Diffracted beams, plane crystal monochromators

Plane crystal monochromator in the diffracted beam. This case is considered in the next two sections. 

The influence of the plane crystal monochromator in the diffracted beam can also be considered in the context of using the conic section. The plane crystal monochromator is located after the receiving slit at the radius R -... 

The measurements of Campbell show the effect very clearly (Fig. 8-36). Crystals M and C are both calcite (CaCOj) M serves as a flat crystal monochromator set to reflect Abe radiation, and C is the crystal being studied. Two fixed counters with wide slits are arranged to receive the transmitted beam T and the beam D diffracted by the transverse planes. Crystal C is then rotated through a small angle about 6g, and the intensities of T and D are measured as a function... 

For some specific applications, they can be associated with traditional monochromators. An illustration of a typical configuration of this kind is shown in Figure 2.16a. A parabohc artificial crystal is irradiated by a divergent source, and the beam diffracted by this element is then diffracted by a monochromator comprised of two or four plane crystals [SCH 95]. The beam resulting from this system, sometimes referred to as a hybrid monochromator, is perfectly monochromatic and much more intense than in the absence of a parabolic artificial crystal. Note, however, that this beam is much wider than the initial beam produced by the source. Therefore, this monochromator is only used for particular types of configurations and these hybrid monochromators are essentially used for certain studies of epitaxial thin films [STO 97]. 

A four-reflection monochromator comprised of plane single crystals is placed between the source and the sample (see Figure 2.36). The beam irradiates the sample, which is placed on a three axes sample holder. The sample holder makes it possible to orient the normal to any family of ctystal planes in the diffraction plane defined by the axis of the incident beam and the directions of the diffracted beams. Since the beam is strictly parallel, it is not necessary to inclnde a slit between the sample and the monochromator. The intensity of the diffracted beams is meastrred by using a detector which moves along a circle, centered on the sample or with a curved position sensitive detector. This last featrrre makes this system a distant relative of the Debye-Scherrer diffractometers, for which the sample is, by definition, the center of the detection circle. 

In some cases, when attempting to improve the monochromaticity of the diffracted beams, a plane back monochromator is inclnded on which the beams diffracted by the film are diffracted once more [FEW 91]. These analyzer crystals are usually made out of silicon or germanium. A back monochromator like this is comprised of one, two or three crystals. For a specific position of the sample,... 

The resulting incident X-ray beam will diverge thus, a series of optics usually follows the X-ray tube. Soller shts are comprised of thin parallel plates. When placed in the beam path, the shts limit the divergence of the beam perpendicular to the plane in which the diffracted beam intensity will be measured. A crystal monochromator or p-filter can also be applied to the incident beam to suppress X-ray fluorescence. 

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

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