Polarization ratios for repeatedly reflected X-ray beams

The polarization ratio for a repeatedly reflected X-ray beam has been calculated, assuming the Darwin formulism, in the case where the diffracting planes of the crystals are parallel. It is shown that polarization ratios lie closer to unity than those obtained using the kinematic approximation. The integrated intensities and polarization ratios from a double-crystal spectrometer are discussed. The polarization ratio for a graphite monochromator has been measured experimentally and shown to be close to the value predicted by the Darwin theory and significantly different from the kinematic value.