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

Y.Sato1), D.Ohsawa2), A.Higashi3), M.Hirabayashi4), Y.Okada4), S.Uehara5), H.Nikjoo6) and F.Soga1)

1) National Institute of Radiological Sciences, 4-9-1 Anagawa, 263-8555 Chiba-Inageku, Japan
2) Kyoto University, Yoshida-Konoe, 606-8501 Kyoto-Sakyoku, Japan
3) Hyogo Ion Beam Medical Center, Shingu, Hyogo, Japan
4) Science University of Tokyo, Yamazaki, 2641 Chiba-Noda, Japan
5) Kyushu University, 3-1-1 Maidashi, 812-8582 Fukuoka-Higashiku, Japan
6) NASA, Johnson Space Center, Houston TX 77058, USA

We measured the energy and angular distributions (7 eV-10 keV and 20-160 degrees) of secondary electrons produced in the collision of 6.0 MeV/u He2+ ions with water vapor. Binary-encounter collision peaks were clearly observed at the calculated energies at angles <90 degree, 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 15%. The energy distribution (SDCS) were also obtained by integrating the DDCS values with respect to the ejected angles, and compared with the empirical model of Rudd. The energy spectrum (SDCS) show good agreement with the model of Rudd in the energy range of secondary electrons <100 eV. However, in the energy range 100 eV to 1 keV experimental spectrum shows large discrepancies by -40%, 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 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 the radius (r) less than 1 nm the dose is very high due mainly to excitation event induced by low-energy electrons. The penumbra shows a well-known r-2 dependence.