Goniometer circle

XR D-5 D/S. As the goniometer circle is horizontal, the table (Figure 9-5) can carry a fairly heavy load of instruments, samples, or accessories without risk of distorting the goniometer angle. Any value of 26 can be set reliably and quickly. Good performance is available at widely... 

Once the user is satisfied with the task queue that he has constructed by updating the run database, he must ensure that the angle readings on the goniometer circles are correct. These values are contained in the interface database. 

Figure 3.33. The schematic of monochromatization of the diffracted beam using a curved crystal monochromator. RS - receiving slit, M - curved monochromator, MS -monochromator scatter slit, D - detector. The dash-dotted arc represents the goniometer circle. The dashed arc shows the focusing circle of the monochromator.

Figure 2.8 Basic Bragg Brentano geometry. The dotted circle centred on the sample position represents the goniometer circle on which the image of the divergent source of X rays is focussed by diffraction from the flat plate sample. Strictly speaking, true focussing only occurs when the sample plate has a curved surface. However, given that the footprint of the beam on the sample plate is considerably smaller than the radius of the focussing circle, the fiat plate approximation works well in practice. The source is usually fixed and to collect the diffraction pattern the sample and detector are rotated by 6 and 26, respectively. An alternative is to fix the sample (usually in the horizontal position, e.g. useful for a liquid sample) and to move both the source and the detector by 6 and 6, respectively.

Cryst measurements with a new model two -circle goniometer, especially adapted to use in chem labs) 21)M.W.Porter R.C.Spiller, Nature 144, 298-302 (1939) (Crystallochemical analysis) 22)R.C.Evans, "Crystal Chemistry , Cambridge Univ Press, London (1939) 23)H.F-Miller S.E.Q.Ashley, Jr., "Crystallochemical Analysis , Data Folder from Pittsfield Lab of Gen Elec Co (1941) 24)A. 

In the diffractometer, the goniometer head and crystal are mounted in a system of movable circles called a goniostat, which allows automated movement of the crystal into almost any orientation with respect to the X-ray beam and the detector (see Figs. 4.21 and 4.22). 

The complete diffractometer consists of a fixed X-ray source, the goniostat, and a movable scintillation-counter detector. The system of circles (Fig. 4.21) allows rotation of the goniometer head (angle ), movement of the head around a circle centered on the X-ray beam (angle x), and rotation of the X circle around an axis perpendicular to the beam (angle o). Furthermore, the detector moves on a circle coplanar with the beam. The axis of this circle coincides with the o-axis. The position of the detector with respect to the beam is denoted by the angle 20. With this arrangement, the crystal can be moved to... 

Figure 4.21 System of circles in diffractometry. The crystal in the center is mounted on a goniometer head.

Figure 2.15. The three different monochromator/sample geometries used in powder diffraction a) flat diffracted beam monochromator, parallel arrangement b) curved diffracted beam monochromator, angular arrangement, and c) flat primary beam monochromator, parallel arrangement. F - focus of the x-ray source, S - sample, M - crystal monochromator, D - detector, Rm - radius of the monochromator focusing circle, Rq - radius of the goniometer focusing circle.

Figure 3.7. Horizontal (left) and vertical (right) orientations of a flat sample. The location of the goniometer axis is shown using a dash-double dotted line with small filled circles at the ends. The dashed line indicates the location of the optical axis, which is the line connecting the focus of the x-ray tube, the receiving slit and the sample surface in the reflection geometry, or the sample center in the transmission geometry at 0 = 20 = 0°.

Figure 3.10. Synchronization of the goniometer arms the x-ray source is stationary while the sample and the detector rotations are synchronized to fulfill the 0-20 requirement (left) the sample is stationary while the source and the deteetor arms are synchronized to realize the 0-0 condition (middle) - this geometry is in common use at present only the detector arm revolves around the goniometer axis in the case of a cylindrical sample (right). F - focus of the x-ray tube indicating the position of the x-ray source arm, D - detector arm, 0 - Bragg angle. The common goniometer axis (which is perpendicular to the plane of the projection) around which the rotations are synchronized is shown as the open circle in each of the three drawings. The location of the optical axis is shown as the dash-dotted line.

Figure 3.24. The length of the projection of the incident beam, L, on the surface of the flat sample in Bragg-Brentano geometry. F - focal point of the x-ray source, DS - divergence slit, R - goniometer radius,

There is a large variety of specimen holders in use, each suited to some particular purpose. The simplest consists of a fixed post to which the specimen is attached with wax or plasticine. A more elaborate holder is required when it is necessary to set a crystal in some particular orientation relative to the x-ray beam. In this case, a three-circle goniometer is used (Fig. 5-7) it has three mutually perpendicular axes of rotation, two horizontal and one vertical, and is so constructed that the crystal, cemented to the tip of the short metal rod at the top, is not displaced in space by any of the three possible rotations. 

Single-crystal specimens may also be examined in a diffractometer by mounting the crystal on a three-circle goniometer, such as that shown in Fig. 5-7, which will allow independent rotation of the specimen and counter about the diffractometer axis and two other axes passing through the specimen. (Incidentally, independent rotation of the specimen about the diffractometer axis is often called an tu, rather than a 6, rotation.) In fact, special single-crystal diffractometers are available, designed solely for the determination of complex crystal structures. 

After the orientation of a crystal is found by x-rays, it is often necessary to rotate it into some special orientation, such as one with <100> along the incident beam, for the purpose of either (a) subsequent x-ray examination in the special orientation, or (b) subsequent cutting along some selected plane. To obtain this orientation, the crystal is mounted in a three-circle goniometer like that shown in Fig. 5-7, whose arcs have been set at zero, and its orientation is determined by,... 

---