X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock

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СКАЧАТЬ the development of the performance of X-ray spectrometers, in particular their energy resolution. For the first instruments, only the distinction between K-, L-, and M-lines was possible. With the improvement of the resolution, a splitting of these lines was then discovered, i.e. α-, β-, and γ-lines could be distinguished. Later, further splittings were detected, which are denoted by indices.

There is also the nomenclature introduced by the IUPAC, which is based on an exact designation of the respective electron levels from which the X-ray lines are generated. A comparison of the designations for the most important X-ray lines can be found in Table 2.2.

      

      2.2.4 Interaction of X-rays with Matter

      X-radiation interacts with matter – it will be scattered and absorbed. This process is described by Lambert–Beer's law.

      (2.5)

with

I	intensity after absorption in a layer
D	thickness of the layer
P	density of the layer
M	mass attenuation coefficient of the layer material
I 0	primary intensity

      The mass attenuation coefficient μ has several contributions. At low energies, the absorption described by the photoionization coefficient τ is dominant; the influence of the scattering characterized by the scattering coefficient σ increases with energy, and in the case of energies >1.2 MeV the electron pair production described by κ gains importance. However, this is outside the range of energy that is of interest for X-radiation.

      (2.6)

All these interactions depend on both the analyzed material and the energy of the radiation, as shown in the diagrams in Figure 2.5 for the elements (a) carbon and (b) lead. They show that in the energy range of about 0.5–40 keV, absorption dominates. Rayleigh scattering contributes a fraction of less than 1% for low energies. At higher energies, Compton scattering dominates the radiation attenuation coefficient.

      2.2.4.1 Absorption

Absorption will be understood merely as the attenuation of the incident X-ray radiation by the ionization of atoms. This process is described by the element- and energy-dependent photoionization coefficient. It increases with higher atomic number and decreases with increasing energy as can be seen in Figure 2.5. This energy dependence can be approximated by τ ≈ E⁻³. The curve does show discontinuities that manifest themselves as jumps. They occur when the energy of the absorbed radiation is sufficient to ionize a new electron shell, i.e. a new interaction mechanism is added. In the diagrams in Figure 2.5, this applies to Pb for the excitation of M, L, and K radiation at energies of approximately 3, 15, and 88 keV, respectively. The energy of the K-absorption edge of C, on the other hand, is 0.283 keV and is therefore not displayed in the diagram.

Table 2.2 Comparison of line designations for the main X-ray lines.

Siegbahn	IUPAC	Siegbahn	IUPAC	Siegbahn	IUPAC
Kα1	K–L3	Lα1	L3–M5	Mα1	M5–N7
Kα2	K–L2	Lα2	L3–M4	Mα2	M5–N6
Kß1	K–M3	Lß1	L2–M4	Mβ	M4–N6
Kß2	K–N3	Lß2	L3–N5	Mγ	M3–N5
Kß3	K–M2	Lß3	L1–M3	Mξ	M4,5–N2,3
Kß4	K–N5,4	Lß4	L1–M2
Kß5	K–M4,5	Lß5	L2–O4,5
		Lß6	L3–N1
		Lγ1	L2–N4
		Lγ2	L1–N2
		Lγ3	L1–N3
		Lγ4	L1–O3
		Lγ5	L2–N1		СКАЧАТЬ В начало < 12 13 14 15 16 17 18 19 20 21 > В конец