10.2.1 Basic Concepts

As discussed in detail in Chapters 5 and 6, radioactive (and other) radiations may have different impacts on the substances. The radiation can be absorbed or scat­tered, and, as a result of the interactions with the radiation, the irradiated substances themselves can emit different radiations, including particles or electromagnetic photons. All these phenomena provide analytical information on the different
structural levels of matter. The changes of intensity of the entering radiation, as well as the type, energy, or energy distribution, and the activity or intensity of the emitted radiation can be used in qualitative, quantitative, structural, and species analysis of the bulk phases, interfaces, and species bound to the surfaces of the substances.

Different types of radiations (namely, photons in the whole range of the electro­magnetic spectrum, electrons and beta particles, neutrons, and positively charged particles) are used for the irradiation. The emitted radiations can also be photons and particles (electrons and positively charged particles). The emission is the result of the interactions between the entering radiation and the nuclei, nuclear field, and orbital electrons.

As seen from the list of the entering and emitted radiations, there are many ana­lytical methods using the interaction of radiation with matter. They can be classi­fied on the basis of the entering and the emitted particles, as summarized in Table 10.2. As usual, the interactions are significantly influenced by the mass and charge of the particles (as discussed in Section 5.1). The classification is made on the basis of the mass of the particles.

For the sake of completeness, Table 10.2 includes the analytical method in which the substance is irradiated with photons with lower energy than the nuclear radiation (e. g., nuclear magnetic resonance, electron spin resonance, infrared, near­infrared, visible, ultraviolet spectroscopy, or dynamic light scattering). Of course, these methods traditionally belong to other disciplines of chemistry, so they are not discussed in detail here. It is important to note, however, that they also utilize the interactions of radiation with matter. In addition, nuclear magnetic resonance can be considered to be a nuclear analytical method in which the magnetic field of spe­cial nuclei is excited by electromagnetic radiation with low energy. At the same time, the highest-energy electromagnetic radiation (gamma photons) also excites the nuclei: the two terminal ranges of the electromagnetic spectrum have an impact on the same part—namely, the nucleus of the atoms.

In each row of Table 10.2, the emitted particles are the same; only their energy is different. The photons or particles within the same energy range are detected with the same methods, independent of the irradiation. As an example, the emission of X-ray photons of X-ray fluorescence analysis (XRF), electron microprobe, and ion (including proton)-induced XRF is mentioned. As will be discussed later in this chapter (Section 10.2.3.1), the X-ray photons (shown in row 4 of Table 10.2) are emitted as a consequence of the electron emission from the K or L electron orbital of the samples to be analyzed (the photoelectric effect, discussed in Section 5.4.4). High-energy gamma photons (shown in row 4 of Table 10.2) are emitted when irra­diation with neutrons or charged particles induces nuclear reactions (such as NAA, PGAA (see Sections 10.2.2.1 and 10.2.2.2), and charged particle activation analysis (CPAA; Section 10.2.5.2)).

As seen in row 5 of Table 10.2, electrons can be emitted after irradiation of the matter with photons, electrons, or ions. When Auger electron emission (see Sections

5.3 and 5.4.4) results from electron emission from the K or L electron orbital of the samples to be analyzed, they are detected and measured by the same techniques,

a, alpha particle; AES, Auger electron spectroscopy; CPINRA, charged particle-induced nuclear reaction analysis; CPAA, charged particle activation analysis; EIID, electron-induced ion desorption; EELS, electron-energy-loss spectroscopy; EMP, electron microprobe; ESCA, electron spectroscopy for chemical analysis; ESR, electron spin resonance; EXAFS, extended X-ray absorption fine structure; IEX, ion-excited X-ray fluorescence spectroscopy; IMMA, ion microprobe mass analyzer; IMXA, ion microprobe X-ray analysis; INS, ion neutralization spectroscopy; IR, infrared spectroscopy; ISS, ion scattering spectrometry; LEED, low-energy electron diffraction; LAMMA, laser microprobe mass analysis; n, neutron; NAA, neutron activation analysis; NEXAFS, near-edge X-ray absorption fine structure; NIR, near-infrared spectroscopy; NMR, nuclear magnetic resonance; p, proton; PGAA, prompt gamma activation analysis; PIXE, particle-induced X-ray emission; RHEED, reflection high-energy electron diffraction; RIBS, Rutherford backscattering spectroscopy; SAM, scanning Auger microanalysis; SANS, small-angle neutron scattering; SEM, scanning electron microscopy; SIMS, secondary ion mass spectroscopy; UPS, ultraviolet photoelectron spectroscopy; UV, ultraviolet spectroscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction analysis; XRF, X-ray fluorescence analysis; XANES, X-ray absorption near-edge structure.

Source: Adapted from Nagy and Konya, with permission from Taylor & Francis.

independent of the irradiation method. On the basis of the emitted radiation, gamma, X-ray, electron, and charge particle spectroscopic methods are classified.

In classical radioanalysis, those methods are used when the emitted radiation originates from the nucleus or the internal electron orbitals. Accordingly, classical analytical methods are the activation analytical methods, especially NAA and XRF. In fact, XRF does not require nuclear processes: the irradiating X-ray photons can be produced by an X-ray tube using the electron transition between the internal electron orbitals of the cathode of the X-ray tubes. Furthermore, the emitted X-ray photons also originate from electron transition between electron orbitals. As dis­cussed previously in this chapter, the emission of the same particles with similar energy requires similar detection and measuring techniques, independent of the irradiation method. For this reason, the methods providing X-ray photons and elec­trons (e. g., electron microprobe, AES, and X-ray photoelectron spectroscopy (XPS)) are also considered to be nuclear analytical methods. At the same time, the photoelectric effect may or may not be accompanied by a nuclear process. Thus, the term “radioanalysis” is used in a very broad sense: all methods may be consid­ered to be radioanalysis in which emitted particles and photons are analyzed.

To summarize, nuclear analysis refers to all types of detection and measurement techniques of the emitted radiation. As mentioned several times previously, radiation can be emitted both from the nucleus and from the orbital electrons. The radiation that originates from the nucleus can be the consequence of nuclear reactions or excita­tion of the nucleus. The radiation that originates from the electron orbitals relates to the excitation and de-excitation of the electrons or ionization. Common characteristics of these methods are that they are selective, sensitive, and frequently indestructible.

The methods listed in Table 10.2 differ in the depth of the introduction of the radiation, the interacting part of the substance, and the number of the interactions of the radiation with matter; and these characteristics determine which properties of the substance can be investigated using a particular method. The depth of the intro­duction of the radiation determines how thick the studied layer is, i. e., whether the properties of bulk phases, the interface, or the species adsorbed on the surfaces may be studied. The mass, charge, and energy of the radiation influences the thick­ness of the studied layer. The layer of absorption of the radiation is usually deeper in the case of light and neutral radiations. The energy of the radiation, however, strongly modifies this general tendency. For example, high-energy X-rays or elec­trons are introduced deeply, so the properties of the bulk can be studied. At small X-ray or electron energies, the structure of the surface layer can be studied.

In Table 10.3, the thickness of the studied layer, the primary information, the typical sensitivity, and the detectable elements and species of the methods included in Table 10.2 are summarized.