Toward measurements of total cross sections for positrons and electrons scattered by potassium and rubidium atoms

We have attempted to measure total cross sections (${\mathit{Q}}_{\mathit{T}}$’s) for positrons and electrons scattered by potassium and rubidium atoms in the energy range 1–102 eV using a beam-transmission technique. The measurements are subject to increasing errors as the projectile energy is reduced below 20 eV due to the incomplete discrimination of our apparatus against projectiles scattered through small angles. Our measured positron-K and -Rb ${\mathit{Q}}_{\mathit{T}}$’s each show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV. The observed peaks may be artifacts due to the incomplete-discrimination problems referred to above, but careful consideration of those problems does not resolve the marked discrepancies between our low-energy positron-Rb results and those obtained in recent five-state close-coupling-approximation calculations.