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*+) (100 eV)(-1) have been measured in deaerated aqueous solutions of 0.25 mM methyl viologen (MV(2+)) containing various concentrations of formate anion (0.01-2 M) after irradiation with six different ion beams (4He(2+), 12C(6+), 20Ne(10+), 28Si(14+), 40Ar(18+) and 56Fe(26+) 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 4He(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 56Fe(26+) radiolysis, the MV*+ yield value changes from 2.2 to 4.1. The other values lie between the yields for 4He(2+) and 56Fe(26+). In addition, G(MV*+) decreases with increasing LET. In the case of 12C(6+) 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 MV(2+)/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) x 10(10) and (5 +/- 0.5) x 10(10) 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.

Pulse radiolysis of 4,4'-bipyridyl aqueous solutions at elevated temperatures: spectral changes and reaction kinetics up to 400 degrees C.

The spectral changes as well as the reaction kinetics of the transient species of 4,4'-bipyridyl (4,4'-bpy) have been experimentally investigated by pulse radiolysis techniques up to 400 degrees C. The results show that the transient species such as OH adduct 4,4'-bpyOH*, monoprotonated electron adduct 4,4'-bpyH*, and doubly protonated electron adduct 4,4'-bpyH2+* have 15-20 nm blue shifts from room temperature to 400 degrees C. For a deaerated neutral solution of 4,4'-bpy in the presence of tert-butyl alcohol, ethanol, or NaCOOH, the doubly protonated electron adduct is the main transient species at room temperature. But at temperatures > 350 degrees C, a monoprotonated form, the N-hydro radical 4,4'-bpyH*, becomes predominant. Interestingly, at room temperature, CO2-* could not efficiently react with 4,4'-bpy, but the reaction was accelerated with increasing temperature; at 350 degrees C, this reaction completed within 2 mus. Using an alkaline solution (pH = 11.5) of 4,4'-bpy in the presence of tert-butyl alcohol, we studied the N-hydro radical 4,4'-bpyH* from room temperature to 400 degrees C at 25 MPa. An estimation of the temperature-dependent G(e(aq)-) at 25 MPa agrees with our previous result with methyl viologen as a scavenger.