TOPAS-nBio validation for simulating water radiolysis and DNA damage under low-LET irradiation

The chemical stage of the Monte Carlo track-structure simulation code Geant4-DNA has been revised and validated. The root-mean-square (RMS) empirical parameter that dictates the displacement of water molecules after an ionization and excitation event in Geant4-DNA has been shortened to better fit experimental data. The pre-defined dissociation channels and branching ratios were not modified, but the reaction rate coefficients for simulating the chemical stage of water radiolysis were updated. The evaluation of Geant4-DNA was accomplished with TOPAS-nBio. For that, we compared predicted time-dependentGvalues in pure liquid water for^·OH, e^-_aq, and H₂with published experimental data. For H₂O₂and H^·, simulation of added scavengers at different concentrations resulted in better agreement with measurements. In addition, DNA geometry information was integrated with chemistry simulation in TOPAS-nBio to realize reactions between radiolytic chemical species and DNA. This was used in the estimation of the yield of single-strand breaks (SSB) induced by¹³⁷Csγ-ray radiolysis of supercoiled pUC18 plasmids dissolved in aerated solutions containing DMSO. The efficiency of SSB induction by reaction between radiolytic species and DNA used in the simulation was chosen to provide the best agreement with published measurements. An RMS displacement of 1.24 nm provided agreement with measured data within experimental uncertainties for time-dependentGvalues and under the presence of scavengers. SSB efficiencies of 24% and 0.5% for^·OH and H^·, respectively, led to an overall agreement of TOPAS-nBio results within experimental uncertainties. The efficiencies obtained agreed with values obtained with published non-homogeneous kinetic model and step-by-step Monte Carlo simulations but disagreed by 12% with published direct measurements. Improvement of the spatial resolution of the DNA damage model might mitigate such disagreement. In conclusion, with these improvements, Geant4-DNA/TOPAS-nBio provides a fast, accurate, and user-friendly tool for simulating DNA damage under low linear energy transfer irradiation.

Aqueous solution of UCl6(2-) in O2 saturated acidic medium: an efficient system to scavenge all primary radicals in spurs produced by irradiation

Absorbance measurements find the yield of the oxidation of U(IV) to be (8.75 +/- 0.05) x 10(-7) mol J(-1) in the (60)Co gamma radiolysis of aqueous solutions containing 4.4 x 10(-3) mol L(-1) U(IV) in the presence of O(2) saturated 2 mol L(-1) Cl(-) at pH = 0. This high value of oxidation yield suggests that all primary radicals formed by water decomposition are scavenged in these solutions. Simulations using a nonhomogeneous stochastic kinetic track model agree with the experimental results and are used to explain the mechanism for scavenging radicals and oxidation of U(IV).

Track effects of heavy ions in liquid water

The various types of ionizing radiation can have widely differing radiation effects due to their variation in track structure. The effects of track structure on the radiation chemistry of water are particularly important because of the fundamental aspects and the wide range of practical applications. This review gives an overview of how the physics of energy-loss processes are responsible for the geometry of the particle track and thereby the final product yields. The radiation chemical effects are discussed in qualitative terms to show how the fundamental relationship between the physics and chemistry of particle tracks leads to the observed products. Special phenomena at very high rates of energy deposition are also covered. Finally, a discussion on the future research trends is given.

OH radicals and oxidizing products in the gamma radiolysis of water

The production of OH radicals in the gamma radiolysis of water has been examined with radical scavenger techniques employing formic acid. OH radical yields were found to vary from 2.4 radicals/100 eV at the low scavenger concentration limit to 4.2 at a formic acid concentration of 3 M. An inverse Laplace transform technique was applied to the scavenger concentration dependence to obtain the temporal dependence of OH radicals in pure water. It was found that the relative decrease in OH radical yields from 200 ps to 3 ns was virtually the same for the transform of the scavenger data and the directly measured time-resolved results. The absolute yields for the time-resolved experiments are about 10% higher than expected from the present results with scavengers. The agreement can be considered to be good, and reasons for the observed difference are given. Approximately 40% of the OH radicals produced lead to the formation of hydrogen peroxide, which is the only other major oxidizing species in the gamma radiolysis of water. The net water decomposition for gamma rays was found to vary from an initial value of 5.6 +/- 0.3 molecules/100 eV to 3.8 +/- 0.2 molecules/100 eV at 1 micros.

Effect of electron energy on the radiation chemistry of liquid water

The dependence of the radiation chemistry of water on electron energy is probed using Monte Carlo track-structure simulation and stochastic modeling of diffusion kinetics. Decreasing the initial electron energy from 1 MeV to 100 keV has little effect on the decay kinetics of e(aq)- and OH. or the formation of H2 and H2O2. In the energy range 100 keV to 1 keV, the initial electron energy has a considerable effect on the chemistry; decreasing the electron energy increases the amount of intratrack reaction, lowering the long-time yields of e(aq)- and of OH. and raising the yields of the molecular products. For electrons of initial energy lower than 1 keV, these trends in the kinetics are reversed; the amount of reaction decreases and there is more e(aq)- and OH. surviving intratrack reactions with less H2 formed. The changes in the radical chemistry and product formation are due to changes in the relative distributions of the reactants produced by the primary ionization and excitation events. The reversal at low energies occurs because the local density of reactive species in low-energy tracks is similar to that in the (larger) spurs of high-energy tracks.

