Partial Atomic and Ionic Stopping Powers of Gaseous Hydrogen for Helium and Hydrogen Beams

The partial stopping power of a target material for an ion or an atom is the energy lost per target particle per ${\mathrm{cm}}^2$ in all types of collisions in which the ion or the atom is the projectile, with the exception of those collisions in which the projectile leaves with its charge changed. Partial atomic stopping powers of target gases for atoms are measured by imposing a strong transverse magnetic field on the stopping cell. For partial ionic stopping-power measurements, the ion beam is held in a circular orbit, closely defined by vanes and slits, as it traverses the stopping cell. The partial stopping powers ${\epsilon }_0$ (for ${\mathrm{He}}^0$ traversing ${\mathrm{H}}_2$ gas), ${\epsilon }_1$ (for ${\mathrm{He}}^{+}$), ${\epsilon }_2$ (for ${\mathrm{He}}^{++}$) have been measured in the kinetic energy range 40-460 keV, and new measurements of the ordinary, or total stopping power of ${\mathrm{H}}_2$ gas for helium from 40 to 220 keV have been made. Using the known charge composition of He beams in ${\mathrm{H}}_2$, it is possible to deduce the fraction of the stopping power losses due to charge changing collisions, and this varies from 37% at 140 keV to 27% at 400 keV. At 120 keV, the total energy loss in completing the charge changing cycle ${\mathrm{He}}^{+}$ → ${\mathrm{He}}^0$ → ${\mathrm{He}}^{+}$ is 95±9 eV, rising to 117±10 at 160 keV.