A high-energy phase retarder for the simultaneous production of right- and left-handed circularly polarized x-rays

The availability of new high-energy synchrotron radiation sources, such as the Advanced Photon Source (APS), have made measurements of the spin-dependent momentum distribution of electrons much easier. Making these experiments feasible, however, requires high-energy x-rays (E > 30 keV) with not only a high flux density but also a high degree of circular polarization. Circularly polarized (CP) x-rays have until now been produced essentially in three ways: (i) use of perfect crystal x-ray phase retarders (XPR), (ii) selection of synchrotron radiation off-axis to the orbital plane, and (iii) specialized insertion devices, such as elliptical multipole wiggler (EMW). While each technique has its specific benefits, high-energy phase retarders coupled with a planar undulator have proved to be versatile, simple to operate, economical to fabricate, and extremely successful at energies below 100 keV. Although they may not match a focused EMW beam in throughput flux at these higher energies [1], they do provide other distinct advantages. Because they are the last optical element prior to the sample, the final photon polarization state is well defined and not subject to the instabilities of the particle orbit. Helicity switching can be incorporated thus reducing beam-related errors in the low count-rate signal.