Focusing monochromators

Focusing monochromators concentrate the diffracted beams into small areas of their focal surface; they are particularly efficient for photographic intensity recording. Crystal monochromators must be capable of resolving the Kα₁−Kα₂ doublet to prevent the production of a doubled diffraction pattern. They therefore require monochromator crystals with a very small mosaic spread and fine or very fine focus X-ray tubes. They reflect the X-rays on the lattice planes very close to the surface. The intensity of the reflected beam depends essentially upon the condition of the crystal surface and upon the matching of the mosaic spread, the aberrations of the focusing geometry and the dimensions of the X-ray source; it depends very little upon the value of the integrated intensity calculated for the mosaic state of the monochromator material. The polarization ratio of the reflected beam is very nearly r = |cos 2θ| as for a perfect crystal. Mirrors set at an angle very close to the critical angle for the Kα radiation do not reflect the Kβ and shorter wavelength components. Absorption reduces the sharpness of the cut-off and the reflectivity near the critical angle. The intensity of the reflected beam is proportional not only to the angular aperture, but also to the reflectivity near the critical angle. The choice between crystal monochromators and mirrors depends mainly upon the size of the specimen: curved crystals give fairly convergent beams (1° to 3°), and mirrors narrow and quasi-parallel ones (2′ to 5′).