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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| X-ray fluorescence | 1/6 | https://en.wikipedia.org/wiki/X-ray_fluorescence | reference | science, encyclopedia | 2026-05-05T10:06:44.876284+00:00 | kb-cron |
X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. When a material is illuminated with high-energy X-rays, its atoms can become excited and emit their own unique, characteristic X-rays—a process similar to how a blacklight makes certain colors fluoresce. By measuring the energy and intensity of these emitted "secondary" X-rays, scientists can identify which elements are present in the sample and in what quantities. Thus, XRF is the basis of a non-destructive analytical technique widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects such as paintings.
== Underlying physics ==
When materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, and may occur if the atom is exposed to radiation with an energy greater than its ionization energy. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way makes the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy (generally lower).
=== Characteristic radiation === Each element has electronic orbitals of characteristic energy. Following removal of an inner electron by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are a limited number of ways in which this can happen, as shown in Figure 1. The main transitions are given names: an L→K transition is traditionally called Kα, an M→K transition is called Kβ, an M→L transition is called Lα, and so on. Each of these transitions yields a fluorescent photon with a characteristic energy equal to the difference in energy of the initial and final orbital. The wavelength of this fluorescent radiation can be calculated from Planck's law:
λ
=
h
c
E
{\displaystyle \lambda ={\frac {hc}{E}}}
The fluorescent radiation can be analysed either by sorting the energies of the photons (energy-dispersive analysis) or by separating the wavelengths of the radiation (wavelength-dispersive analysis). Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material. This is the basis of a powerful technique in analytical chemistry. The typical form of the sharp fluorescent spectral lines obtained in the wavelength-dispersive method is illustrated in Figure 2 (see Moseley's law).
=== Primary radiation sources === In order to excite the atoms, a source of radiation is required, with sufficient energy to expel tightly held inner electrons. Conventional X-ray generators, based on electron bombardment of a heavy metal (i.e. tungsten or rhodium) target are most commonly used, because their output can readily be "tuned" for the application, and because higher power can be deployed relative to other techniques. X-ray generators in the range 20–60 kV are used, which allow excitation of a broad range of atoms. The continuous spectrum consists of "bremsstrahlung" radiation: radiation produced when high-energy electrons passing through the tube are progressively decelerated by the material of the tube anode (the "target"). A typical tube output spectrum is shown in Figure 3. For portable XRF spectrometers, copper target is usually bombarded with high energy electrons, that are produced either by impact laser or by pyroelectric crystals. Alternatively, gamma ray sources, based on radioactive isotopes (such as 109Cd, 57Co, 55Fe, 238Pu and 241Am) can be used without the need for an elaborate power supply, allowing for easier use in small, portable instruments. When the energy source is a synchrotron or the X-rays are focused by an optic like a polycapillary, the X-ray beam can be very small and very intense. As a result, atomic information on the sub-micrometer scale can be obtained.
=== Dispersion === In energy-dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-state detector which produces a "continuous" distribution of pulses, the voltages of which are proportional to the incoming photon energies. This signal is processed by a multichannel analyzer (MCA) which produces an accumulating digital spectrum that can be processed to obtain analytical data. In wavelength-dispersive analysis, the fluorescent X-rays emitted by the sample are directed into a diffraction grating-based monochromator. The diffraction grating used is usually a single crystal. By varying the angle of incidence and take-off on the crystal, a small X-ray wavelength range can be selected. The wavelength obtained is given by Bragg's law:
n
⋅
λ
=
2
d
⋅
sin
(
θ
)
{\displaystyle n\cdot \lambda =2d\cdot \sin(\theta )}
where d is the spacing of atomic layers parallel to the crystal surface.
=== Detection ===