Partial cross-section measurements for ionization of helium by positron impact.

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