Measurements of absolute reflectivities of mosaic crystals and their wavelength dependence

The absolute reflectivities of a Pb (111) and a Cu (220) crystal in transmission have been measured with very good resolution by a comparison of the diffracted and forward transmitted intensities at two wavelengths, 1.825 and 1.226 Å. When appropriate absorption corrections are made, the theoretical maximum value of 0.5 is obtained for the peak reflectivity of Pb (111) at 1.825 Å. At the shorter wavelength, both the peak reflectivity and the effective mosaic width are reduced. The results obtained with etched crystals can be explained by well known formulae involving secondary extinction. The results for unetched crystals can be satisfactorily accounted for by a model in which there are layers with three different mosaic widths. The corresponding formulae for this model are derived.