Ionisation of noble gases by positron impact

The positron induced absolute single and double ionisation cross sections f or the noble gases Ar, Kr and Xe have been determined in the energy region from below threshold to similar to 1000 eV. With the use of an admixture of He to the target gas normalisation was performed by comparison to known si ngle ionisation cross sections for He. Special interest has been taken in t he energy gap of 6.8 eV below the direct double ionisation threshold energy , where only double ionisation with positronium formation is possible. This has been done to study the significance of double ionisation with Ps forma tion for the heavy noble gases.