Total-cross-section measurements for positron and electron scattering by O2, CH

Abstract

Total cross sections (${Q}_{T}$'s) have been measured for 1--500-eV positrons and electrons scattered by ${\mathrm{O}}_{2}$, ${\mathrm{CH}}_{4}$, and ${\mathrm{SF}}_{6}$ using a beam-transmission technique. The positron ${Q}_{T}$'s are compared with the corresponding electron ${Q}_{T}$'s for each target gas. It is found that the positron ${Q}_{T}$'s are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as ${\mathrm{SF}}_{6}$. The positron ${Q}_{T}$ curve for ${\mathrm{CH}}_{4}$ reveals a significant increase in the vicinity of its positronium formation threshold, while the ${\mathrm{O}}_{2}$ and ${\mathrm{SF}}_{6}$ curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron ${Q}_{T}$'s for these gases. Interesting similarities are found in a comparison of the present positron and electron ${Q}_{T}$ curves for ${\mathrm{CH}}_{4}$ with prior comparison measurements for argon.