Showing papers by "W. E. Kauppila published in 1997"

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

Abstract: We have measured total (${\mathrm{Q}}_{\mathrm{T}}$ ) and positronium formation (${\mathrm{Q}}_{\mathrm{Ps}}$ ) cross sections for 1--302-eV positrons (${\mathrm{e}}^{+}$ 's) and ${\mathrm{Q}}_{\mathrm{T}}$ 's for electrons (${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ 's) scattered by hydrogen atoms and molecules. A beam transmission technique is used to measure ${\mathrm{Q}}_{\mathrm{T}}$ '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. ${\mathrm{Q}}_{\mathrm{Ps}}$ 's are measured using the same scattering cell by detecting coincidences of 511-keV annihilation \ensuremath{\gamma} 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 ${\mathrm{e}}^{+}$ -H ${\mathrm{Q}}_{\mathrm{T}}$ 's and ${\mathrm{Q}}_{\mathrm{Ps}}$ 's agree very well with theoretical calculations. Comparisons of the present ${\mathrm{e}}^{+}$ and ${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$ -H ${\mathrm{Q}}_{\mathrm{T}}$ 's show a merging to within 20% for energies above 12 eV.

Measurements of positron-methane differential scattering cross sections

Abstract: Relative differential cross sections (DCS's) have been measured for 4 to 200 eV ${\mathrm{e}}^{+}$-${\mathrm{CH}}_{4}$ quasielastic scattering (elastic scattering plus rotational and vibrational excitations) from 30\ifmmode^\circ\else\textdegree\fi{} to 135\ifmmode^\circ\else\textdegree\fi{} in a crossed-beam experiment. The observed DCS behavior, including diffraction effects at low energies, is very similar to that found earlier for ${\mathrm{e}}^{+}$-Ar elastic scattering. Comparisons between (${\mathrm{e}}^{+}$,${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$)-(${\mathrm{CH}}_{4}$,Ar) DCS measurements suggest that observed diffraction effects may be correlated with the sign of the net interaction potential and the degree of spherical symmetry of the target.