Fundamental interactions of positrons with hydrogen atoms

In a crossed beam experiment an intense positron beam is scattered by a beam of hydrogen atoms. Ionized atoms, i.e. protons, are detected with a quadrupole mass analyser (QMA) and distinguished from ions of higher masses. Further forwardly scattered positrons are detected by a channeltron electron multiplier (CEM) in time correlation with the QMA pulses. While the QMA signal rate is proportional to the sum of the positronium formation and impact ionization cross sections the correlated rate only depends on impact ionization events. After proper normalization the difference of QMA counts and correlated counts yields the cross section for positronium formation. Results will be presented. Recently the experiment has been modified to enable the measurement of doubly differential cross sections. Angle-dependent and energy-loss dependent impact ionization cross sections can be measured as well as differential elastic scattering probabilities. Initial tests with an electron beam were performed successfully. Preliminary data on differential cross sections are expected to be available