Process X-ray fluorescence

X-rays can be emitted from a sample by bombarding it with electrons, alpha particles, or with other X-rays. When electrons or alpha particles are used as the excitation source, the process is called X-ray emission or particle-induced X-ray emission (PIXE). This is the basis of X-ray microanalysis using an electron microprobe (Chapter 14) or an SEM. An alpha particle X-ray spectrometer (APXS) is currently on the Mars Curiosity rover collecting data on Martian rock composition. 

When the excitation source is a beam of X-rays, that is, photons, the process of X-ray emission is called fluorescence. This is analogous to molecular fluorescence discussed in Chapter 5 and atomic fluorescence discussed in Chapter 7, because the wavelength of excitation is shorter than the emitted wavelengths. The beam of exciting X-rays is called the primary beam the X-rays emitted from the sample are called secondary X-rays. The use of an X-ray source to produce secondary X-rays from a sample is the basis of XRF spectroscopy. The primary X-ray beam must have a that is shorter than the absorption edge of the element to be excited. 

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Two of the techniques appHed in this category are Auger electron process and X-ray fluorescence process. A schematic in Figure 7.5 shows the processes for these two techniques. These two processes compete with one another, and their relative abundance is dependent on the atomic number of the absorber. In general fluorescence is more pronounced for heavier elements, whereas the Auger process is more prominent for lighter elements. 

Figure 12.2 Schematic of the X-ray fluorescence process. For this classic image of a medium-size atom are displayed different possible electronic reorganizations when an electron in the K shell is ejected from the atom by an external primary excitation X-ray, creating a vacancy. The different K, L and M transitions are not significantly affected by the nature of the chemical combination in which the atom is found. In this way the sulphur line Kui changes from 0.5348 nm for to 0.5350 nm for S°. This difference of around 1 eV is comparable to the natural linewidth for X-ray radiation.

The x-ray fluorescence spectrometer consists of three main parts the excitation source, the specimen presentation apparatus, and the x-ray spectrometer. The function of the excitation source is to excite the characteristic x-rays in the specimen via the x-ray fluorescence process. The specimen presentation apparatus holds the specimen in a precisely defined position during analysis and provides for introduction and removal of the specimen from the excitation position. The x-ray spectrometer is responsible for separating and counting the x-rays of various wavelengths or energies emitted by the specimen. In this book the term x-ray spectrometer denotes the collection of components used to disperse, detect, count, and display the spectrum of x-ray photons emitted by the specimen. When referring to the entire instrument, including excitation source, sample presentation apparatus, and x-ray spectrometer, the term x-ray fluorescence spectrometer will be used. In this latter sense the term x-ray fluorescence analyzer is sometimes encountered in the literature. 

For each photoelectron that leaves the surface, an atom with a core hole is left behind in a highly excited state, which relaxes both by radiative and nonradiative processes. In a radiative recombination process, the core hole is filled in an electronic transition from a core level of lower binding energy or a valence level. The surplus energy is released by the emission of an X-ray photon, in a so-caUed X-ray fluorescence process. In this process, the emitted photon has a lower energy than the exciting photon and dipole selection rules apply for both, excitation and de-excitation. Conversely, Auger processes are nonradiative de-excitation channels... 

Figure Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

Figure 8.21 The competitive processes of X-ray fluorescence (XRF) and Auger electron emission...

An alternative mechanism of excess energy release when electron relaxation occurs is through x-ray fluorescence. In fact, x-ray fluorescence favorably competes with Auger electron emission for atoms with large atomic numbers. Figure 16 shows a plot of the relative yields of these two processes as a function of atomic number for atoms with initial K level holes. The cross-over point between the two processes generally occurs at an atomic number of 30. Thus, aes has much greater sensitivity to low Z elements than x-ray fluorescence. 

A development in the 1960s was that of on-line elemental analysis of slurries using x-ray fluorescence. These have become the industry standard. Both in-stream probes and centralized analyzers are available. The latter is used in large-scale operations. The success of the analyzer depends on how representative the sample is and how accurate the caUbration standards are. Neutron activation analyzers are also available (45,51). These are especially suitable for light element analysis. On-stream analyzers are used extensively in base metal flotation plants as well as in coal plants for ash analysis. Although elemental analysis provides important data, it does not provide information on mineral composition which is most cmcial for all separation processes. Devices that can give mineral composition are under development. 

