Focusing monochromator

The point F is either the focai point on an X-ray tube or the focai point of a focusing monochromator. 

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.

Preparation for X-ray Analysis. Lattice constants are calculated from patterns obtained on powder samples with a Norelco diffractometer using monochromatic radiation (AMR-202 Focusing Monochromator) from a high-intensity copper source. The crystals are powdered with a diamond mortar and pestle, and the powder passed through a 74-/ sieve. Accurate lattice constants are calculated from the x-ray data. 

An isolation valve is used to maintain a permanent vacuum of approximately 10 torr in the analysis chamber. This chamber can be fitted with a variety of excitation sources. The traditional X-ray sources. A1 and Mg. can be supplied by a tube where the anticathode comprises two faces. It is possible to use a focusing monochromator with an X-ray source, reducing the spread of energy levels arriving at the sample. This type of source is u.scd for high precision studies but requires flat, conducting samples (see below), A loss in intensity of approximately a factor ten compared to standard sources means that they arc rarely employed in the study of catalysts. 

The chief value of the focusing monochromator lies in the fact that all the monochromatic rays in the incident beam are utilized and the diffracted rays from a considerable area of the crystal surface are all brought to a focus. This leads to a large concentration of energy and a considerable reduction in exposure time compared to the unbent-crystal monochromator first described. However, the latter does produce a semiparallel beam of radiation, and, even though it is of very low intensity, such a beam is required in some experiments. 

The combination of a focusing monochromator and a focusing camera is known as a Guinier camera, pioneered by Guinier in the late 1930s. Later investigators produced... 

Fig. 6-16 Cameras used with focusing monochromators. Only one diffracted beam is shown in each case. After Guinier [G.IO].

With a diffractometer one has the option, which does not exist with a powder camera, of placing a crystal monochromator in the diffracted, rather than the incident, beam. Figure 7-28 shows such an arrangement. The diffracted beam from the specimen comes to a focus at the receiving slit S, diverges to the focusing monochromator M, and comes to a focus again at the counter slit S2. Counter, crystal, and slits are mounted on one support and rotate as a unit about the diffractometer axis. 

G.IO Andre Guinier. X-Ray Crystallographic Technology (Lx>ndon Hiiger and Watts, 1952). Excellent treatment of the theory and practice of x-ray diffraction. The title is not fair to the book, which includes a considerable body of theory and detailed experimental technique. The theory and applications of the reciprocal lattice are very well described. Includes treatments of focusing monochromators, small-angle scattering, and diffraction by amorphous substances. 

Monochromator in the incident beam. Figure 6.24 shows beam paths in the diffractometer with an incident-beam focusing monochromator. The rays from... 

Figure 2.11 Focusing monochromator, using a bent and ground crystal.

RELATIONSHIP BETWEEN THE REFLECTION OF X RAYS AND THE DEGREE OF PERFECTION OF CRYSTALS IN FOCUSING MONOCHROMATORS ... 

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. 

When discussing the conditions for focusing an x-ray beam in focusing monochromators. 

Figure 2 shows the relationship between Im and the integrated width of the reflection curve Wf forw = 3 10" rad, which represents about 0.2 mm of the linear width of the tube focus forij = 70 mm. This corresponds approximately to the geometrical operating conditions of focusing monochromators in normal x-ray diffractometers. The quantity g Q /p was assumed to be 2 10" (curve 1), the value corresponding to LiF (200 reflection), and 1 10" (curve 2), corresponding to Ge (111) for Cu radiation. 

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. 

The results obtained may be used as a basis for making the correct choice of crystals with respect to degree of perfection in making focusing monochromators and in choosing the method of preparing them. 

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