Sequential radiation chemical reactions in aqueous bromide solutions: pulse radiolysis experiment and spur model simulation

Abstract

Pulse radiolysis experiments were carried out to observe transient absorptions of reaction intermediates produced in N₂O- and Ar-saturated aqueous solutions containing 0.9–900 mM NaBr. The most important species among the reaction intermediates are BrOH˙⁻ and Br₂˙⁻, which commonly have absorption peaks around 360 nm. The experimental results were compared with the results of simulation based on a spur diffusion model. Each of several complicated sequential radiation-induced chemical reactions was carefully considered, optimizing its rate constant within a range of reported values, including experimental uncertainty. All the experimental results, covering a wide variety of conditions, were able to be universally reproduced by the simulation, assuming a reaction not yet reported, 2BrOH˙⁻ → Br₂ + 2OH⁻, with a rate constant of 3.8 × 10⁹ M⁻¹ s⁻¹, which is significant only within 10 μs for rather high bromide concentrations (>10 mM). Primary G values, which are yields after sufficient diffusion from the spur to the perimeter region during 100 ns, of major water decomposition products, as well as of the reaction intermediates, were calculated for N₂O- and Ar-saturated conditions as a function of NaBr concentration. Such comprehensive information on primary G values allows one to predict radiation-induced chemical change by considering only homogeneous chemical kinetics.