X-ray primary

X-ray fluorescence A method of analysis used to identify and measure heavy elements in the presence of each other in any matrix. The sample is irradiated with a beam of primary X-rays of greater energy than the characteristic X-radiation of the elements in the sample. This results in the excitation of the heavy elements present and the emission of characteristic X-ray energies, which can be separated into individual wavelengths and measured. The technique is not suitable for use with elements of lower atomic number than calcium. 

Fig. 20. Primary x-ray line and Bremsstrahlung background excited by bombardment with 15 keV electrons, (a) Linear scale plot, (b) Logarithmic scale...

The primary x-ray source is usually a sealed tube with a Rh anode. The tube usually has an end window so that the sample is as close to the anode as possible. This arrangement ensures that the surface of the sample is irradiated with a beam with nearly uniform intensity. 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

The primary X-ray beam is directed onto the solid surface in grazing incidence. The angle of incidence is kept below the critical angle at which total reflection occurs. The critical angle is given by... 

Sealed conventional fine structure tubes with Mo, W, Cu, or Cr anodes are used as primary X-ray sources, as well as rotating anode tubes, or synchrotron radiation. The maximum energy of the X-ray quanta determines the range of elements acces-... 

The cross-section of the primary X-ray beam is extended and not an ideal point. This fact results in a blurring of the recorded scattering pattern. By keeping the cross-section tiny, modern equipment is close to the point-focus collimation approximation - because, in general, the features of the scattering patterns are relatively broad. Care must be taken, if narrow peaks like equatorial streaks (cf. p. 166) are observed and discussed. The solution is either to desmear the scattering pattern or to correct the determined structure parameters for the integral breadth of the beam profile (Sect. 9.7). 

X-ray radiation wavelength—that is, 1/ X. When the crystal is rotated, the reciprocal lattice rotates with it and different points within the lattice are brought to diffraction. The diffracted beams are called reflections because each of them can be regarded as a reflection of the primary X-ray beam against planes in the crystal. 

Furthermore, under controlled bombardment conditions, peak intensity measurements may be used for a quantitative determination of the appropriate element. Measurements of the characteristics and intensity of primary X-rays produced by electron bombardment constitute the basis of electron probe microanalysis. Figure 8.33 illustrates the complex nature of the reactions initiated by the impact of an electron beam on a target. As a consequence of this complexity it has proved extraordinarily difficult to make fully quantitative measurements, and it is only recently with the widespread application of dedicated computers and sophisticated software that this has become possible. 

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. 

Primary X-rays are produced by the bombardment of a suitable target with a stream of accelerated electrons. A typical X-ray generator uses an evacuated tube into which the target (e.g. tungsten) projects as a cooled anode together with a tungsten filament cathode. At a potential of 20-50 keV electrons are emitted from the cathode and bombard the... 

Electron probe microanalysis functions by direct examination of the primary X-rays produced when the specimen is used as a target for an electron beam. Focused electron beams allow a spot analysis of a 1 pm3 section of the specimen. One current development employs the electron beam within a scanning electron microscope to provide both a visual picture of the surface of the sample and an elemental analysis of the section being viewed. Spectra obtained from primary X-rays always have the characteristic emission peaks superimposed on a continuum of background radiation (Figure 8.32). This feature limits the precision, sensitivity and resolution of electron probe microanalysis. 

Primary X-ray spectrum for Cu with the absorption edge of a nickel filter superimposed showing how it may be used to isolate the long wavelength emission peak. 

Alternative sources of primary X-rays now include synchrotron radiation (Pollard et al., 2007 290). The synchrotron is a large electron accelerator which produces electromagnetic radiation across the entire spectrum, with high spectral purity and very high beam intensity. At specific stations around the storage ring, particular sections of the electromagnetic spectrum are selected... 

Figure 2.9 Interaction of the primary X-ray beam with a solid sample. See text for discussion. (After Jenkins, 1974 Fig. 3-3. John Wiley Sons Limited. Reproduced with permission.)...

XRF spectrometry is based on the principle that primary X-rays (from an X-ray tube or radioactive source) are incident upon a sample and create inner shell (K, L, M) vacancies in the atoms of the surface layers. These vacancies de-excite by the production of a secondary (fluorescent) X-ray whose energy is characteristic of the elements present in the sample. Some of these characteristic X-rays escape from the sample and are counted and their energies measured. Comparison of these energies with known values for each element (e.g., Van Grieken and Markowicz 1993, Parsons 1997) allow the elements present in the sample to be identified and quantified. 

The reflection of characteristic X-rays described in this note seems to be reasonably well explained by the assumption (mentioned in our first paper) that a primary X-ray excites secondary, characteristic X-rays in atoms, with a certain time lag, approximately the same for all atoms of one kind. It is not necessary to assume that the primary ray has wave-lengths, but only that it must have been produced by a voltage above certain critical values. If the secondary rays have wave-lengths they will interfere and produce beam s in the required directions. This point of view, however, does not appear to be compatible with the law of the conservation of energy-applied to the processes going on in individual atoms. 

In principle, the difference between X-ray fluorescence spectrometry and electron-probe microanalysis lies in the fact that the analytical information is provided, in the first case, by secondary, fluorescence X-rays, and in the second by primary X-rays, emitted as a result of the impact of the electron beam on the sample s electrons. 

Ruppert, M., Panjikar, S., Barleben, L. and Stockigt, J. 2006. Heterologous expression, purification, crystallization and primary X-ray analysis of raucaffricine glu-cosidase, a plant enzyme specifically involved in Rauvolfia alkaloid biosynthesis. Acta Crystallographica Section F-Structural Biology and Crystallization Communications, 62 257-260. 

Jauncey has recently described a corpuscular quantum theory of the scattering of polarized X-rays, in which a formula for the intensity of the scattering in any direction is derived. From this formula an expression for the linear scattering coefl cient per unit solid angle in any direction 4> may be obtained. In particular this linear scattering coefficient for the case where plane polarized X-rays are scattered in the plane of the electric vector of the primary X-rays is given by... 

Figure 13.1—X-ray fluorescence. The sample, when excitated by a primary X-ray source, emits fluorescence that can be detected according to two modes 1) simultaneous detection by a cooled diode that detects the energy of a single photon 2) sequential detection of the emitted wavelengths (using a "6,26 goniometer assembly).

Liquid samples do not require preparation before analysis. However, it is preferable to transform solid samples, especially if the matrix is not well known. In fact, depending on the incident angle of the primary X-ray radiation, the sample thickness can be variable from only a few angstroms (for a grazing angle of radiation) to half a millimetre (see Fig. 13.4). Thus any superficial heterogeneity will translate as variations in the results. 

Figure 13.6—Schematic of a cooled Si Li detector. The high quantum yield of this detector allows the use of primary X-ray sources of low power (a few watts or a radioisotope source).

Compton effect. A small fraction of the primary X-ray excitation beam is scattered in the form of radiation whose wavelength depends on the angle of observation. This radiation is superimposed on the X-ray fluorescence spectrum. The shift in angstroms between the two wavelengths (excitation and Compton) is given by ... 

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.

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