TRANSFER OF CIRCULAR-POLARIZATION IN BRAGG CRYSTAL X-RAY MONOCHROMATORS

In this paper, the state of polarization of a quasi-monochromatic beam diffracted by a perfect crystal in the Bragg geometry is described in the framework of standard concepts of optics: the polarization transfer function (PTF), the coherence matrix, the Stokes decomposition, and the Mueller matrix. From simulations based on the dynamical theory of X-ray diffraction, we analyze how the phase and amplitude of the sigma and pi-polarization components vary during several successive reflections on perfect single crystals. Different crystal arrangements are then compared from the point of view of their polarization transfer properties, i.e. the conventional double crystal (+, -) monochromator, the antiparallel (dispersive) four-crystal setting and another four-crystal configuration with two ''twisted'' two-crystal modules. The latter is discussed carefully as it makes it possible, in principle, to recover high circular polarization rates (even at Bragg angles approaching 45-degrees) but at the expense of increasing losses in the transmitted intensity. Of particular interest is the comparison of the polarization transfer properties of different crystal pairs, e.g. Si(111) or Ge(111) single crystals. Finally, we call attention to the potential advantages of using asymmetric reflections.