The 'effective charge' of fast helium ions in metals

Within this project, the stopping cross section ε of 14 metallic elements: Ag, Al, Au, Bi, C, Ca, Ge, Li, Pb, Rb, Si, Sn, Sr, and Y have been measured. For some of these materials no data were available up to now. Protons and helium ions with energies from roughly 75 keV to 750 keV have been used. The measurements were performed in backscattering geometry. For ε, an error of between 3 % and 7 % is assumed. An existing UHV-chamber has been modified (the vacuum system has been improved to give a base pressure of 5x10-10 mbar, and a water-cooled evaporation chamber to produce targets in situ has been attached) to allow measurements on highly reactive elements (for example, Rb). By a new method, the target thickness can be determined on-line with an accuracy of 3 %. The backscattering yields of two targets, which are alternately exposed to the beam, are compared to each other. Our results are compared to four of the most frequently used tabulations. For hydrogen projectiles, discrepancies of up to 50 % are found. The ratio R of ε-values for helium ions and protons of equal velocity gives the 'effective charge' of the helium projectile. The assumption that R does not depend on target properties holds only to first approximation. At least for low projectile velocities it can be shown, that R depends primarily on the plasma density of the target metal. (author)