In a crossed-beam experiment the angle-integrated ionization cross section for both positron and electron impact on atomic hydrogen (excluding positronium formation) has been measured in the energy region of 17.5 to 600 eV. Below 450 eV the positron cross section is larger than the electron one, at 50 eV by about a factor of 2. Absolute cross sections are obtained by comparing the electron data with literature values. At energies above 30 eV all theoretical predictions lie below our measured cross sections for positron-impact ionization.

• Received 29 December 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.1019