Dottorato in FISICA

Titolo della tesi: Development of ionization models in Geant4-DNA to simulate the effect of cosmic rays on molecules in the atmosphere

Cosmic rays play a crucial role in atmospheric chemistry by generating ions, which influence atmospheric chemistry leading to key processes such as aerosol formation, cloud microphysics. Despite their importance, current atmospheric models rely on macroscopic parameterizations that lack the molecular-scale accuracy needed for detailed chemical calculations. A more precise understanding of ion-driven reactions requires particle-molecule interaction models that explicitly account for the molecular structure of atmospheric gases. This study represents the first step toward extending the Geant4-DNA toolkit for atmospheric applications. The primary goal of this work was to develop and validate new interaction models for electrons and positrons in N2, O2, and CO2, covering the energy range 10 eV – 10 GeV. The models include elastic scattering, ionization, electronic excitation, and positronium formation processes. Cross-section models were optimized for the target molecules, and both differential and total cross sections were benchmarked against theoretical calculations and experimental data. The implemented models accurately reproduce analytical predictions, and stopping power and range calculations show agreement within 5% with the ESTAR database up to 1 MeV and within 10% up to 1 GeV. To test the validity of these models in a realistic atmospheric scenario and to demonstrate their capabilities, we coupled the CORSIKA code for extensive air showers simulations with a dedicated Geant4 appication tracking secondary particles down to low energies. The ionization yield at 10 km altitude was compared with literature data, particularly with the electromagnetic component of the ionization yield reported by Usoskin et al. and Mishev and Velinov. The calculated ionization yield is fully compatible with the electromagnetic component of the literature data, although the statistics used are too low to make definitive predictions. This work marks a significant contribution to the field of climate modeling by introducing an innovative approach for characterizing ion production in the atmosphere and addressing key challenges in atmospheric chemistry. The newly implemented interaction models provide a molecular-level description of electron and positron interactions in atmospheric gases, necessary to link radiation transport to chemical reaction calculations. Future efforts will focus on improving computational efficiency, increasing statistics, and incorporating missing secondary contributions to further enhance the accuracy of ionization modeling in the upper atmosphere.