The temperatures varied from 25 °C to 150 °C. (B) The same setup as (A) was used, however, the temperature was set to 25 °C while the pH varied from 5 to 9. The results were compared with published experimental and simulated work.Main results.The IRT method in GEANT4-DNA was successfully modified to simulateG-values for radiolytic species at different temperatures and pH values. Our temperature-dependent results agreed with experimental data within 0.64%-9.79%, and with simulated data within 3.52%-12.47%. The pH-dependent results agreed well with experimental data within 0.52% to 3.19% except at a pH of 5 (15.99%) and with simulated data within 4.40%-5.53%. The uncertainties were below ±0.20%. Overall our results agreed better with experimental than simulation data.Significance.Modifications in the GEANT4-DNA code enabled the calculation ofG-values for radiolytic species at different temperatures and pH values.

Keywords: GEANT4-DNA; radiation chemical yield; radiation chemistry; water radiolysis.

Figure 1.

$\text{G}\left(e_{aq}^{-}\right)$ at the temperature range from 25 °C to 150 °C simulated in this work and comparison with the published work. The results were compared with the published experimental data (Lin et al 2004, Elliot and Bartels 2009), and simulation results from Monte Carto track structure codes IONLYS-TRACION and IONLYS-TRACELE (Plante 2011b) and from TRACIRT (Hervé du Penhoat et al 2000).

Figure 2.

$\text{G}{(}^{•}\text{OH})$ and $\text{G}\left({\text{H}}_2{\text{O}}_2\right)$ were simulated in this study within the temperature range of 25 °C to 150 °C. These simulated values were then compared to published experimental data (Elliot and Bartels 2009), as well as simulation results obtained from Monte Carlo track structure codes IONLYS-TRACION and IONLYS-TRACELE (Plante 2011b), and TRACIRT(Hervé du Penhoat et al 2000).

Figure 3.

Time-evolution of G-values for generated reactive species at different temperatures from 25 °C to 150 °C for incoming electron energy of 1 MeVand pH value of 7.

Figure 4.

$\text{G}\left(e_{aq}^{-}\right)$ and $\text{G}\left(e_{aq}^{-}+{\text{H}}^{•}\right)$ at different pH values from 5 to 9 simulated in this work and the comparison with published values. The results were compared with the published pulse radiolysis experimental results (Sehested et al 1970, Spinks and Woods 1990) and simulation results from Monte Carlo track structure codes IONLYS-IRT (Autsavapromporn et al 2007), IONLYS-TRACION and IONLYS-TRACELE (Plante 2011b).

Figure 5.

$\text{G}{(}^{•}\text{OH})$ and $\text{G}\left({\text{H}}_2{\text{O}}_2\right)$ at different pH values from 5 to 9 were simulated in this work and compared with the published work. The results were compared with the published experimental results (Spinks and Woods 1990) and simulation results from Monte Carlo track structure codes IONLYS-IRT (Autsavapromporn et al 2007), IONLYS-TRACION and IONLYS-TRACELE (Plante 2011b).

Figure 6.

Time-evolution of G-values for generated reactive species at different pH values from 5 to 9 with the incoming electron energy of 1 MeV and temperature of 25 °C.