X-rays emission intensity

The basic model of the factor analysis method as applied here assumes that the x-ray emission Intensity of any specified element Is a linear sum of the quantity of that element found In the minerals present at that sample location ... 

X-ray emission intensities were obtained from the spectra by using a digital filtering method which is available in the standard TN5500 software. For light elements up to, say, bromine, the only lines suitable for analysis are the Ka and K/9 emissions, and the Ka s were used as in the work of Cliff and Lorimer (16). Where overlap occurred between the Ka of one element and the K/9 of another, an experimentally determined correction was used our corrections were in good agreement with those measured by Heinrich et al. (32). 

Table 2 X-ray emission intensity ratios for standard samples...

A solid surface is bombarded with monochromatic electrons, and small but abrupt changes in the resulting total x-ray emission intensity are detected as the energy of the electrons is varied... 

EDS linescans in combination with EPMA for chromium and silicon indicate that the two elements are uniformly spatially distributed throughout the Cr-PB material. However, the examination of the Cr-K10 samples by these methods revealed the existence of fragments with different ratios of X-ray emission intensity. Although the variations observed are within a few percent, this fact reflects a less uniform distribution of chromium in the samples obtained from K10 montmorillonite. 

Energy-dispersive x-ray microanalysis was used to qualitatively verify the graded profile. A cross-sectioned sample, polished to a 1pm finish, was prepared. The measurement was conducted from the near-surface region to the center of the sample with step size of 50 pm using a JEOL 35C scanning electron microscope. The x-ray emission intensities for TiKot, AlKa, and ZrLot were collected at each point. 

X-ray fluorescence spectrometry is based upon the excitation of a sample by X-rays. A primary X-ray beam excites secondary X-rays (X-ray fluorescence) which have wavelengths characteristic of the elements present in the sample. The intensity of the secondary X-rays is used to determine the concentrations of the elements present by reference to calibration standards, with appropriate corrections being made for instrumental errors and the effects the composition of the sample has on its X-ray emission intensities. Alternatively, the X-rays may be detected without... 

The x-ray spectroscopic technique is essentially nondestructive, and measurable signals can be obtained from as little as 1 mg of specimen. Optimum specimen sizes range from 0.1 to 5 g of solid material, and the technique is equally applicable to solids, liquids, and even gases. A wide range of well-analyzed calibration standards has become available over the past few years and this, combined with the development of fundamental -type algorithms relating x-ray emission intensity and elemental composition, is allowing the use of the x-ray method in a wide variety of analytical disciplines. 

Table 1.3 lists various radioisotopes used as excitation sources for XRF analysis. To perform the qualitative and quantitative XRF analysis based on a radioisotope excitation, one should know the relative intensities and the precise energies of the X- or y-rays emitted by the source. Verma and Pal (1987) have calculated the K and L X-ray emission intensities for some radio nuclides ( Ce, " Ce, Eu, Dy, °Tb, Yb, and Np) using... 

The relative intensities of X-ray emission lines from targets varies for different elements. However, one can assume a ratio of KaJKa2 = 2 for the commonly used targets. The ratio of Ko2lKjii from these targets varies from 6 to 3.5. The intensities of Kj32 radiations amount to about 1 percent... 

The ionosphere is subject to sudden changes resulting from solar activity, particularly from solar emptions or flares that are accompanied by intense x-ray emission. The absorption of the x-rays increases the electron density in the D and E layers, so that absorption of radio waves intended for E-layer reflection increases. In this manner, solar flares dismpt long-range, ionospheric bounce communications. 

X-radiation can also be induced by high energy (several Me proton beams from ion accelerators. Such particle-induced x-ray emission (PIXE) (284) is useful for thin samples and particulates, having detection Hmits of g. Intense synchrotron x-ray sources have found appHcations in... 

The X-ray emission process followii the excitation is the same in all three cases, as it is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. 

Figure 1-19, which contains some of the results obtained by Faessler and Goehring,41 shows what information about chemical constitution the x-ray emission process can yield under favorable circumstances. In this simple case, wavelength shift and line intensity are both useful, the latter in the less obvious way explained below. 

Figure 4-10 gives intensity distributions for crystals used in x-ray emission spectrography during 1958 in the authors laboratory. Each of the patterns shows some broadening. Only in the case of the sodium chloride crystal with the major flaw was the broadening serious enough to produce interference with the Ka lines of adjacent elements. 

For purposes of x-ray emission spectrography, the Cauchois transmitting crystal has the serious drawback of greatly attenuating the x-ray beam. It must therefore be ruled out for x-ray beams of long wavelength at all but the highest intensities, and for the determination of traces, where the intensities are low even under the best conditions. 

Before turning to Method II, we shall discuss the variation with film thickness in the intensity of a characteristic line produced in a film by x-ray excitation. The discussion that follows is significant also for x-ray emission spectrography, and the ideas are explicit or implicit in the work of Glocker and Schreiber.11... 

The modification improves performance and is interesting in connection with x-ray emission spectrography (Chapters 7, 8, and 9). It consists in measuring the intensity of tin Ka relative to that of scattered x-rays entering the detector from an analyzing crystal set for the reflection of x-rays 2.2 A in wavelength. As the tin coating becomes thicker, increased attenuation of the x-rays scattered by the iron cause s the intensity ratio to increase more rapidly than does the intensity of tin Ka. Table 6-3 contains performance data for the Quantrol on Method II (modified). The instrument can also be set up to use industrially a modification of Method III. 

Sherman compares calculated and observed intensities for a number of known samples in some of which the enhancement components predominate over the intensities by direct excitation. The agreement obtained is usually within a few per cent, and this would be satisfactory even for considerably simpler problems. To be sure, the calculations do not give concentrations from measured intensities. But the fact that intensities can be satisfactorily calculated from known concentrations means that absorption and enhancement effects are thoroughly understood, and that x-ray emission spectrography is on a firm foundation. 

The discussion preceding Equation 7-5 points to the weight-fraction as the logical unit for the x-ray emission spectrography of infinitely thick samples. Were all complications absent, one might expect proportionality between analytical-line intensity and weight-fraction such that... 

Minerals generally present difficult problems in chemical analysis, and these problems grow more serious when the elements being determined are as difficult to separate as are those named above. The time and effort that x-ray emission spectrography can save are therefore great, but there are obstacles to be surmounted. Among these are (1) Absorption and enhancement effects are often serious. (2) The element of interest may be present at low concentration in a matrix that is unknown and variable. (3) Satisfactory standards are not always easy to obtain. (4) Simple equipment sometimes does not resolve important analytical lines- completely. (5) Sample preparation and particle size often influence the intensities of analytical lines Class II deviations (7.8) can be particularly serious with minerals. 

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