Electron energy-loss distributions in solid, dry DNA

Experimentally derived optical constants and X-ray attenuation cross sections were used to construct the complete dipole oscillator strength distribution for solid, dry DNA. Monte Carlo simulations of the energy loss by electrons of initial energy 5 keV to 1 MeV in DNA were performed using cumulative inelastic cross sections obtained from a formulation incorporating the constructed dipole oscillator strength distribution. The energy-loss distribution, the most probable energy loss and the mean energy loss for electrons in DNA are compared to those for liquid water, gaseous water and gaseous hexane. For the most part, the calculations show that electron energy loss in DNA is very similar to that in liquid water; however, it is quite different from both gaseous water and gaseous hexane. The mean energy losses for a 1 MeV incident electron in DNA, liquid water, gaseous water and gaseous hexane are 57.9, 56.8, 50.9 and 38.4 eV, respectively. The large differences found between the predictions for DNA and for the gaseous media bring into serious question calculations of radiation-induced damage in DNA which make use of cross sections for gaseous media. Stopping powers and continuous-slowing-down approximation ranges for the media for electrons are also presented.

Yields of hydroxyl radical and hydrated electron scavenging reactions in aqueous solutions of biological interest

Equations describing the yield of scavenging reactions of the hydroxyl radical and of the hydrated electron in aqueous solutions of biological interest are presented. These equations are shown to be easy to use and they accurately predict the absolute yields of radiolytic products. Examples given include the radiolysis of aqueous solutions of glycylglycine, thymine, and DNA as functions of their concentrations. The effects of nitrous oxide and of varying concentrations of added radioprotectors are also shown. Application of the equations to other systems as well as their use in the estimation of heterogenous effects is discussed.

Concerning plasmon excitation in liquid water

The existence of plasmon excitation produced by the interaction of fast charged particles in liquid water is examined critically from the theoretical point of view. By the analysis of the Fano index at different excitation energies and by comparing the real and imaginary parts of the dielectric function for both the liquid and the gas at unit density, we find no convincing evidence for plasmon excitation in the condensed phase of water. A density and a condensation effect on the dielectric function are observed. Implications of this finding for diffusion kinetics are discussed briefly. A procedure for the determination of the real and imaginary parts of the dielectric function from the dipole oscillator strength distribution is presented.

Molecular product formation in the electron radiolysis of water

The time dependence of the formation of a molecular product in radiation chemistry is linked to the yield of the product formed in scavenging experiments by a Laplace transform relationship. Kinetic modeling with deterministic methods is used to show that such a relationship can be used to describe the molecular product (H2 and H2O2) formation following the fast-electron radiolysis of water and of aqueous solutions. Experimental yields are fitted using an appropriate empirical function, and the time dependence of the yields of the molecular products in the absence of a scavenger is derived using the Laplace relationship.

Comparison of stochastic and deterministic methods for modeling spur kinetics

Stochastic and deterministic kinetic methods have been used to model the temporal evolution of spatially nonhomogeneous clusters of reactants resulting from the dissociation of one to six water molecules into either H3O+, OH, and e-aq, or H atoms and OH radicals. When the ionic fragmentation initially producing H3O+, OH, and e-aq is considered, the stochastic and deterministic methods predict similar time dependences for the decay of the reactive species; however, the two methods suggest very different product yields. For a two-dissociation spur, the deterministic treatment overestimates both the H2 and the H2O2 yields by about 70%. The error decreases to less than 15% for a spur with six water dissociations. For a distribution of spurs representing a high-energy electron track, the differences in the predicted yields of reactants are less than 6% at 0.1 microseconds, but the stochastic and deterministic predictions for the yields of H2 and H2O2 differ by about 50%. The kinetics of spurs produced by the fragmentation of water to H atom and OH radical shows discrepancies in both the reactant and the product yields. The size of the discrepancy decreases as the number of H/OH pairs increases, and the predictions of the two techniques are almost the same for clusters of six water dissociations.

The production of OH radicals in the radiolysis of water with 4He ions

Formic acid solutions of 1, 10, 100, and 1000 mM have been irradiated with 4He ions of 5 to 25 MeV, and the production of OH radicals has been determined by measuring the yield of CO2. The differential OH radical yields were obtained from the observed energy dependencies; with 25 MeV 4He ions they range from 1.91 to 3.48 molecules/100 eV for formic acid concentrations of 1 to 1000 mM, respectively. The OH radical yields decrease with decreasing particle energy, and at the maximum LET (230 eV/nm) they range from 0.30 at 1 mM to 0.82 molecules/100 eV at 1000 mM. These values are only 15 to 20% of that found with fast electrons. The OH radical yields are relatively more dependent on formic acid concentration at higher 4He ion energies. The average time dependencies of the OH radical from 7.7 ns to 7.7 microseconds were estimated from the formic acid concentration dependencies at various 4He energies. In terms of absolute yields, there is a considerable variation in the yields of OH radicals with time at the highest energies, but at the maximum LET the OH radical yields are nearly invariant with time after about 10 ns.