Experimental Cross Sections for Charge-Changing Collisions ofHe+andHe++Ions Traversing Gases

Abstract

Ion beams of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{++}$, in the kinetic energy range 100 to 450 kev, are held in arcs of circular orbits in a magnetic field. When a few microns of gas are admitted the beam is attenuated by charge-changing collisions, since with change of charge the ion is lost from its orbit. Cross sections for such charge-changing collisions are designated by ${\ensuremath{\sigma}}_{\mathrm{if}}$, where $i$ is the initial positive ionic charge in electron units and $f$ the charge after the collision. (${\ensuremath{\sigma}}_{10}+{\ensuremath{\sigma}}_{12}$) has been directly measured for ${\mathrm{He}}^{+}$ in hydrogen, helium, and air, and (${\ensuremath{\sigma}}_{21}+{\ensuremath{\sigma}}_{20}$) for ${\mathrm{He}}^{++}$ in the same gases.Other observers have measured the equilibrium ratio ${\mathrm{He}}^{+}$/${\mathrm{He}}^{++}$ attained in a field-free beam after many collisions, and combining these data with our observations allows calculation of ${\ensuremath{\sigma}}_{10}$ and ${\ensuremath{\sigma}}_{12}$ separately if one assumes ${\ensuremath{\sigma}}_{02}$ negligible compared to the other cross sections. The electron loss cross sections ${\ensuremath{\sigma}}_{12}$ increase with energy for helium and air throughout the measured region and are of the order ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$; ${\ensuremath{\sigma}}_{12}$ for hydrogen vs energy shows a broad maximum at about 370 kev and 0.98\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ per hydrogen atom.The capture cross sections ${\ensuremath{\sigma}}_{10}$ decrease rapidly in the measured energy range and in the region 200-450 kev those measured in helium agree, within the estimated experimental error, with theoretical calculations of H. Schiff.In attempting the resolution of the sums (${\ensuremath{\sigma}}_{21}+{\ensuremath{\sigma}}_{20}$) into the separate cross sections it is found that the errors in the measurements accumulate to such an extent that the individual values become very unreliable. An auxiliary experiment designed to increase the accuracy of our knowledge of the separated capture cross sections ${\ensuremath{\sigma}}_{20}$ and ${\ensuremath{\sigma}}_{21}$ is in progress.