Effect of multiple ionization on the radiolysis of liquid water irradiated with heavy ions a theoretical study using Monte-Carlo simulations
Water makes up a predominant part of the milieu of living tissue, and, not surprisingly, plays a central role for understanding the interaction of ionizing radiation with biological systems. Most aqueous radiation chemistry studies have involved low-linear energy transfer (LET) radiation, such as [superscript 60]Co [gamma]-rays or fast electrons. A survey of the literature shows that the radiolysis of liquid water at low LET is generally well understood. However, at high LET, several reported data have not hitherto been quantitatively explained : (1) The primary yield of hydroperoxyl/superoxide anion (HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-], p[kappa][subscript a] = 4.8) radicals increases with increasing LET, a behavior that is contrary to the other radical yields. As yet, the origin of these HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] radicals is not clearly established, even though they are the major radical species produced at high LET; (2) The primary yield of hydrogen peroxide rises with increasing LET to a maximum, after which it falls. No suitable explanation for the presence of such a decrease in H[subscript 2]O[subscript 2] yields at high LET has been offered; (3) The exact details of the mechanism by which high-LET radiations are very efficient for the inactivation of tumoral hypoxic cells, are still not well known. One possible explanation for the decreased radiobiological oxygen enhancement ratio (OER) at high LET is offered by the generation in situ of an oxygenated microenvironment around the tracks of more densely ionizing radiations (the so-called"oxygen-in-the-track" hypothesis). This work has been originally motivated by the hypothesis proposed by FERRADINI and JAY-GERIN (1998) that multiple ionization (MI) of water would be responsible for the large HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] yield produced in liquid water subject to heavy-ion irradiation. The purpose of this study is to test the validity of this hypothesis. To this aim, Monte Carlo track structure simulations are used to calculate the G-values of the various radiolytic species, including O[subscript 2], generated in the radiolysis of deaerated liquid water by several different types of radiation ([superscript 1]H[superscript +], [superscript 4]He[superscript 2+], [superscript 12]C[superscript 6+], and [superscript 20]Ne[superscript 9+] ions) over a wide range of LET up to [tilde] 900 keV/[micro]m, at neutral pH and in 0.4 M H[subscript 2]SO[subscript 4] (pH 0.46) solutions at 25ÀC. It is found that, upon incorporating the mechanisms of double, triple, and quadruple ionizations of water in the calculations, a quantitative agreement between theory and experiment can be obtained. In particular, in neutral (pH 7) solutions, our results reproduce very well the large increase observed in [Special characters omitted.] at high LET. Under the conditions of this study, the mechanisms of triple and quadruple ionizations make only a minor contribution to the yield of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-]. With the exception of protons, our calculations also simultaneously predict a maximum in [Special characters omitted.] around 100-200 keV/[micro]m in accord with experiment. For each irradiating ion considered, this maximum occurs precisely at the point where [Special characters omitted.] begins to rise sharply, suggesting, in agreement with experiments, that the yields of HO[subscript 2][superscript ¨]/O[subscript 2][superscript ¨-] and H[subscript 2]O[subscript 2] are closely linked. Moreover, the incorporation of MI in our simulations has only little effect on the variation of the computed [Special characters omitted.] and G[subscript ¨OH] values as a function of LET. In the case of acidic solutions irradiated by [superscript 12]C[superscript 6+] and [superscript 20]Ne[superscript 9+] ions, our results also predict a well-defined maximum in the curve of [Special characters omitted.] as a function of LET of [tilde] 1.4 molec./100 eV ([tilde] 45% greater in magnitude than that found in neutral water) around 180-200 keV/[micro]m, in good agreement with experiment. Finally, our simulation results show a steep increase in the initial and primary yields of O[subscript 2] with increasing LET. For example, for 24-MeV [superscript 12]C[superscript 6+] ions (LET [tilde] 500 keV/[micro]m), the initial in situ track concentration of oxygen is estimated to be about 3 orders of magnitude higher than the concentration of O[subscript 2] found in typical human cells. Such results, which largely plead in favor of the"oxygen in the heavy-ion track" hypothesis, could have profound consequences in radiobiology and in particular explain the observed reduction in the oxygen enhancement ratio (OER) with increasing LET. In conclusion, our results strongly support the importance of the role of MI in the heavy-ion radiolysis of water at high LET. They suggest that MI, although infrequent relative to single ionization events, is very efficient chemically.