Biophysical investigation on the initial process of heavy-ion impact

Particle irradiation therapy of deep cancers is now in an active progressive practice because of its property to focus the dose due to Bragg peak formation. Around the peak (6-25 MeV/u), the energy loss occurs by the ionization and excitation of biomaterials (mainly, water) and concomitantly, many secondary electrons are released. The released secondary electron rays are thus intimately concerned with the initial process of heavy ion impact, and the spatial energy distribution of the electrons plays an important role for exerting the biological effects. The purpose of the present study is to evaluate the precise double differential cross section (DDCS) for production of the secondary electrons to compare with the pre-existed theory. For this, 6.0 MeV/u C⁴⁺ ion was impacted on the water vapor to measure DDCS producing 10 eV-10 keV electrons released in a broad space angle of 20-60 deg. The observable DDCS energy spectrum gave a broad electron loss to continuum (ELC) peak specific to poly-charged incident electrons. The integral calculus (SDCS, single DCS) of DDCS with the incident angle gave, at the region of low energy range (10-700 eV), the decrease of released electrons due to shield by ion potential derived from orbital electrons within the incident ions. At the energy range, SDCS ratio of incident C⁴⁺/C⁶⁺ agreed with the value calculated from Z² scaling law modified with the effective charge of C⁴⁺: the cross section of incident 6.0 MeV/u C⁴⁺ for water-K-LL Auger electron release was found to be ca.3.33 x 10^-18 cm² vs calculated value ca.3.7584 x 10^-18 cm². (T.T.)