Measurement of positronium formation in positron collisions with hydrogen atoms

The positronium-formation cross section was measured for positron collisions with atomic hydrogen in a crossed-beam experiment. Measurements were made in the energy range of 13 eV≤E≤205 eV. At E=15±2.8 eV we observed a maximum cross section of (4.3±0.8)×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$; because of averaging over an energy-resolution function of 6.5 eV FWHM, the observed maximum is a lower limit of the true maximum. For E≥75 eV the cross section is zero within our error margin of 0.3×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$. No other measurements but numerous theoretical predictions are available for comparison.