Computational Study of Low Energy Electrons Through Amorphous Ice and Gaseous Phase Water

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Abstract

The thermalization processes of low energy electrons are important in a number of fields. The low energy secondary electron accounts for significant amounts of energy dissipation from a high energy radiation particle. Many of these primary radiation particles are of technological and medical significance, including the nuclear industry, astrochemistry, astrobiology, biology and hadron therapy. Understanding these low energy processes is crucial to understanding any science that is concerned with radiation effects in materials. In order to accurately predict the effects of radiation, radiation track chemistry including the contribution of low energy electrons first must be understood. The ballistic simulations of the electron on a collision by collision basis allow scientific study of thermalization processes not available to experimentalists; and can illuminate the intricacies of these processes. A computational Monte Carlo simulation approach was developed to calculate various parameters of the thermalization process of the low energy electron. Multiple simulation method types were used to allow future comparison of various factors to experimental data. The method types vary from computationally inexpensive, but unrealistic, to a very computationally expensive but realistic simulation process. Experimental cross-sections for the gas and amorphous ice phases of water are used for sub excitation electrons of energies in the range of 1-20 eV and â‰Â¤100 eV, respectively. The density normalized gas phase is used to compare to other cross-sectional data. The amorphous ice cross-sections are used to represent the liquid phases of water. The thermalization distances, average number of interactions and the range of the low energy electron as it thermalizes in these two mediums were obtained from all the method types. The contribution of the DEA process for electron attenuation in water is calculated as a function of initial electron energy. There is a divide in the scientific community as to the role and total contribution of this process to water radiolysis products and the following chemical stage after irradiation, particularly its role in the generation of excess molecular hydrogen from water and water-oxide interfaces. The percentage of secondary electrons generated at a specific initial energy that terminate their radiation tracks as either a thermalized electron, hydrated electron (or the precursor to e – ) or those that aq become captured in the DEA resonance is calculated for the first time for the purpose to determine the significance of this process. Finally, the effect of the coulomb field from a sibling ionized water radical cation on a thermalizing electron is the next step to understanding the thermalization of the low energy electron in condensed water. This calculation opens the door to simulations on the geminate recombination kinetics of fully thermalized electrons. The dielectric constant of water is taken at its high and low frequency limit to understand the effect of the strength of the coulomb field from the parent cation. Monte Carlo simulations of the decay of the hydrated electron in water with its cation partner can be compared to experimental decay curves. Preliminary predictions of the experimental electron initial kinetic energy after ionization and before localization of solvation are predicted by computing a variety of decay curves for the hydrated electron and comparing with normalized experimental decay signals.

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Accompanying CD with Appendix E: Dose Plots is a slideshow of all the Dose Distribution plots generated during this PhD. It consists of some 1,300+ slides.

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