Proton Irradiation

Published in Scientific Reports, Dr. Kyle Reeves and Professor Yosuke Kanai report their theoretical study of electronic excitation dynamics in liquid water under proton irradiation. In the so-called electronic stopping process, a highly energetic proton induces a time-varying field that is highly localized and heterogeneous on the molecular scale, and massive electronic excitations are produced as a result of the field-matter interaction.

Using their new first-principles quantum dynamics simulation methodology, they revealed molecular details of how electrons are dynamically excited via energy transfer from highly energetic protons on the atto/femto-second time scale. Compared to water molecules in the gas phase, molecules' electronic responses are strongly suppressed in liquid water, and the molecules undergo an ionization process along paths of the energetic irradiating protons. The simulation also shows that the lone-pair electrons on oxygen atoms are primarily responsible for the electronic excitations.

These molecular-level findings represent an important step toward developing a comprehensible understanding of the overall water radiolysis under proton irradiation, which is central to various modern technologies ranging from proton beam cancer therapies to nuclear power plants. The works is supported by the NSF and the DOE's INCITE award.