Editing X-ray fluorescence (section)

==Underlying physics==
[[File:X-ray fluorescence simple figure.svg|thumb|Figure 1: Physics of X-ray fluorescence in a schematic representation.]]

When materials are exposed to short-[[wavelength]] X-rays or to gamma rays, ionization of their component [[atom]]s 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 [[atomic orbital|orbital]]s 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 [[electron hole|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).

[[File:XRFScan.jpg|thumb|300px|right|Figure 2: Typical wavelength dispersive XRF spectrum]]
[[File:TubeSpectrum-en.svg|thumb|300px|right|Figure 3: Spectrum of a rhodium target tube operated at 60 kV, showing continuous spectrum and K lines]]

===Characteristic radiation===
Each element has electronic orbitals of [[characteristic X-ray|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 [[Siegbahn notation|given names]]: an L→K transition is traditionally called [[K-alpha|K<sub>α</sub>]], an M→K transition is called K<sub>β</sub>, an M→L transition is called L<sub>α</sub>, 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 postulate|Planck's Law]]:

:<math> \lambda = \frac{h c}{E} </math>

The fluorescent radiation can be analysed either by sorting the energies of the photons ([[energy-dispersive X-ray spectroscopy|energy-dispersive]] analysis) or by separating the wavelengths of the radiation ([[wavelength-dispersive X-ray spectroscopy|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]]. Figure 2 shows the typical form of the sharp fluorescent spectral lines obtained in the wavelength-dispersive method (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 generator]]s, 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 bombared with high energy electrons, that are produced either by impact laser or by pyroelectric crystals.<ref>Kawai, Jun. "Pyroelectric X-Ray Emission." X-Ray Spectroscopy for Chemical State Analysis. Singapore: Springer Nature Singapore, 2022. 107-133.</ref><ref>{{Cite journal|url=https://pubs.aip.org/aip/jap/article-abstract/97/10/104916/317055/High-energy-x-ray-production-with-pyroelectric?redirectedFrom=fulltext|title=High-energy x-ray production with pyroelectric crystals &#124; Journal of Applied Physics &#124; AIP Publishing|journal=Journal of Applied Physics |date=11 May 2005 |volume=97 |issue=10 |doi=10.1063/1.1915536 |last1=Geuther |first1=Jeffrey A. |last2=Danon |first2=Yaron |url-access=subscription }}</ref>

Alternatively, gamma ray sources, based on [[radioactive isotopes]] (such as <sup>109</sup>Cd, <sup>57</sup>Co, <sup>55</sup>Fe, <sup>238</sup>Pu and <sup>241</sup>Am) can be used without the need for an elaborate power supply, allowing for easier use in small, portable instruments.<ref>{{Cite journal | url=https://inis.iaea.org/collection/NCLCollectionStore/_Public/02/006/2006036.pdf | title=Radioisotope X-Ray Fluorescence Spectrometry | journal=Technical Reports Series | number=115 }}</ref>

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 X-ray spectroscopy|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]]:

:<math> n \cdot \lambda = 2 d \cdot \sin(\theta)</math>

where ''d'' is the spacing of atomic layers parallel to the crystal surface.

===Detection===
[[File:20180221-OSEC-LSC-0055 (39518128545).jpg|thumb|A portable XRF analyzer using a [[silicon drift detector]]]]
In energy-dispersive analysis, dispersion and detection are a single operation, as already mentioned above. [[Proportional counter]]s or various types of solid-state detectors ([[PIN diode]], Si(Li), Ge(Li), [[silicon drift detector]] SDD) are used. They all share the same detection principle: An incoming X-ray [[photon]] ionizes a large number of detector atoms with the amount of charge produced being proportional to the energy of the incoming photon. The charge is then collected and the process repeats itself for the next photon. Detector speed is obviously critical, as all charge carriers measured have to come from the same photon to measure the photon energy correctly (peak length discrimination is used to eliminate events that seem to have been produced by two X-ray photons arriving almost simultaneously). The spectrum is then built up by dividing the energy spectrum into discrete bins and counting the number of pulses registered within each energy bin. EDXRF detector types vary in resolution, speed and the means of cooling (a low number of free charge carriers is critical in the solid state detectors): proportional counters with resolutions of several hundred eV cover the low end of the performance spectrum, followed by [[PIN diode]] detectors, while the Si(Li), Ge(Li) and SDDs occupy the high end of the performance scale.

In wavelength-dispersive analysis, the single-wavelength radiation produced by the monochromator is passed into a chamber containing a gas that is ionized by the X-ray photons. A central electrode is charged at (typically) +1700 V with respect to the conducting chamber walls, and each photon triggers a pulse-like cascade of current across this field. The signal is amplified and transformed into an accumulating digital count. These counts are then processed to obtain analytical data.

===X-ray intensity===

The fluorescence process is inefficient, and the secondary radiation is much weaker than the primary beam. Furthermore, the secondary radiation from lighter elements is of relatively low energy (long wavelength) and has low penetrating power, and is severely attenuated if the beam passes through air for any distance. Because of this, for high-performance analysis, the path from tube to sample to detector is maintained under vacuum (around 10 Pa residual pressure). This means in practice that most of the working parts of the instrument have to be located in a large vacuum chamber. The problems of maintaining moving parts in vacuum, and of rapidly introducing and withdrawing the sample without losing vacuum, pose major challenges for the design of the instrument. For less demanding applications, or when the sample is damaged by a vacuum (e.g. a volatile sample), a helium-swept X-ray chamber can be substituted, with some loss of low-Z (Z = [[atomic number]]) intensities.