Goniometer head

Step 1. A single crystal with the largest dimension equal to about 0.01 mm to 0.3 mm is mounted on a glass fiber which in turn is mounted on a copper pin. The copper pin is placed on a goniometer head which in turn is placed on the goniometer (Fig. 13). The crystal is positioned manually in the center of the goniometer. In this position, the crystal is always in the center of the monochromated incident x-ray beam (whose diameter is about 1.0 mm). 

The Pyrex tube was suspended, with capillary down, in a small-holed rubber stopper which, in turn, was fastened to a goniometer head by a length of stout copper wire. The solid material within the capillary was photographed in a cold room (4°C.) using copper x-radiation, a camera with radius 5 cm., and oscillation range 30°. The effective camera radius was established by superimposing a powder spectrum of NaCl during an exposure of the sample the lattice constant for NaCl at 4°C. was taken to be 5.634 A. 

With the cold nitrogen stream diverted, the loop-mounted crystal is positioned on the goniometer head. 

Figure 4.4 Procedure for shock-cooling a crystal using a gaseous cryogen. (a) The goniometer head is assembled on the X-ray camera and the cold gaseous nitrogen stream is centred on the eucentric point of the camera, (b) The stream is deflected with some appropriate obstruction.

Goniometer head with magnetic mount Procedure (Fig. 4.5)... 

Liquid nitrogen the goniometer head and plunged into a liquid nitrogen... 

Mount a pin with a loop on the goniometer head 12. Choose the cryoprotectant that gives no ice rings. 

Measurement of interfacial angles, and graphical representation. The most accurate method of measuring the angles between crystal faces is an optical one, which makes use of the reflection of light by the plane faces. The crystal is mounted on the stem of a goniometer head (Fig. 13) by means of wax, shellac, or plasticine a beam of parallel... 

The classical method of mounting crystals is to transfer them into a fine glass capillary along with a droplet of the mother liquor. The capillary is then sealed at both ends and mounted onto a goniometer head (see Fig. 4.20, and Chapter 4, Section III.D), a device that allows control of the crystal s orientation in the X-ray beam. The droplet of mother liquor keeps the crystal hydrated. 

In all forms of data collection, the crystal is mounted on a goniometer head, a device that allows the crystallographer to set the crystal orientation precisely. The goniometer head (Fig. 4.20) consists of a holder for a capillary tube containing the crystal two arcs (marked by angle scales), which permit rotation of the crystal by 40° in each of two perpendicular planes and two dovetailed sledges, which permit small translations of the arcs for centering the crystal on the rotation axis of the head. 

Protein crystals, either sealed in capillary tubes with mother liquor or flash-frozen in a fiber loop, are mounted on the goniometer head, which is adjusted to center one face of the crystal perpendicular to the X-ray beam and to allow rotation of the crystal while maintaining centering. Flash-frozen crystals are held in a stream of cold nitrogen gas emerging from a reservoir of liquid nitrogen. 

Figure 4.20 Goniometer head, with capillary tube holder at top. The tool (right) is an Allen wrench for adjusting arcs and sledges. Photo courtesy of Charles Supper Company.

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.

The precession camera (Fig. 4.23), although the more complicated in its motion, produces the simplest diffraction pattern. X rays enter through the black tube at left to strike the crystal, mounted in a goniometer head. Beyond the crystal are an annular-screen holder (smaller black square) and a film holder (larger black square). The remaining machinery moves crystal, screen, and film in a precessing motion about the X-ray beam. 

Figure 4.23 Precession camera with mounted goniometer head. To see a precession photograph, refer to Fig. 2.6. Photo courtesy of Charles Supper Company.

The total thickness d = dA + dB of a sheet is directly given by the Bragg spacing of the X-ray patterns, or directly measured on the electron micrographs (to obtain an accurate value of d by electron microscopy one must use electron micrographs provided by sections perpendicular to the planes of the sheets, this is easy with an electron microscope equipped with a goniometer head). 

V-UV Application Specular Reflection by Crystal Faces. If one focuses the incoming beam 70 onto a single crystal face, the specular UV-vis reflectivity (and its polarization) can be measured. The crystal is mounted on a goniometer head the orientation of the crystal axes relative to the instrumental axes must be predetermined separately on an X-ray diffractometer. 

The whole apparatus, which was 10 cm tall, was mounted on a goniometer head, this being fixed to the 0-tumtable of the diffractometer. This arrangement allowed the sample to be exactly located at the center of rotation of the diffractometer. The body of the cell holder was of aluminum, clad in cadmium foil. The sample and solution were cooled using the water circulation ports illustrated, the temperature within the cell being monitored by means of a metal junction thermometer. 

Single crystals may be mounted on a goniometer head. Depending on the problem, 90° or 180° arrangements may be used. 

FIGURE 14.3 Goniometer head holding a crystal. (Oxford Diffraction Ltd.)... 

Crystals that are unaffected by air, moisture, or light are usually mounted directly on a quartz or glass fiber, which is inserted into a goniometer head (Figure 14.3). If the crystal is sensitive to moisture or air, a sealed capillary tube may be used. Suitable apparatus can be used when nonambient temperatures and pressures are needed. The diamond anvil has been developed for high pressure. 

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