Ionization of argon and krypton by positron impact

Total single and double ionization cross sections for argon and krypton by positron impact have been measured for projectile energies ranging from thr eshold to similar to 1 keV. The single ionization cross sections are seen t o behave as expected, showing large contributions from positronium formatio n at low energies. In the double ionization cross sections there seems to b e a lack of positronium formation at threshold, while a clear contribution from the process is seen at intermediate energies.