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The performance of a concentrating X-ray monochromator, proposed by Fankuchen (1937), in which the beam is monochromatized by reflexion from a crystal surface inclined at an angle [varphi] to the Bragg reflecting planes, has been studied experimentally by investigating the variation of the intensity of the reflected beam as a function of [varphi] and, for [varphi] = 0, of the surface treatment of the crystal. It is found that the ratio of the reflected to the incident intensity reaches a maximum for a certain value of [varphi], and that this maximum is always less than twice the ratio for [varphi] = 0. An explanation in terms of surface inhomogeneities is suggested, and the formulae derived on these assumptions are found to fit the experimental curves well. The reflected intensity varies widely with the surface treatment applied to the crystal. With calcite the reflexion is enhanced by rubbing the surface with the wooden end of a pencil or by grinding very lightly on the finest sandpaper, but the use of any coarser abrasive decreases the intensity of reflection. The practical application of this kind of monochromator is discussed, and a method of providing a very narrow beam of monochromatic radiation is suggested.
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