K-shell Core Electron Excitations in Electronic Stopping of Protons in Water from First Principles

Understanding the role of core electron excitations in liquid water under proton irradiation has become increasingly important due to the growing use of ion beams in radiation oncology as an alternative to the X/γ-ray photon radiation. Using a first-principles, non-equilibrium simulation approach based on real-time TDDFT, we accurately determine the electronic stopping power over a wide range of proton velocities. The result agrees with available experimental data and also with the SRIM model. Explicit treatment of the K-shell, 1s electrons of oxygen atoms in water was found to be necessary to accurately predict the electronic stopping power for large velocities. The simulations also reveal that excitations of K-shell core electrons influence the valence electron excitation itself, and the separate treatment of valence and core electron excitation is not satisfactory for large velocities.