Stopping power and ranges of electrons, protons and alpha particles in liquid water using the Geant4-DNA package

This paper presents stopping power and ranges of electrons, protons, and alpha particles in liquid water, calculated using the latest Geant4-DNA processes implemented in the Geant4 Monte Carlo simulation toolkit. Inelastic cross sections are obtained using the first Born approximation and semi-empirical formulas like Rudd's model for ionisation and the Miller and Green formula for excitation. Elastic collisions and vibrational excitations are considered for tracking electrons until complete thermalisation (0.025 eV). A speed scaling procedure with an effective charge screening term was used to compute alpha particle and heavy ion cross sections. Geant4-DNA simulations were carried out using thin liquid water volumes to determine the linear energy loss (dE/dX), while larger volumes were used to obtain the particle range. While results converge for highly energetic particles, differences are observed for low energies when the applied theoretical models begin to diverge from each other. Results show a good agreement between the analytical calculations obtained from the models, the Geant4-DNA Monte Carlo simulation predictions and the data published in the ICRU reports. Geant4-DNA processes apply to the following energy ranges: 0.025 eV-1 MeV for electrons, 100 eV-100 MeV for protons and 1 keV-400 MeV for alpha particles in liquid water, however since experimental data for very low energies is scarce and very difficult to obtain these processes could not be thoroughly validated so they are recommended for energies above 1 eV for electrons, 1 keV for protons and 10 keV for alpha particles. Relativistic, highly charged ions were implemented in our own “house” version of the code and will be available in future releases of Geant4.