Measuring process parameters on full-scale plants is notoriously difficult, but is needea for control. Usually few of the important variables are accessible to measurement. Recycle of material makes it difficult to isolate the effects of changes to individual process units in the circuit. Newer plants have more instrumentation, including on-line viscosimeters [Kawatra and Eisele, International ]. Mineral Processing, 22, 251-259 (1988)], mineral composition by on-line X-ray fluorescence, belt feeder weighers, etc., but the information is always incomplete. Therefore it is helpful to have models to predict quantities that cannot be measured while measuring those that can. 

Over the last seventeen year s the Analytical center at our Institute amassed the actual material on the application of XRF method to the quantitative determination of some major (Mg, Al, P, S, Cl, K, Ti, Mn, Fe) and trace (V, Cr, Co, Ni, Zn, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pb, Th, U) element contents [1, 2]. This paper presents the specific features of developed techniques for the determination of 25 element contents in different types of rocks using new Biaiker Pioneer automated spectrometer connected to Intel Pentium IV. The special features of X-ray fluorescence analysis application to the determination of analyzed elements in various types of rocks are presented. The softwai e of this new X-ray spectrometer allows to choose optimal calibration equations and the coefficients for accounting for line overlaps by Equant program and to make a mathematic processing of the calibration ai ray of CRMs measured by the Loader program. 

New process technologies (Ref 53) such as jet mills (Fig 2) and co-precipitation (Ref 97) may allow safe compounding of sensitive or toxic formulations. New analytical tools such as neutron radiography (Ref 92) afford improved non-destructive testing of devices. X-ray fluorescence (Ref 93) and neutron activation (Ref 94) provide quantitative analysis of pyrotechnic compns and their trace contaminants... 

Accompanying the photoemission process, electron reorganisation can result in the ejection of a photon (X-ray fluorescence) or internal electronic reorganisation leading to the ejection of a second electron. The latter is referred to as the Auger process and is the basis of Auger electron spectroscopy (AES). It was Harris at General Electric s laboratories at Schenectady, USA, who first realised that a conventional LEED experiment could be modified easily to... 

The ionized atom that remains after the removal of the core hole electron is in a highly excited state and will rapidly relax back to a lower energy state by one of two routes, namely X-ray fluorescence (Section 5.1.2) or by transferring the energy to an electron in another orbit, which, if it has sufficient energy, will be ejected into the vacuum as Auger emission. An example of the latter process is illustrated in Figure 5.29. 

When primary X-rays are directed on to a secondary target, i.e. the sample, a proportion of the incident rays will be absorbed. The absorption process involves the ejection of inner (K or L) electrons from the atoms of the sample. Subsequently the excited atoms relax to the ground state, and in doing so many will lose their excess energy in the form of secondary X-ray photons as electrons from the higher orbitals drop into the hole in the K or L shell. Typical transitions are summarized in Figures 8.35 and 8.36. The reemission of X-rays in this way is known as X-ray fluorescence and the associated analytical method as X-ray fluorescence spectrometry. The relation between the two principal techniques of X-ray emission spectrometry is summarized in Figure 8.37. 

The competitive relaxation process known as the Auger effect involves radiationless transitions and the ejection of valency electrons. X-ray fluorescence spectrometry has developed as a... 

It is obvious that the simpler a method of analysis, the easier it will be to automate. Non-destructive methods which involve a minimum of sample treatment are the most attractive. X-ray fluorescence, for example, has been successfully applied to the continuous monitoring and control of process streams. However, the scope of automated analysis is wide and methods have been designed with a basis in nonspecific properties (pH, conductance, viscosity, density) as well as those characteristic of the che-... 

Non-destructive analysis is especially valuable in an on line situation. X-ray fluorescence has above all become of major importance for the analysis of inorganic process streams. Cement production is an example of the successful application of this technique. The X-ray analyser can be used for the simultaneous assay of the various feedstocks (iron ore, clay and limestone) for Fe203, A1203, Si02 and CaO. In turn the signals from the analyser are used to control the feedstock supplies to the blending mill and to maintain an optimum product composition. 

The lifetime of the core-ionized atom is measured from the moment it emits a photoelectron until it decays by Auger processes or X-ray fluorescence. As the number of decay possibilities for an ion with a core hole in a deep level (e.g. the 3s level) is greater than that for an ion with a core hole in a shallow level (e.g. the 3d level), a 3s peak is broader than a 3d peak. 

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