X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock

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       A large number of different X-ray spectrometry instruments are available, each of which is designed for specific analytical tasks. A more detailed discussion of the individual instrument types can be found in Section 4.3.

       The radiation sources used in laboratory analyses today are X-ray tubes. In the past, isotope sources have been used as well. Another radiation source includes synchrotrons. Their radiation properties, i.e. the high beam brilliance, the polarization of the synchrotron radiation, or the possibility of generating monoenergetic radiation by means of appropriate X-ray optics, allow the use of very dedicated measuring geometries and measurement methods. As a result, new analysis methods are often developed at these radiation sources, which subsequently can be transferred into routine practice. However, these special analyses methods are not discussed within this scope since the very high instrumental effort and the limited availability of measurement time at these sources restrict their routine use.Another type of interesting X-ray source are plasmas in which atoms are ionized by extremely high temperatures. These atoms then emit X-radiation when transferred back to the ground state. It is usually radiation in the lower energy range. Because the plasma is often generated by a laser impact, these sources can be pulsed and consequently be used for time-resolved studies. However, these sources are not yet suitable for real routine use.

2.2 X-ray Radiation and Their Interaction

      2.2.1 Parts of an X-ray Spectrum

      X-radiation is electromagnetic radiation in the energy range of approximately 0.1–100 keV or with wavelengths in the range of approximately 25 to 0.01 nm. X-radiation is therefore characterized either by its energy E or by its wavelength λ. Both quantities are mutually transferable through the following relationship:

      (2.1)

      X-ray radiation can be generated by several processes. A continuous broadband spectrum is emitted by the stepwise deceleration of highly energetic charged particles but also by highly ionized plasmas (bremsstrahlung). Line-like spectra (characteristic radiation) are generated when transitioning excited atoms back into the ground state, if the energy difference of the energies involved is within the range described above. For the excitation of X-rays in the laboratory scale, accelerated charged particles, i.e. electrons or protons, as well as high-energy ionizing radiation, i.e. X-rays themselves are used. In the beginning, radioactive elements were also used as radiation sources in laboratory equipment. However, these sources are now used not very often because of the high safety requirements.

      The most common way of producing X-ray radiation is the deceleration of accelerated electrons. This is used in X-ray tubes and electron microscopes. The deceleration of the electrons results in both a continuous spectrum and a line-like spectrum.

The continuous spectrum results from the deceleration of the electrons on the tube target by scattering on the atomic nuclei. The intensity of the emitted radiation is described in detail by Kramers' law.

      (2.2)

      with

I cont	intensity of the emitted radiation
K	proportionality coefficient
I	electron current
Z	atomic number of the decelerating material
E 0	maximum energy of the electrons

      If this process is carried out in an X-ray tube, the generated radiation must be directed at the sample through an exit window. The exit window of the tube absorbs the low-energy parts of the primary spectrum. This process is described by the following addition to Kramers' law, in which μ is the mass attenuation coefficient, ρ is the density, and d is the thickness of the tube window:

      (2.3)

The proportions of this relation are shown in Figure 2.1. It shows the continuous spectrum (emission) generated at the target, the low transmission through the tube window in the low-energy range (transmission) as well as the resulting radiation emitted by the tube (tube output).

      In addition to the continuous radiation, there are also line-like spectral components in the energy range of X-rays, which are generated by electron transitions in an atom. For this purpose, an internal electron level must be ionized by an energy input, which is higher than the binding energy of the electron. This excitation is possible by radiation, i.e. electron, proton, or even X-rays themselves or by the generation of a high-energy plasma. The resulting electron vacancy in an inner shell is filled by electrons of outer shells, in order to transfer the atom to a stable state again. These transitions can occur from different electron levels, resulting in a series of X-ray lines being emitted. The energy level differences depend on the type of the emitting atom; therefore, this radiation is called the characteristic radiation. The labeling of these lines starts with the designation of the primary vacancy, i.e. when the innermost K-shell is ionized it is K-radiation, when the L-shell is primarily ionized it is L-radiation, etc.

Figure 2.1 Parts of the continuous spectrum of an X-ray tube.

Figure 2.2 Line energy as a function of the atomic number.

      The energies of the electron levels depend mainly on the number of protons of the atom, i.e. on the atomic number. This means that the energy differences depend on the type of the atoms. These energy differences are described by Moseley's law.

(2.4)

      with

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E	=	energy difference between the electron levels
Z	=	atomic number of the atom
C1, C2	=	constants