HELICITY SWITCHABLE HIGH ENERGY X-RAY PHASE RETARDER. J. C. Lang, G. Srajer, S. Shastri, D. Haeffner, D. Mills, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439

We have developed a high-energy (E > 50 keV) phase retarder capable of producing both left- and right-handed circularly polarized photons simultaneously. This phase retarder was constructed from a single Ge crystal in a monolithic Bragg-Laue design, similar to that studied previously (D. M. Mills, Nucl. Instr. and Meth. A266, 531 (1988)). This previous phase retarder while able to produce photons with a relatively high degree of circular polarization (P_c>90%), had a major drawback in it's inability to conveniently reverse the photon helicity. The phase retarder in this study overcomes this limitation by simultaneously exciting the (220) and (2-20) crystalographically equivalent and perpendicular reflections. Each of these reflections induces an equal and opposite phase retardation between the [[sigma]] and [[pi]] wave fields in the Laue portion of the crystal, thus producing two x-ray beams with opposite senses of helicity. These beams are spatially separated by approximately 2 mm therefore helicity reversal is easily achieved by chopping between the two beams. A major advantage of this technique is that it retains the non dispersive geometry between the preliminary monochromator (Bragg portion) and the Laue phase retarding crystal therefore preserving the maximum possible throughput of the optic. Details of the construction and characterization of this optic will be presented.