Ionisation of molecular hydrogen by positrons

A method based on extraction and detection of the produced ions in time correlation with the scattered positrons is used to measure the positron impact ionization cross section (PIICS) and the positronium formation cross section (PFCS). Maximum values of 1.7 x 10 to the -16th sq cm for the PIICS at about 63 eV and 2.3 x 10 to the -16th sq cm for the PFCS at about 20 eV are obtained. While the PIICS is less than the electron impact ionization cross section near threshold, it is greater than it at intermediate energies. The results provide evidence for a structure in the elastic cross section at the positronium formation threshold.