We propose a method for polarising antiprotons in a storage ring by means of
a polarised positron beam moving parallel to the antiprotons. If the relative
velocity is adjusted to v/c≈0.002 the cross section for spin-flip is
as large as about 2⋅1013 barn as shown by new QED-calculations of
the triple spin-cross sections. Two possibilities for providing a positron
source with sufficient flux density are presented. A polarised positron beam
with a polarisation of 0.70 and a flux density of approximately 1.5⋅1010/(mm2 s) appears to be feasible by means of a radioactive 11C
dc-source. A more involved proposal is the production of polarised positrons by
pair production with circularly polarised photons. It yields a polarisation of
0.76 and requires the injection into a small storage ring. Such polariser
sources can be used at low (100 MeV) as well as at high (1 GeV) energy storage
rings providing a time of about one hour for polarisation build-up of about
1010 antiprotons to a polarisation of about 0.18. A comparison with other
proposals show a gain in the figure-of-merit by a factor of about ten.Comment: 13 pages, 8 figures; v2: minor language and signification corrections
v3: (14 pages, 12 figures) major error, nonapplicable polarisation transfer
cross sections replaced by the mandatory spin-flip cross section