Plane crystal monochromators

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... 

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 -... 

Figure 2.11. Diagram showing how a plane crystal monochromator works...

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]. 

The beam line optics consists of a bent Si (111) monochromator. Various crystals with different oblique-cut angles are available, namely a=0°, 6.0°, 7.8°, 9.5°, 11.4°, 13.7° and 16.5° to allow an energy range of 5-25 keV to be covered, by appropriate choice of crystal. There is a bent plane fused quartz mirror (Satow, Mikuni, Kamiya and Ando 1989). 

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. 

The XAS spectrometer is similar to a UV-visible system in that it consists of a source, a monochromator, and a detector. The most favorable XAS source, synchrotron radiation, is tunable to different wavelengths of desirable high intensity. A laboratory instrument for analysis of solids and concentrated solutions may use a rotating anode source (further described in Section 3.3). The monochromator for X-ray radiation usually consists of silicon single crystals. The crystals can be rotated so that the wavelength ( i) of the X-rays produced depends of the angle of incidence (0) with a Bragg lattice plane of... 

FIGURE 2.22 Bragg reflections from a double-crystal monochromator. From the Bragg equation, nX=2ds,m6, t/for the planes of the crystal stays constant, so changing the angle changes the wavelength of the X-rays reflected. 

Ge(l 11) is generally preferred over Si(220) as monochromator crystal due to its higher structure factor. Depending on the nature of the sample one may wish to expand or compress the beam in the horizontal plane by a Fankuchen cut. A compression is particularly useful for the study of small samples (e.g. fibres). The compression is given by ... 

The existing instruments for resonance scattering experiments may be classified according to the monochromator system. As synchrotron radiation is highly polarized in the plane of the orbit, the vertical reflection by the monochromator crystals is preferred. Under these conditions the polarization factor remains nearly constant. With reference to this design feature, the following arrangements exist or are under construction ... 

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