X-rays spectrum

X-ray Electromagnetic radiation of wave length c. 1 k. X-rays are generated in various ways, including the bombarding of solids with electrons, when they are emitted as a result of electron transitions in the inner orbits of the atoms bombarded. Each element has a characteristic X-ray spectrum. 

Samples were tested on in a melt of salts (75% Na SO, 25% NaCl) at 950°C in an air atmosphere for 24 hours. Micro X-rays spectrum by the analysis found that the chemical composition of carbides of an alloy of the ZMI-3C and test alloys differs noticeably. In the monocarbide of phase composition of an alloy of the ZMI-3C there increased concentration of titanium and tungsten is observed in comparison with test alloys containing chemical composition tantalum. The concentration of more than 2% of tantalum in test alloys has allowed mostly to deduce tungsten from a mono carbide phase (MC) into solid solution. Thus resistance of test alloys LCD has been increased essentially, as carbide phase is mostly sensitive aggressive environments influence. The critical value of total molybdenum and tungsten concentration in MC should not exceed 15%. 

In addition to cleanliness (contamination effects), surface morpholt and the alteration of composition during specimen preparation can cause serious artifacts in microanalysis. In some older instruments, the microscope itself produces undesirable high-energy X rays that excite the entire specimen, making difficult the accurate quantitation of locally changing composition. Artifacts also are observed in the EDS X-ray spectrum itself (see the article on EDS). 

Measurements of the characteristic X-ray line spectra of a number of elements were first reported by H. G. J. Moseley in 1913. He found that the square root of the frequency of the various X-ray lines exhibited a linear relationship with the atomic number of the element emitting the lines. This fundamental Moseley law shows that each element has a characteristic X-ray spectrum and that the wavelengths vary in a regular fiishion form one element to another. The wavelengths decrease as the atomic numbers of the elements increase. In addition to the spectra of pure elements, Moseley obtained the spectrum of brass, which showed strong Cu and weak Zn X-ray lines this was the first XRF analysis. The use of XRF for routine spectrochemical analysis of materials was not carried out, however, until the introduction of modern X-ray equipment in the late 1940s. 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

In FIXE the X-ray spectrum represents the integral of X-ray production along the path length of the decelerating ion, as mediated by X-ray absorption in the mate-... 

Helium-ion induced X-ray spectrum from anodized tantalum (fluencei 1.5 X 10 HeVcmV ... 

In an electron-excited X-ray spectrum the discrete X-ray lines are superimposed on a continuous background this is the well-known bremsstrahlung continuum ranging from 0 to the primary energy Eq of the electrons. The reason for this continuum is that because of the fundamental laws of electrodynamics, electrons emit X-rays when they are decelerated in the Coulomb field of an atom. As a result the upper energy limit of X-ray quanta is identical with the primary electron energy. 

In EDXS the so-called spectrum-image method [4.122] can also be employed. A series of spectra is taken from a scanned rectangular field resulting in a data cube with its upper plane as the scanned x-y area and the third axis as the X-ray spectrum. Comprehensive information about the chemical composition and element distribution is extractable from this data set by subsequent processing. 

Relative intensities and displacement of the various lines in the Kq X-ray spectrum of magnesium relative to the Kd.s line (adapted by permission of Physical Electronics Corp. from Ref. [21])... 

Rontgen-priifung, /. X-ray testing or examination. -rfihre, /. Rontgen tube. X-ray tube, -spektralanalyse, /. X-ray spectrum analysis, -spektrum, n. X-ray spectrum, -strahlen, m.pl. Rontgen rays, X rays. 

The gas tube has two advantages that may be of overriding importance in special cases. As Figure 1-1 shows, it is easy to vary the target metal so that an x-ray spectrum characteristic of a particular metal can often be generated as needed. Furthermore, spectral purity can be maintained because the risk of target contamination is small in a gas tube properly operated. 

Fig. 1-3. The continuous x-ray spectrum. Note that the short-wavelength limit (Eq. 1-2) is 0.248 A for 50 kv and 0.620 A for 20 kv. A spectrum from a rectified a-c tube would have the peak displaced to the right and for a given input energy would have less x-ray output. (After Ulrey, Phys. Rev. [2], 11, 401.)...

In most ordinary cases, the disadvantages of x-ray excitation are more than compensated by the absence of the disadvantages peculiar to electron excitation, by the great convenience of Coolidge tubes (1.3), and by the absence of the large background count to which the continuous x-ray spectrum excited by electrons gives rise (1.5). 

The discussion just concluded is largely implicit in the earlier discussion of the excitation of a continuous x-ray spectrum by electron bombardment (4.1). Note that x-rays behave differently when they are used for excitation. An x-ray penetrates with little or no loss of energj" until it is absorbed, and it is the more likely to penetrate to greater depth (in regions of continuous abiorption) the greater its energy (or shorter its wavelength). 

