Efficient positronium formation by slow positrons incident on solid targets

A beam of slow positrons in the few-eV region is directed at bakable solid targets in a vacuum of (2-4) × 10⁻⁸ Torr. Measurements of the 2γ/3γ annihilation-yield ratio indicate the conversion of the incident positrons into positronium. The conversion efficiency depends on the target material and incident positron energy, varying from less than 25% at 300 K to 40-85% at 900 K.