AN 18-STATE CALCULATION OF POSITRON-HYDROGEN SCATTERING

We report results for positron scattering by ground-state atomic hydro gen in the energy range 0 to 80 eV. The calculations have been perform ed in an 18-state Ps(1s, 2s, ($) over bar 3 ($) over bar s, ($) over b ar 4 ($) over bar s, 2p, ($) over bar 3 ($) over bar p, ($) over bar 4 ($) over bar p, ($) over bar 3 ($) over bar d, ($) over bar 4 ($) ove r bar d) + H(1s, 2s, ($) over bar 3 ($) over bar s, ($) over bar 4 ($) over bar s, 2p, ($) over bar 3 ($) over bar p, ($) over bar 4 ($) ove r bar p, ($) over bar 3 ($) over bar d, ($) over bar 4 ($) over bar d) approximation where the pseudostates (denoted by a bar) have been tak en from Fon et al. Cross sections are presented for elastic scattering , positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational num bers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the e lastic cross section is now known to better than 10%. The cross sectio n for positronium formation is dominated by capture into the Is state and is in fairly good agreement with the measurements of Weber et al. The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al but are a little larger than other theoretical estimates at the higher energies. There is also a ge neral theoretical problem concerning the degree to which the total cro ss sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the meas urements of Jones et al.