High-LET Radiolysis of Liquid Water with 1H+, 4He2+, 12C6+, and 20Ne9+ Ions: Effects of Multiple Ionization

Monte Carlo simulations are used to investigate the effects of multiple ionization of water molecules on the yields of formation of free radical and molecular species, including molecular oxygen, in the radiolysis of pure, deaerated liquid water by using different types of radiation (1H+, 4He2+, 12C6+, and 20Ne9+ ions) up to approximately 900 keV/microm, at neutral pH and 25 degrees C. Taking into account the double, triple, and quadruple ionizations of water, the primary (or "escape") yields (at 10(-6) s) of the various radiolytic species (G(e(aq)-), G(H*), G(H2), G(*OH), G(HO2*/O2*-), and G(H2O2) are calculated as a function of the linear energy transfer (LET) of the radiation. Our results quantitatively reproduce the large increase observed in G(HO2*/O2*-) at high LET. Under the conditions of this study, the mechanisms of triple and quadruple ionizations contribute only weakly to the production of HO2*/O2*-. With the exception of protons, our calculations also simultaneously predict a maximum in G(H2O2) corresponding to the LET of approximately 4.5-MeV helium ions (approximately 100 keV/microm) and approximately 110-MeV carbon ions (approximately 180 keV/microm). This maximum occurs where G(HO2*/O2*-) begins to rise sharply, suggesting, in agreement with previous experimental data, that the yields of HO2*/O2*- and H2O2 are closely linked. Moreover, our results show a steep increase in the initial and primary yields of molecular oxygen with increasing LET, giving support to the "oxygen in heavy-ion tracks" hypothesis. By contrast, it is found that, in the whole LET range considered, the incorporation of multiple ionization in the simulations has only little effect on the variation of our computed G(e(aq)-), G(H*), G(H2), and G(*OH) values as a function of LET. As expected, G(e(aq)-) and G(*OH) decrease continuously with increasing LET. G(H*) at first increases and then decreases at high LET. Finally, G(H2) monotonically rises with increasing LET. Our calculated yield values compare generally very well with experiment.

Low-energy electron penetration range in liquid water

Monte Carlo simulations of electron tracks in liquid water are performed to calculate the energy dependence of the electron penetration range at initial electron energies between 0.2 eV and 150 keV, including the subexcitation electron region (<7.3 eV). Our calculated electron penetration distances are compared with available experimental data and earlier calculations as well as with the results of simulations using newly reported amorphous ice electron scattering cross sections in the range approximately 1-100 eV.

Monte-Carlo calculation of the primary yields of H2O2 in the 1H+, 2H+, 4He2+, 7Li3+, and 12C6+ radiolysis of liquid water at 25 and 300°C

Monte-Carlo simulations are used to calculate the primary yield of hydrogen peroxide (GH2O2) of the radiolysis of pure, deaerated liquid water as a function of linear energy transfer (LET) of the incident radiation over the range ~0.3–100 keV µm–1, at 25 and 300°C. The radiations include 1H+, 2H+, 4He2+, 7Li3+, and 12C6+ ions with energies from 0.17 MeV to 3.6 GeV. At 25°C, it is found that our GH2O2 values, calculated with protons of different initial energies, show a monotonic increase as a function of LET, in agreement with the commonly assumed expectation of an increase in molecular yields with increasing LET. Our calculated H2O2 yields at 300°C increase significantly faster with LET than do their corresponding 25°C values, showing that the temperature dependence of GH2O2 at higher LET is less than for low-LET radiation. We also report our results on the temporal variations of the H2O2 yields, in the interval ~1 × 10–13 – 1 × 10–6 s, at 25 and 300°C and for the different types of radiation considered. Finally, we find that for incident ions of equal LET > 10 keV µm–1, GH2O2 decreases as the ion velocity increases, from protons (or deuterons) to carbon ions. These differences produced in GH2O2 by changing the type of radiation are explained by the greater mean energy of secondary electrons from the higher velocity ions, which penetrate to a greater average distance from the actual particle track, with a corresponding decrease in molecular yields. Our calculated GH2O2 values compare generally well with the experimental data available from the literature and are also in good accord with the predictions of deterministic diffusion-kinetic model calculations reported earlier.Key words: liquid water, radiolysis, primary yields, hydrogen peroxide (H2O2), linear energy transfer (LET), accelerated protons and heavy ions, temperature, Monte-Carlo simulations.

