Methods for the Simulation of the Slowing of Low-Energy Electrons in Water.

A computational Monte Carlo simulation approach for modeling the thermalization of low-energy electrons is presented. The simulation methods rely on, and use, experimentally based cross sections for elastic and inelastic collisions. To demonstrate the different simulation options, average numbers of interactions and the range of low-energy electrons with initial energies ranging from 1 to 20 eV are calculated for density normalized gaseous water. Experimental gas-phase cross sections for (subexcitation) electrons of energies in the range of 1-20 eV were taken from the compilation of Hayashi. The ballistic collision-by-collision simulations provide information on the intricacies of the thermalization processes not available experimentally. © 2018 Wiley Periodicals, Inc.

Note: Establishing α-particle radiation damage experiments using the Dalton Cumbrian Facility's 5 MV tandem pelletron.

Evaluating the radiation stability of mineral phases is a vital research challenge when assessing the performance of the materials employed in a Geological Disposal Facility for radioactive waste. This report outlines the setup and methodology for efficiently allowing the determination of the dose dependence of damage to a mineral from a single ion irradiated sample. The technique has been deployed using the Dalton Cumbrian Facility's 5 MV tandem pelletron to irradiate a suite of minerals with a controlled α-particle ((4)He(2+)) beam. Such minerals are proxies for near-field clay based buffer material surrounding radioactive canisters, as well as the sorbent components of the host rock.

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).

Stochastic simulation of gamma radiolysis of acidic ferrous sulfate solution at elevated temperatures.

The effect of temperature on the oxidation of Fe2+ in the gamma radiolysis of acidic ferrous sulfate solutions has been modelled using the stochastic IRT method, incorporating simulated electron track structures. There is an 11% increase in the Fe3+ yield in aerated 0.4 M H2SO4 solution on going from 298 to 573 K (25 to 300 degrees C). The H2 yield in aerated solution increases about 24% from 0.41 at 298 K to 0.51 at 573 K. In the absence of oxygen, the increase in the yields of both Fe3+ and H2 is about 12%. Calculated yields are consistent with available experimental data at room, and elevated temperatures. Simulations for heavy water (D2O) show similar temperature effects to those found for ferrous sulfate solutions in light water (H2O).

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.

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.