Strong suppression of the positronium channel in double ionization of noble gases by positron impact

Positron-induced double ionization of helium and neon has been studied at energies from threshold to 900 eV. A remarkable difference between the near-threshold behavior of the single and double ionization cross sections is found: Single ionization is dominated by positronium (Ps) formation, while for double ionization the Ps channel is absent or strongly suppressed. Absolute cross sections for the two targets have been derived and are compared to a modified Rost-Pattard parametrization. Good agreement is found, showing that this type of model can be extended from single to double ionization.