Self-radiolysis of tritiated water. 3. The ˙OH scavenging effect of bromide ions on the yield of H2O2 in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

Abstract

Monte Carlo track chemistry simulations were used to determine the yields (or G-values) of hydrogen peroxide in the radiolysis of neutral water and dilute aqueous bromide solutions by low linear energy transfer (LET ∼ 0.3 keV μm⁻¹) radiation (e.g., γ-rays from ⁶⁰Co, fast electrons or high-energy protons) and tritium β-particles (mean LET ∼ 6 keV μm⁻¹) at 25 °C. We investigated the influence of Br⁻ ions, as selective scavengers of ˙OH radical precursors of H₂O₂, on the inhibition of G(H₂O₂) for these two types of radiation. Studying this system under a wide range of Br⁻ concentrations (5 × 10⁻⁷ to 0.2 M) and using a well-accepted mechanism for radiolysis in the presence or absence of air, we examined the chemical changes in the scavengeability of H₂O₂ produced by 300 MeV irradiating protons (used in this work to reproduce the effects of ⁶⁰Co γ/fast electron radiolysis) and tritium β-electron radiolysis. We found that these changes could be related to differences in the initial spatial distributions of radiolytic species (i.e., the structure of the electron tracks, the low-energy β-electrons of tritium depositing their energy almost entirely as cylindrical “short tracks” and the energetic Compton electrons produced by γ-radiolysis forming mainly spherical “spurs”), in full agreement with previous experimental and theoretical work. Simulations showed that the short track geometry of higher LET tritium β-electrons in both water and aqueous bromide solutions favored a clear increase in G(H₂O₂) compared to ⁶⁰Co γ-rays. Moreover, the presence of oxygen was seen to scavenge hydrated electrons (e_aq⁻) and H˙ atoms on the ∼10⁻⁷ s time scale, thereby protecting H₂O₂ from further reactions with these species in the homogeneous stage of radiolysis. This protection against e_aq⁻ and H˙ atoms therefore led to an increase in the long time H₂O₂ yields, as seen experimentally. Finally, for both deaerated and aerated solutions, the H₂O₂ yield in tritium β-radiolysis was found to be more easily suppressed than in the case of cobalt γ-radiolysis, and interpreted by the quantitatively different chemistry between spurs and short tracks. These differences in the scavengeability of H₂O₂ precursors in passing from 300 MeV irradiating protons to tritium β-electron irradiation were in good agreement with experimental data, thereby lending strong support to the picture of tritium β-radiolysis in terms of short tracks of high local LET.