X-ray intensities

Wilson A J C 1949 The probability distribution of X-ray intensities Acfa Crystallogr.2 318-21... 

Figure 4.7 Various representations of the properties of a mixture of crystalline and amorphous polymer, (a) The monitored property is characteristic of the crystal and varies linearly with 0. (b) The monitored property is characteristic of the mixture and varies linearly with 0 between and P, . (c) X-ray intensity is measured with the sharp and broad peaks being P. and P., respectively.

Table 14. Selected Values of the K and Electron-Binding Energies, K- and L2-Shell Fluorescent Yields, and /K x-Ray Intensity Ratio ...

Atomic number, Z Binding energy keV Fluorescent yield X-ray intensity ratio, IN/Kb... 

The keystone of practical quantitative electron probe microanalysis is Castaing s first approximation, which relates the concentration for a constituent in the unknown to the concentration in a standard in terms of the ratio of X-ray intensities generated in the target ... 

Neglecting unimportant geometric factors,the integrated X-ray intensity diffracted from a thin film is... 

Ni standard reference sample. When the X-ray intensity sinks below 75% of the original intensity, the tube must be replaced with a new one. 

For an electron-transparent specimen the absorption and fluorescence correction parts can often be neglected, this is the so-called thin-film criterion introduced by Cliff and Lorimer [4.118]. Thus, for a thin specimen containing two elements A and B yielding the net X-ray intensities I a and 1b, the concentration ratio reduces to ... 

Although the agreement between the observed x-ray intensities and those calculated from the aforementioned model is not perfect, the latter is thought to be essentially correct. The composition of a gas hydrate of Structure I is then characterized by... 

During World War II and thereafter, the methods of x-ray detection were improved until it is now a matter of simple routine to measure relative x-ray intensity easily and precisely. This improvement, which was accelerated by the rapid progress in nuclear physics, has promoted a rapidly growing appreciation of the great advantages that can attend the application of x-ray absorption and emission to chemical analysis. In their rush to make these applications, analytical chemists have occasionally made discoveries predictable from earlier work, usually by physicists, in the field of x-rays. 

To the analytical chemist, scattering is important mainly because it increases total absorption, and because it often leads to an increase in the background observed when x-ray intensities are measured. The extent to which a sample scatters x-rays (or y-rays) can sometimes be used as a means of analysis. 

The effects in question are often translated into electric currents, pulsed or continuous. For the convenient reading or recording of these currents, complex electronic circuitry (2.3) may be needed. Modern methods of measuring x-ray intensity are therefore primarily a concern of the experimental physicist. Nevertheless, the analytical chemist must know something about them because x-ray detectors are now among the tools of his trade. This chapter, which cannot hope to do justice to modern x-ray detection, will attempt to provide him with an acceptable minimum of knowledge. 

The principal functions performed by electronic circuitry in the measurement of x-ray intensity are listed below. 

To illustrate these considerations, and to introduce a detector in which measured x-ray intensity is given by an electric current, we shall use experimental results obtained on the simple laboratory photometer described in Section 3.5. The general approach is broadly applicable in absorptiometry with polychromatic beams. 

For a detector that gives current i proportional to x-ray intensity I (i.e., for a linear detector), Equation 1-4 may be written... 

Fig. 3-3. Attenuation and filtering of polychromatic x-rays by aluminum. Variation of effective wavelength with thickness. The effective wavelengths shown in tin figure correspond to the measured mass absorption coefficients. The change ir effective wavelength accounts for the deviations from the (dashed) straight lines The x-ray intensities used gave 210 /xamp through 0.0127-cm aluminum (curve A) 3200 /xamp through 0.381-cm aluminum (curve B). (Liebhafsky, Smith, Tanis, anc Winslow, Anal. Chem., 19, 861.)...

Fig. 5—4. Analysis by the absorption-edge method. The solid lines are photometric measurements of a photograph of the x-ray intensity as a function of angle. The concentration is calculated from the ratio of these lines extrapolated to the absorption edge. Table 5-4 gives some typical results.

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