Positrons ionize helium atoms either by impact ionization, resulting in three outgoing particles, or by positronium formation. We determined cross sections for both processes for incident-positron energies ranging from the respective thresholds to 1000 eV. The cross section for impact ionization by positrons (${\sigma }_{\mathrm{ion}}^{+}$) exceeds the corresponding electron cross section (${\sigma }_{\mathrm{ion}}^{-}$) below 500 eV. On the high-energy side of its maximum, the positronium-formation cross section (${\sigma }_{\mathrm{Ps}}$) lies above all theoretical predictions.

• Received 16 July 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.3031