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

Formation yields of methyl viologen cation radicals G(MV center dot+) (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 (He-4(2+), C-12(6+), Ne-20(10+), Si-28(14+), Ar-40(18+) and Fe-56(26+) with incident energies varying from 0.6 to 28 GeV) provided by the Heavy Ion Medical Accelerator in Chiba (HEMAC) 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 center dot+) increases with increasing formate concentration. In He-4(2+) radiolysis, it increases from 5.7 to 7.1 as the concentration of formate is increased from 0.01 to 2 M, while in Fe-56(26+) radiolysis, the MV center dot+ yield value changes from 2.2 to 4.1. The other values lie between the yields for He-4(2+) and Fe-56(26+). In addition, G(MV center dot+) decreases with increasing LFT. In the case of C-12(6+) radiolysis, G(MV center dot+) increases with increasing energy of the carbon ions front 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 center dot+, MV center dot+ + (OH)-O-center dot and (COO)-C-center dot + (OH)-O-center dot have been introduced in the reaction scheme. After optimization, the rate constants of these latter two reactions are determined to be (3 +/- 0.5) x 10(10) and (5 +/- 0.5) X 10(10) M (1) s(-1), respectively. By contrast, the reaction between e(aq)(-) and MV center dot+ is too slow to affect G(MV center dot+). 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. (C) 2008 by Radiation Research Society.