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X-ray emission induced by charged particles, also known as particle-induced X-ray emission (PIXE), is a powerful analytical technique for the elemental and chemical characterization of materials. PIXE is based on the principle that when a charged high-energy particle beam, such as a proton or helium ion, interacts with a sample atom, it can eject an inner-shell electron. This vacancy is then filled by an electron from a higher energy shell, and an X-ray photon is emitted with an energy characteristic of the specific element and the particular transition that occurred.
In a recent study published in the Journal of Analytical Atomic Spectrometry, researchers used PIXE to investigate the chemical sensitivity of the Kα X-ray emission of aluminum (Al) and its compounds (1). They used 2 MeV protons and 3 MeV He ions to excite the X-ray emission from thick pelletized samples of Al, Al2O3, Al2S3, AlN, and AlPO4. The X-ray emission was measured using a wavelength-dispersive spectrometer with a flat diffraction crystal.
The Kα X-ray emission line is the most prominent X-ray line emitted by an element, and it is often used in PIXE analysis. However, the Kα X-ray line can be split into multiple components, known as multiple ionization satellites (MIS), due to the simultaneous ionization of multiple electrons from the atom. The energy and intensity of the MIS peaks are sensitive to the chemical environment of the atom, which makes it useful for chemical analysis.
The researchers observed that the energy and intensity of the Kα MIS peaks were different for the different Al compounds. This indicates that the chemical environment of the Al atom has a significant effect on the Kα X-ray emission. The researchers also found that the energy shift of the Kα1,2 peaks showed a linear decrease with the increase of the calculated effective charge on Al atoms. This suggests that the chemical shift of the Kα peaks can be used to determine the effective charge on the Al atom, which is a measure of its chemical environment.
The results of this study demonstrate the importance of studying the chemical effects of the Kα X-ray band for low-Z elements. The researchers suggest that future work should focus on constructing an extensive and accurate database of diagram lines and related MIS for proton and He ion excitation in the MeV energy range. This database could be used to improve the accuracy of PIXE analysis for chemical characterization.
Fazinic, S; Mihalic I. B; Miokovic, A, Ramos, M. R; Petric, M. Influence of Chemical Effects on Al High-Resolution Kα X-ray Spectra in Proton and Alpha Particle Induced X-ray Spectra. J. Anal. At. Spectrom., 2023, 38, 2179–2187.