Yamashita, S., Katsumura, Y., Lin, M., Muroya, Y., Miyazaki, T., Murakami, T., Meesungnoen, J. and Jay-Gerin, J-P. Water Radiolysis with Heavy Ions of Energies up to 28 GeV. 3. Measurement of G(MV^· ) in Deaerated Methyl Viologen Solutions Containing Various Concentrations of Sodium Formate and Monte Carlo Simulation. Radiat. Res. 170, 521–533 (2008).

Formation yields of methyl viologen cation radicals G(MV^· ) (100 eV)⁻¹ have been measured in deaerated aqueous solutions of 0.25 mM methyl viologen (MV² ) containing various concentrations of formate anion (0.01–2 M) after irradiation with six different ion beams (⁴He² , ¹²C⁶ , ²⁰Ne¹⁰ , ²⁸Si¹⁴ , ⁴⁰Ar¹⁸ and ⁵⁶Fe²⁶ with incident energies varying from 0.6 to 28 GeV) provided by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Science (NIRS) in Japan. The sample solutions are irradiated at the incident energy of the ions using 1-cm irradiation cells. Corresponding LET values cover the range from 2.2 to 185 eV/nm. G(MV^· ) increases with increasing formate concentration. In ⁴He² radiolysis, it increases from 5.7 to 7.1 as the concentration of formate is increased from 0.01 to 2 M, while in ⁵⁶Fe²⁶ radiolysis, the MV^· yield value changes from 2.2 to 4.1. The other values lie between the yields for ⁴He² and ⁵⁶Fe²⁶ . In addition, G(MV^· ) decreases with increasing LET. In the case of ¹²C⁶ radiolysis, G(MV^· ) increases with increasing energy of the carbon ions from 135 to 400 MeV/nucleon, i.e., with decreasing LET from 21 to 11 eV/nm. In parallel to the above measurements, Monte Carlo simulations of the radiolysis of the MV² /formate solutions have been performed. Ionic strength effects on reactions between charged species are taken into account. To reproduce the experimental results, previously unreported reactions such as e⁻_aq MV^· , MV^{· ·}OH and ^·COO^{− ·}OH have been introduced in the reaction scheme. After optimization, the rate constants of these latter two reactions are determined to be (3 ± 0.5) × 10¹⁰ and (5 ± 0.5) × 10¹⁰ M⁻¹ s⁻¹, respectively. By contrast, the reaction between e⁻_aq and MV^· is too slow to affect G(MV^· ). On the basis of these calculations, characteristics of intratrack reactions induced by heavy-ion beams are discussed in reference not only to the scavenger method used for measurement of water decomposition product yields but also to the differences in the relative spatial distribution of the reactants as well as the places where their intratrack reaction occurs within the geometry of the ion track structure.