Measurements of total and positronium formation cross sections for positrons and electrons scattered by hydrogen atoms and molecules

We have measured total (Q_T ) and positronium formation (Q_Ps ) cross sections for 1–302-eV positrons (e⁺ 's) and Q_T 's for electrons (e^- 's) scattered by hydrogen atoms and molecules. A beam transmission technique is used to measure Q_T 's where the projectile beam passes through a low-temperature scattering cell containing a mixture of hydrogen atoms and molecules generated in an adjacent radio-frequency discharge region. Q_Ps 's are measured using the same scattering cell by detecting coincidences of 511-keV annihilation γ rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the walls of the scattering cell in which the Ps is formed. The present e⁺ -H Q_T 's and Q_Ps 's agree very well with theoretical calculations. Comparisons of the present e⁺ and e^- -H Q_T 's show a merging to within 20% for energies above 12 eV.