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.