Secondary Electrons from Water Vapor with the Impact of 6.0 MeV/u He2+ Ions: Atomic Data and their Application to Biomedical Investigations

Abstract

We measured the energy and angular distributions (7 eV–10 keV and 20°–160°) of secondary electrons produced in collisions of 6.0 MeV/u He2+ ions with water vapor. Binary‐encounter collision peaks were clearly observed at the calculated energies at angles of <90°, as well as the K‐LL Auger peak of oxygen at about 500 eV for all angles. From these measurements, the doubly differential cross sections (DDCS) of electron emissions were deduced with an estimated uncertainty of ±13%. The energy distribution (SDCS) was also obtained by integrating the DDCS values with respect to the ejected angles, and compared with an empirical model of Rudd. The energy spectrum (SDCS) showed good agreement with the model of Rudd in the energy range of secondary electrons of <100 eV. However, in the 100–1000 eV energy range, the experimental spectrum shows significant discrepancies, smaller by 30%, and is nearly twice greater in the region >3keV. To assess the new cross sections, these values were incorporated in the kurbuc Monte‐Carlo track structure code system for a simulation of secondary electrons. Radial dose distributions for 6.0 MeV/u He2+ ions were obtained by analyzing the tracks generated by the code kurbuc using the new DDCS values. In the core with a radius (r) of less than 1 nm, the dose is very high due mainly to excitation events, induced by low‐energy electrons. The penumbra shows a well‐known r−2 dependence.