Monochromator, crystal focusing

There are distinct advantages in using a focusing camera in conjunction with a curved-crystal focusing monochromator the convergent... 

Fig. 70. Focusing cameras, (a) Seeman-Bohlin type 1.0AH — 1.0BR = 180°—20. (b) Asymmetric arrangement to give reflections at 20 < 90°. (c), (d), (e), and (/) Arrange ments of curved-crystal focusing monochromator (0) with focusing camera.

There are three accessories used to produce monochromatic radiation metal foil filters, crystal monochromators, and focusing mirrors. An element with atomic number Z can be used as a selective filter for radiation produced by an element of atomic number Z+ 1. For example, a nickel (Z=28) absorption filter, may be used to cut out the Cu KjS (Z=29 for Cu) radiation, leaving only Cu Ka radiation.Not all white radiation, however, is eliminated by this method. Alternatively a single-crystal monochromator may be used. An intense Bragg reflection from the monochromator crystal is used as the incident beam for X-ray diffraction studies. Focusing mirrors, designed to produce a beam that is not only monochromatic but also convergent, may be used. In this case the incident beam is doubly deflected by two perpendicular mirrors. 

If the monochromating crystal is bent but not cut, some concentration of energy will be achieved inasmuch as the reflected beam will be convergent, but it will not converge to a perfect focus. 

Even though intensity is decreased during diffraction by a monochromator, a KP filter is not needed because the monochromator is set to diffract only Aa radiation. As a result, and because of the focusing action of the monochromator, the intensity of a diffraction line at the counter can actually be higher with a monochromator than without, particularly if the monochromating crystal is graphite. 

Various optical elements can be placed in the beam path to tailor the characteristics of the X-ray beam. These can work by diffraction e.g. a monochromator crystal), reflection e.g. a mirror), or absorption e.g. a filter or slits). A monochromator is used to select a particular wavelength, a mirror can focus the beam or suppress higher harmonics, and filters can be used to remove unwanted radiation. 

The monochromator crystal need not always be flat as illustrated in Figures 2.9 and 2.10 but may be bent or otherwise shaped in such a way to have the reflected rays from different parts of the crystal focus at a single line or a single point. In this way a large gain in the monochromatized beam flux can be achieved. There are a... 

The use of an organic monochromator made the reflected beam practically nondivergent in the horizontal plane. The vertical divergence was 2° when two 4-mm-high entry slits were used. The focusing arrangement was such that the counter slit, the center of the surface of the sample, and the center of the surface of the monochromator crystal (acting as the source of rays) were located on the same circle. 

The results are described of a study of the relationship between the reflection intensity of x rays and the degree of perfection of crystals used in focusing monochtomatois. An expression has been obtained which gives the variation in the intensity with the integrated reflection width and the size of the focus of the radiation source. Analytical conditions ate presented for estimating the optimal mosaic parameters (size and disorientation angle of the blocks), which the monochromator crystal should satisfy to obtain the maximum reflection intensity. 

The effect of the degree of perfection of monochromator crystals on the basic characteristics of focusing monochromators, and particularly on the intensity of the reflected beam, has not received its due attention in the literature. For example, it has been considered [5] that tubes with small focus dimensions and the most perfect crystals must be used to obtain a narrow intense beam of monochromatic radiation. However, measurements made on quartz of different degrees of perfection [6] showed that the maximum intensity of a monochromatic beam is observed for "average" degrees of perfection of the monochromator crystal. 

The reflection intensity is zero for rays for which the angle a is higher in absolute value than the integrated width of the reflection from the monochromator crystal. The effective width of the focus area whose intensity will make a contribution to the reflected beam will evidently decrease with decreasing integrated width of the reflection curve, i.e., the more perfect the monochromator crystal. Consequently, to obtain the maximum intensity for the reflected monochromatic beam, it is necessary to establish a definite optimum relationship between the effective width of the focus and the integrated width of the reflection curve of the monochromator crystal. 

Let the intensity distribution at the focus be represented by the function G(o ), the reflectivity of the monochromator crystal by R(J, a, X), and the spectral intensity distribution of the primary beam by i(X - Xg) " 16 reflection intensity from the monochromator is proportional to the product of these functions [6]. The total intensity of the monochromator beam when the monochromator is placed at the Bragg angle (t> = t>g) is then defined by the following expression ... 

The dependence of the change in Iju on the focus parameters of the x-ray tube and the degree of perfection of the monochromator crystal can be traced for a specific model without affecting the generality of the qualitative conclusions. Let the intensity distribution at the focus be represented by a Gaussian function G (a) = Gg exp [— Here, the total power of the... 

As may be seen from Fig. 2, the intensity fimction of a beam reflected from the focusing monochromator passes through a maximum, whose value increases with the reflectivity of the crystal. With a decrease in reflectivity of the monochromator crystal, the maximum is displaced toward lower values of Wr. 

In the Debye - Scherrer diffractometer with monochromator (Fig. 29). the line focus of the X-ray tube, the monochromator crystal, and the detector window all lie on the focusing circle of... 

This camera (see Fig. 10) uses a cross between the Franks and Guinier optics [3]. The monochromator crystal is used to select K i and focuses the beam in the horizontal direction. The glancing incidence mirror focuses rays in the orthogonal direction. This camera can resolve repeat spacings beyond the 100 nm mark. This camera is not commercially available, although component parts can be purchased. For those with the... 

Figure 6.22. The source of the scanning x-ray spot in the PHI XPS microprobe is an electron beam scanning over an aluminum anode. Emitted x-rays are monochromated and focused onto the sample surface by a curved quartz crystal. (From PHI [366] reproduced with permission.)...

The line width of the X-ray source is on the order of 1 eV for A1 or Mg Ka sources but can be reduced to better than about 0.3 eV with the use of a monochromator. A monochromator contains a quartz crystal which is positioned at the correct Bragg angle for A1 Ka radiation. The monochromator narrows this line significantly and focuses it onto the sample. It also cuts out all unwanted X-ray satellites and background radiation. An important advantage of using a monochromator is that heat and secondary electrons generated by the X-ray source cannot reach the sample. 

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