Polarized positron beams

Another important feature of e e machines is that the magnetic guide field together with the synchrotron radiation lead to transverse polarizations of the beams, with electron (positron) polarization antiparallel (parallel) to the magnetic field. The polarization arises because the synchrotron radiation induces up-down spin transitions which, for the positron, say, are larger for down—>up than for up-+down, where up means along the guide field B. 

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