Energy Loss and Effective Charge of He, C, and Ar Ions below 10 MeV/amu in Gases

The energy loss associated with the passage of typical heavy ions through various gases has been measured as a function of gas thickness. The ions enter an absorber cell with initial energies of about 10 MeV/amu, held fixed by a magnetic analyzer; the final energy of the ions is determined by analyzing the emergent beam with a magnetic spectrograph. Range-energy curves are obtained for He, C, and Ar ions in ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and Ar and for C ions in C${\mathrm{H}}_{4}$ and He. Over-all accuracy is about 1%. The mean ionization potentials of ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and Ar are determined to be 18.3\ifmmode\pm\else\textpm\fi{}2.6, 79\ifmmode\pm\else\textpm\fi{}7, and 190\ifmmode\pm\else\textpm\fi{}17 eV, respectively. From the stopping power for C and Ar ions relative to that for He ions, their fractional effective charge is computed. Plots of the square of the fractional effective charge as a function of the ratio of the velocity of an ion to that of its own first $K$ electron are in agreement with similar plots for heavy ions in aluminum and oxygen, with one exception, the case of Ar ions in ${\mathrm{H}}_{2}$.