Low-energy elastic scattering of positrons on argon

Elastic scattering of positrons from argon has been calculated for incident-positron energies of up to 300 eV. The target atom is represented by a frozen core in a continuum relativistic Hartree-Fock calculation with dipole- and quadrupole-polarization corrections. The cutoff parameter $r_0$ is fitted to reproduce measured values of the scattering length. Scattering lengths near -4 a.u. give good agreement of the calculations with recent measurements of total, differential, and momentum-transfer cross sections below 3 eV. Relativistic and positron-polarization effects are found to be much smaller than for electrons. The relativistic effects are dominated by the relativistic contraction of the target atom.