Experimental Electron-Transfer Cross Sections for Collisions of Oxygen Ions in Argon, Nitrogen, and Helium at Energies of 7-40 MeV

In the energy range 7-40 MeV, electron-capture and electron-loss cross sections have been determined for incident oxygen ions of charges +2 to +8 passing through argon, nitrogen, and helium. Thin-target conditions have been used in all cases. The analysis technique used to extract the cross sections allows relative cross sections to be obtained without consideration of the measurement of the gas target thickness. Multiple-transfer cross sections can thereby be determined reliably in the presence of cross sections three orders of magnitude greater. Double, triple, and quadruple transfers were observed. The electron-loss cross sections appear to pass through maxima when the velocity of the ion is in the vicinity of the velocity of the electron to be lost. Single-electron-capture cross sections have magnitudes as large as ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 40 MeV in argon, and depend on velocity approximately as ${V}^{\ensuremath{-}n}$, where $n$ lies between 3 and 6 for the heavy gases. The importance of capture from inner shells of the heavy gases is inferred by comparison with helium, in which $n$ is 8 or 9 and the cross sections are relatively small. The possibility of capture into the $K$ shell of the oxygen ion is suggested by the systematic differences observed in capture by +7 and +8 oxygen ions from capture by lower-charged ions.