Evaluation of water decomposition by irradiation of heavy ion beam and development of the visualization of the ion track in matter

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Water radiolysis has been investigated with heavy ions having energies up to 28GeV produced from the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Sciences (NIR). Beams of ^4He^<2+>, ^<12>C^<6+>, ^<20>Ne^<10+>, ^<28>Si^<14+>, ^<40>M^<18+> and ^<56>Fe^<26+> with respective energies of 150, 400, 300, 490, 500 and 500MeV/u corresponding LET values of 2.2, 13, 30, 54, 92 and 183eV/nm, respectively, were taken for the irradiation. The LET changes in sample solutions can be neglected due to their high energies for the irradiation of 1 cm cells. Primary g values have been determined for three important products, hydrated electron (e^-_<aq>), hydroxyl radical (●OH) and hydrogen peroxide (H2O2) as track segment yields (Differential yields) under the condition of neutral pH.With increasing LET, the g values of e^-_<aq> and ●OH decrease from 2.4 and 2.6 in ^4He^<2+> radiolysis to 0.9 and 1.1 numbers/100 eV in ^<56>Fe^<26+> radiolysis, respectively. It wa … More s also found that the primary g value of _<e-aq> is smaller than that of ●OH for any type of ion beam. For the ^<12>C^<6+> beam, other energies such as 290, 220, 135MeV/u were taken for the irradiation to investigate the effects of type or atomic number of ions on the measurement yields. Furthermore, effects of dissolved oxygen on enhancement of H_2O_2 production have also been investigated with aerated NaNO_3 solutions. The presence of dissolved oxygen caused 15-35% enhancement in H_2O_2 yields for all beams.As a second chemical system, we chose methyl viologen (MV^<2+>) solution in the presence of various concentration of formate and measured the yield of methyl viologen radical cation (MV^<●+>). The G(MV^<●+>) increases with increasing formate concentration. In ^4He^<2+> radiolysis, G(MV^<●+>) increases from 5.7 to 7.1 as the concentration of formate is increases from 10mM to 2M, while in ^<56>Fe^<26+> radiolysis, the MV^<●+> yield changes from 2.2 to 4.1. Other values lie between these ^4He^<2+> and ^<56>Fe^<26+> data. G(MV^<●+>) decreases with increasing LET. In the case of C^<6+> radiolysis, G(MV^<●+>) increases with increasing energy of the carbon ion from 135 to 400 MeV/u corresponding to the decrease of the LET of 21 to 11 eV/nm.In parallel to above measurement, Monte-Carlo simulations of the radiolysis of MV^<2+>/formate solutions have been performed. Ionic strength effects on reactions between charged species are taken into account. To reproduce the experimental results, unreported reactions such as MV^<・+> + ●OH and COO^<・-> + ●OH were introduced in the reaction scheme. After optimization, the rate constants of the reactions have been determined to be 3x10^<10> and 5x10^<10>M^<-1>s^<-1>, respectively. On the contrary, reaction between e^-_<aq> and MV^<・+> is too slow to affect G(MV^<・+>).Furthermore, reproducibility of the Monte Carlo and Spur diffusion calculations was compared. Less