Equilibrium Charge-State Populations of Carbon Ions from 2 to 10 MeV/amu inH2,N2, Ar, and Ni

Measurements of the equilibrium fractions of ${\mathrm{C}}^{6+}$, ${\mathrm{C}}^{5+}$, and ${\mathrm{C}}^{4+}$ ions as a function of ion energy have been made using a gas cell with differentially pumped exit slits and a magnet which spatially separates ions emerging from the slits in different charge states. The separated ion beams are intercepted with different thicknesses of foil and stopped in a junction counter, and charge-state populations are determined from the relative number of counts in each peak of the resulting pulse-height spectrum. Estimated errors are as small as ±0.002 in the population and ±0.05 MeV/amu in the energy. The rms charge of the ions is found to be a function of the material through which they are passing. The rms charge in gaseous ${\mathrm{N}}_2$ and Ar is higher than in solids of neighboring atomic number at ion energies of 3 MeV/amu, but is the same in these gases and in the solids at 8 MeV/amu. At all energies measured, the charge in ${\mathrm{H}}_2$ is anomalously high. The increased charge in solids is ascribed to a large electron-loss cross section at low energies, which shortens the time between collisions to the extent that the loss cross section is affected by excitation of the electron of the carbon ion. Estimates based on a crude model indicate that electron capture by the carbon ions takes place predominantly from the $K$ shell of ${\mathrm{N}}_2$ and the $L$ shell of Ar. The anomalous charge in ${\mathrm{H}}_2$ is apparently due to the absence of a shell from which capture is highly probable.