Xray Fluorescence

X-ray fluorescence (XRF) is a non-destructive analytical technique based on the emission of characteristic secondary, or fluorescent, X-rays from a material irradiated with high-energy X-rays or gamma rays. It is widely used for elemental analysis in disciplines ranging from geochemistry and materials science to forensic investigation, archaeology and the study of cultural heritage objects, including paintings and murals. Because individual elements emit fluorescent radiation of distinctive energies, XRF provides direct insight into the composition of complex materials.

Underlying Physics

When high-energy X-rays or gamma rays strike matter, they may ionise atoms by ejecting tightly bound inner-shell electrons. The resulting vacancies render the atom electronically unstable, causing electrons in outer shells to fall into the lower-energy positions. Each downward transition emits a photon with energy equal to the difference between the initial and final orbitals. This produces radiation characteristic of the element.
Fluorescence in this context refers to the process in which energy absorbed as high-energy radiation is re-emitted as radiation of generally lower energy. Each element has a defined set of possible electronic transitions, commonly classified using Siegbahn notation. For example, an L→K transition is labelled Kα, while an M→K transition is labelled Kβ. The wavelength of the emitted photon can be calculated using Planck’s postulate, and its energy determines the position of the spectral line.
The intensity of each characteristic line is proportional to the abundance of the corresponding element in the sample. This relationship forms the basis of quantitative analysis.

Primary Radiation Sources

Effective excitation of fluorescence requires radiation with energy high enough to expel inner-shell electrons. Conventional X-ray tubes are the most frequently used source; these produce bremsstrahlung radiation together with characteristic lines from the target material, typically tungsten or rhodium. Operating voltages between 20 and 60 kV enable analysis of a broad range of elements.
Portable XRF instruments often employ copper targets, with high-energy electrons generated by mechanisms such as impact ionisation, laser excitation or pyroelectric crystals. Radioisotope sources—including cadmium-109, cobalt-57, iron-55, plutonium-238 and americium-241—can also be used, eliminating the need for a powered tube and enabling compact device design.
When synchrotron sources or polycapillary optics are used, the X-ray beam can be focused to extremely small dimensions, allowing sub-micrometre spatial resolution for highly localised analysis.

Dispersion Methods

Two principal approaches exist for analysing the emitted fluorescent radiation:

• Energy-dispersive X-ray spectroscopy (EDXRF) measures the energy of incoming photons directly. A solid-state detector produces electrical pulses proportional to photon energies. A multichannel analyser sorts these into a digital spectrum.
• Wavelength-dispersive X-ray spectroscopy (WDXRF) separates radiation by wavelength using a diffraction crystal. By altering the angle between the incident beam, the crystal and the detector, specific wavelengths are selected according to Bragg’s law. WDXRF provides extremely sharp spectral lines with high resolution.

Detection Systems

Energy-dispersive systems integrate dispersion and detection into a single step. Common detectors include proportional counters, PIN diodes and more advanced solid-state detectors such as Si(Li), Ge(Li) and silicon drift detectors (SDDs). These operate by collecting charge carriers generated when an X-ray photon ionises atoms in the detector medium. High detector speed is crucial to ensure that charge generated by different photons is not conflated; peak-length discrimination helps eliminate pulse pile-up.
In WDXRF, the monochromated radiation enters a gas-filled chamber. Ionisation events produce cascades of charge collected across a high-voltage field and converted into digital counts. Both methods accumulate a spectrum that relates photon energy or wavelength to intensity.

X-Ray Intensity Considerations

Only a small proportion of absorbed radiation is re-emitted as fluorescence, making the secondary signal relatively weak. Light elements (low atomic number) emit low-energy photons that have limited penetrating power and are strongly attenuated by air. Consequently, high-precision instruments maintain the beam path under vacuum—typically around 10 Pa—to reduce absorption. Maintaining moving parts and allowing sample exchange within a vacuum chamber presents significant engineering challenges.
For samples sensitive to vacuum conditions, a helium-flushed chamber provides a compromise, improving low-energy transmission relative to air at the cost of some performance.

Chemical Analysis Applications

The first documented proposal to use a primary X-ray beam for fluorescent analysis appeared in 1928. The method is now broadly used in industrial process control, environmental monitoring, metallurgy and cultural heritage conservation. The lightest routinely detectable element is beryllium (Z = 4), although accurate quantification of elements lighter than sodium (Z = 11) requires sophisticated background and inter-element corrections.
XRF is particularly valuable as a non-destructive technique. In the analysis of artworks, for example, XRF permits identification of pigments, underdrawings and restoration layers without harming the object.

Energy-Dispersive Spectrometry

In energy-dispersive X-ray spectrometers (EDS or EDX), silicon-based detectors—often lithium-drifted silicon (Si(Li)) or high-purity silicon wafers—measure photon energies. These detectors contain a silicon junction with a high bias voltage across it. Lithium drifting creates a thick intrinsic layer by compensating residual acceptor impurities, enabling efficient charge collection. Modern SDDs offer improved resolution, faster processing and reduced cooling requirements, making them suitable for both laboratory and portable instruments.