Goniometers typical

Sample Chamber and Detector. The pressure in the sample chamber is typically 10-6torr, although UHY may be required for some experiments. The samples are usually mounted on a five-axis goniometer, so that a series of samples may be loaded and analysed sequentially. The goniometer can tilt and rotate the samples relative to the direction of the incident beam. Comparing spectra obtained at different incident and exit beam angles provides fuller characterization of the sample composition as a function of depth. The samples can be electrical insulators... 

Over 97% of the ions in a beam can be channeled in a crystal. This figure can be achieved for a well-collimated beam of MeV light ions, for which dmin is of order 0.1 A, directed along a low-index direction onto a single crystal sample. Typically, the sample is mounted on a goniometer which allows different low-index directions to be brought parallel to the beam during an experiment. The 2 or 3% of nonchanneled ions are those that hit the ends of the atomic rows at the surface or are scattered from surface disorder. 

Figure 13.9—Schematic of a sequential, crystal-based spectrometer and the spectrum obtained using the sequential method with an instrument having a goniometer. The Soller slit collimator, made of metallic parallel sheets, collimates the primary X-ray beam emitted by a high power source (SRS 300 instrument, reproduced by permission of Siemens). A typical spectrum of an alloy, obtained by an instrument of this category, having an LiF crystal (200) with 26 angle in degrees as the abscissa and intensity in Cps as the ordinate). Model Philips PW2400 Spectrum, reproduced with permission of VALDI-France.

On the other hand, the simplicity of the goniometer arms motion in a horizontal plane, when the sample is located in a vertical plane (Figure 3.7, right), is offset by the need of more complicated sample preparation to ensure that it stays in place and does not fall of This is usually achieved by side packing the sample holder or by mixing a powder with a binder (e.g., x-ray amorphous and chemically inert petroleum jelly, oil, grease or varnish), which typically increase preferred orientation or background, respectively (see section 3.5 for more details on sample preparation). 

Overall, powder diffractometers equipped with point detectors offer the best resolution of the resulting powder diffraction data. While the instrumental resolution increases with the increasing goniometer radius, the intensity of the diffracted beam unfortunately decreases because the incident beam produced by an analytical x-ray tube is always divergent. Therefore, typical goniometer radii vary between 150 and -300 mm. 

X-ray powder differaction data for Naproxen is determined by De Camp, W.H. (11). X-ray powder differaction pattern was obtained by the use of Cu Ka radiation on a Philips goniometer. The X-ray tube was typically operated at 40 KV and 20 mA. Detection was effected with a Nal (Tl) scintillation counter coupled to a pulse-height analyser. 

Although the power of the single crystal method is evident, it suffers from a couple of significant limitations. Firstly, a single crystal of sufficient size, several mm in each dimension, with a typical volume of 50 mm, is necessary. Secondly, a specialised NMR probe incorporating a goniometer is required for the well-defined rotation of the sample, and such equipment is available in only a handful of laboratories worldwide. If, however, both the crystal and the equipment are available, this kind of study yields very precise measurements of parameters that are not available from diffraction techniques. 

In the X-ray cell, immediately after mounting in the X-ray goniometer and being put under potential control, the cyclic voltammetry (CV) was measured for comparison with literature results. After the initial cycling, the electrode potential was transferred to computer control for the duration of the X-ray experiment. Although with time the features in the CV became distorted as the electrolyte made contact with the polycrystalline back and sides of the crystal, the general features were always observable for the duration of the experiment (typically up to a maximum of 24 h). 

Fig. 1 Typical x-ray powder diffractometer (Phillips XL 300). Key components are identified, including the trans-former/power supply, x-ray source, goniometer, and detector...

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