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Monte Carlo calculation of the primary radical and molecular yields of liquid water radiolysis in the linear energy transfer range 0.3-6.5 keV/micrometer: application to 137Cs gamma rays

Monte Carlo simulations of the radiolysis of neutral liquid water and 0.4 M H(2)SO(4) aqueous solutions at ambient temperature are used to calculate the variations of the primary radical and molecular yields (at 10(-6)s) as a function of linear energy transfer (LET) in the range approximately 0.3 to 6.5 keV/micrometer. The early energy deposition is approximated by considering short (approximately 20-100 micrometer) high-energy (approximately 300-6.6 MeV) proton track segments, over which the LET remains essentially constant. The subsequent nonhomogeneous chemical evolution of the reactive species formed in these tracks is simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of water under various conditions. The results obtained are in good general agreement with available experimental data over the whole LET range studied. After normalization of our computed yields relative to the standard radical and molecular yields for (60)Co gamma radiation (average LET approximately 0.3 keV/micrometer), we obtain empirical relationships of the primary radiolytic yields as a function of LET over the LET range studied. Such relationships are of practical interest since they allow us to predict a priori values of the radical and molecular yields for any radiation from the knowledge of the average LET of this radiation only. As an application, we determine the corresponding yields for the case of (137)Cs gamma radiation. For this purpose, we use the value of approximately 0.91 keV/micrometer for the average LET of (137)Cs gamma rays, chosen so that our calculated yield G(Fe(3+)) for ferrous-ion oxidation in air-saturated 0.4 M sulfuric acid reproduces the value of 15.3 molecules/100 eV for this radiation recommended by the International Commission on Radiation Units and Measurements. The uncertainty range on those primary radical and molecular yields are also determined knowing the experimental error (approximately 2%) for the measured G(Fe(3+)) value. The following values (expressed in molecules/100 eV) are obtained: (1) for neutral water: G(e(-)(aq)) = 2.50 +/- 0.16, G(H(.)) = 0.621 +/- 0.019, G(H(2)) = 0.474 +/- 0.025, G((.)OH) = 2.67 +/- 0.14, G(H(2)O(2)) = 0.713 +/- 0.031, and G(-H(2)O) = 4.08 +/- 0.22; and (2) for 0.4 M H(2)SO(4) aqueous solutions: G(H(.)) = 3.61 +/- 0.09, G(H(2)) = 0.420 +/- 0.019, G((.)OH) = 2.78 +/- 0.12, G(H(2)O(2)) = 0.839 +/- 0.037, and G(-H(2)O) = 4.46 +/- 0.16. These computed values are found to differ from the standard yields for (60)Co gamma rays by up to approximately 6%.

Damage induced by 1-30 eV electrons on thymine- and bromouracil-substituted oligonucleotides

The impact of low-energy (1-30 eV) electrons on self-assembled monolayers of heterogeneous oligonucleotides chemisorbed on a gold surface has been investigated by mass spectrometry of desorbed neutral species in an attempt to understand the consequences of secondary electron damage in a short sequence of a DNA single strand. We demonstrate that the most intense observable neutral species (CN, OCN and/or H(2)NCN) desorbed from Cy(6)-Th(3) and Cy(6)-(BrdU)(3) oligos are related to primary fragmentation of the bases induced by electron impact. The dependence of the neutral species desorption on electron energy shows typical signatures of dissociative electron attachment initiated by the formation of shape- and core-excited resonances (i.e. single-electron and two-electron- one-hole transitory anions, respectively). Substitution of dTh by BrdU increases the production of neutral fragments by as much as a factor of about 3 for the entire electron energy range. When the distribution of secondary electrons along radiation tracks in H(2)O is taken into account, we show that the probability for electron damage to heterogeneous oligonucleotides is enhanced by a factor of 2.5-3 for electron energies below 20 eV for both sensitized and unsensitized strands.

La radiolyse de l'eau et des solutions aqueuses : historique et actualité

Experiments showing that water is decomposed by the action of high-energy radiations date back to the first days of the discovery of radioactivity, a century ago. On the occasion of this anniversary, we have attempted to give a comprehensive account of the radiation chemistry of water and its solutions since its origin, with special emphasis on the various physical and chemical stages that led to the present state of this science. To this aim, we describe the effect of different intervening factors on the molecular and radical yields, including dissolved solute concentration, pH, radiation intensity (or dose rate), type and energy of the radiation, presence of oxygen, temperature, phase, and pressure. We also discuss briefly the chemical behavior of the free radicals produced in radiolyzed aqueous solutions. A good, albeit incomplete, description of the phenomena is obtained that leads to various perspectives concerning, on the one hand, the development of this science and, on the other hand, its potential for applications.Key words : radical chemistry, dilution curve, water, hydrated electron, hydroxyl and superoxide radicals, free radicals, radiolysis, chain reactions, molecular and radical yields, cell survival, linear energy transfer.