Charge States of Heavy-Ion Beams Passing through Gases

Abstract

Measurements are reported of the fraction of an ion beam in the various charge states (-1, 0, +1, +2) after equilibrium has been established between competing electron capture and loss reactions. The ions ${\mathrm{H}}^{+}$,${\mathrm{He}}^{+}$, ${\mathrm{N}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{A}}^{+}$ were passed through the gases hydrogen, helium, nitrogen, oxygen, air, neon, and argon. The energy range studied was 20 kev to 250 kev. Under conditions of equilibrium between the singly ionized and neutral states, the velocity dependence of the charge ratios is given by $\frac{{\ensuremath{\sigma}}_{01}}{{\ensuremath{\sigma}}_{10}}={K}_{1}{v}^{m}$, where ${\ensuremath{\sigma}}_{\mathrm{if}}$ is the cross section for the transition between initial charge $i$ and final charge $f$, $v$ is the velocity, and ${K}_{1}$ and $m$ are constants for each ion-stopping gas combination. The corresponding charge ratio for equilibrium between doubly and singly ionized states is given by $\frac{{\ensuremath{\sigma}}_{12}}{{\ensuremath{\sigma}}_{21}}={K}_{2}{(v\ensuremath{-}{v}_{c})}^{m}$, where ${K}_{2}$, ${v}_{c}$, and $m$ are constants for each ion-stopping gas combination. The values of the constants ${v}_{e}$ and $m$ are tabulated. Negative ions are detectable only in the hydrogen beams. They constitute approximately 1 percent of the beam at 30 kev, and decrease in number rapidly at higher energies.