In 1951Castaing8 published results to show that an electron microscope could be converted into a useful x-ray emission spectrograph for point-to-point exploration on a micron scale. The conversion consisted mainly in adding a second electrostatic lens to obtain a narrower electron beam for the excitation of an x-ray spectrum, and adding an external spectrometer for analysis of the spectrum and measurement of analytical-line intensity. Outstanding features of the technique were the small size of sample (1 g cube, or thereabouts) and the absence of pronounced absorption and enhancement effects, which, of course, is characteristic of electron excitation (7.10). Castaing8 gives remarkable quantitative results for copper alloys the results in parentheses are the quotients... 

The Compton scattering cannot be neglected, but it is independent of molecular structure. Then, fitting experimental data to formulas from gas phase theory, the concentration of excited molecules can be determined. Another problem is that the undulator X-ray spectrum is not strictly monochromatic, but has a slightly asymmetric lineshape extending toward lower energies. This problem may be handled in different ways, for example, by approximating its spectral distribution by its first spectral moment [12]. 

This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. 

Another excellent example of the use for method development and validation appears in Morrison and Richardson (1996). Their laboratory was analyzing many samples of Li ore and related samples for Ba, among other elements, using a routine XRF procedure. The reference sample chosen as a control sample for the run was the zinnwaldite ZWC (Govindaraju et al. 1994), for which analyses produced a value approximately twice the reference value. Investigation of that result identified a Rb overlap in the X-ray spectrum that had not previously been observed in use of the method. 

Fig. 8.41 Left. Comparison of SNR of 14.4 keV Mossbauer spectra, taken with a Si-PIN detector system (MER instrument four diodes) and with a SDD detector system (advanced MIMOS instrument only one diode chip) Right. X-ray spectrum of a basalt (Ortenberg basalt see [366, 371], taken with a high resolution Si-drift detector system at ambient pressure (1 atm), demon-...

Figure 2.1. The X-ray spectrum of molybdenum, showing Ka and Kp lines superimposed upon the continuous spectrum. The quantum energy is shown in the upper scale the intensity is in arbitrary units.

The spectrometer is necessarily quite large, and a complicated mechanism has to be precision engineered in order to enable 0 to be altered while keeping both the crystal and the detector on the Rowland circle. In order to cover the whole X-ray spectrum a range of crystals with different lattice spacings is required, which may be interchanged automatically. 

The comparison of coronal and photospheric abundances in cool stars is a very important tool in the interpretation of the physics of the corona. Active stars show a very different pattern to that followed by low activity stars such as the Sun, being the First Ionization Potential (FIP) the main variable used to classify the elements. The overall solar corona shows the so-called FIP effect the elements with low FIP (<10 eV, like Ca, N, Mg, Fe or Si), are enhanced by a factor of 4, while elements with higher FIP (S, C, O, N, Ar, Ne) remain at photospheric levels. The physics that yields to this pattern is still a subject of debate. In the case of the active stars (see [2] for a review), the initial results seemed to point towards an opposite trend, the so called Inverse FIP effect , or the MAD effect (for Metal Abundance Depletion). In this case, the elements with low FIP have a substantial depletion when compared to the solar photosphere, while elements with high FIP have same levels (the ratio of Ne and Fe lines of similar temperature of formation in an X-ray spectrum shows very clearly this effect). However, most of the results reported to date lack from their respective photospheric counterparts, raising doubts on how real is the MAD effect. 

Herzog and Janke (46) found that the intensity of the X-ray spectrum of the wood and of the cellulose in the wood was the same. Thus, they suggested that cellulose is not chemically combined with the lignin of the wood. 

Fig. 3.30. X-ray spectrum of the supernova remnant N49 in the LMC, aged between 5000 and 10 000 yr, taken with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory, showing H-like and He-like K-shell lines of abundant light elements and some L-shell lines of iron, after Park et al. (2003).

Fig. 3.31. X-ray spectrum taken from the XMM-Newton and Chandra X-ray observatories of the inner part of the Centaurus cluster of galaxies, where the metallicity is roughly twice solar, showing the iron L- and K-shell features at energies of 1.2 and 6.8 keV repectively. The curve is a two-component fit to the continuum with temperatures of 0.7 and 1.5 keV. After Sanders and Fabian (2006). Courtesy Andy Fabian.

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. 

The alternative approach to detection and analysis incorporates a solid state detector and a multichannel pulse height analysis system. The crystals used are of silicon (of the highly pure intrinsic type), or the lithium drift principle (p. 463 etseq.) is utilized. All emitted radiations are presented to the detector simultaneously and a spectrum is generated from an electronic analysis of the mixture of voltage pulses produced. Chapter 10 contains a more detailed account of pulse height analysis and solid state detectors. Production of an X-ray spectrum in this way is sometimes known as energy dispersive analysis ofX-rays (EDAX) and where an electron microscope is employed as SEM-EDAX. 

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