Results of the Bielefeld-Brookhaven e+-H experiment

Positronium formation (Ps) cross sections for positrons impinging on atomic hydrogen were measured in the impact energy range from 13 to 255 eV at the High Intensity Positron (HIP) beam at Brookhaven National Laboratory (BNL). The Ps-formation cross section was found to rise rapidly from the threshold at 6.8 eV to a maximum value of (2.98±0.18) ×10^–16 cm² for 15 eV positrons. By 75 eV it drops below the detection limit of 0.17×10^–16 cm² which is the present level of statistical uncertainty. A more elaborate data analysis was performed leading to somewhat smaller cross sections than originally published [1]. During the course of this experiment further relative data on impact-ionization cross sections were accumulated.