How does x ray fluorescence work
A range of application notes available on our XRF Resource pages illustrate the application and suitability of micro-XRF within these fields. X-rays form part of the electromagnetic spectrum, and are characterized by energies lying between ultra-violet and gamma radiation.
Wavelengths are typically in the range 0. X-rays are widely used in society today including for medical imaging in hospitals and baggage screening at airport security gates. Within science their properties are integral to many elemental and structural analytical techniques. On reaching a material, some of the x-rays will be absorbed, and some scattered — if neither process occurs, the X-rays will be transmitted through the material.
When absorption occurs, the X-rays interact with the material at the atomic level, and can cause subsequent fluorescence — it is this X-ray Fluorescence which forms the basis of XRF spectroscopy, and the process is discussed in more detail in the next section. This scattering can occur both with and without loss of energy, called Compton and Rayleigh scattering respectively.
X-ray Fluorescence XRF can be considered in a simple three step process occurring at the atomic level:. The energy difference between the expelled and replacement electrons is characteristic of the element atom in which the fluorescence process is occurring — thus, the energy of the emitted fluorescent X-ray is directly linked to a specific element being analyzed.
It is this key feature which makes XRF such a fast analytical tool for elemental composition. In general, the energy of the emitted X-ray for a particular element is independent of the chemistry of the material. For example, a calcium peak obtained from CaCO 3 , CaO and CaCl 2 will be in exactly the same spectral position for all three materials. Since most atoms comprise a number of electron orbitals eg, K shell, L shell, M shell a number of possible fluorescent transitions are possible.
For example, interaction of X-rays with an atom with K, L and M shells could result in a hole forming in the K shell, which is then filled by an electron from the L shell or from the M shell. In either case, these are termed K transitions. Alternatively, a hole could be formed in the L shell, subsequently filled by an electron from the M shell termed an L transition. Thus, for a single element, a number of XRF peaks are possible, and typically these will all be present in the spectrum, with varying intensities.
They form a characteristic fingerprint for a specific element. The absorption of X-rays by a particular material varies according to energy of the X-rays. As a rule of thumb, low energy X-rays are absorbed more than high energy photons. In order to expel an electron from one of the orbitals, the X-ray energy must exceed the binding energy of that electron — however, if the X-ray energy is too high, then the coupling between X-ray and electron is inefficient, and only a few electrons will be knocked out.
As the X-ray energy reduces, and approaches the electron binding energy, so the yield of expelled electrons increases. Just below this binding energy, a drop in absorption is observed, since the energy is not sufficient to emit electrons from that shell, and is too high in energy to emit electrons from the lower energy shells.
As explained in a previous slide, not all the incident X-rays result in fluorescence. The actual time required for a measurement will depend on the nature of the sample and the levels of interest.
High percentage levels will take a few seconds while part-per-million levels will take a few minutes. Contact Bruker today to find out more about XRF applications or to schedule a free demonstration of our instruments at your worksite. Handheld XRF Applications:.
Here is a detailed breakdown of the process: An x-ray beam with enough energy to affect the electrons in the inner shells of the atoms in a sample is created by an x-ray tube inside the handheld analyzer. The x-ray beam is then emitted from the front end of the handheld XRF analyzer. The x-ray beam then interacts with the atoms in the sample by displacing electrons from the inner orbital shells of the atom.
This displacement occurs as a result of the difference in energy between the primary x-ray beam emitted from the analyzer and the binding energy that holds electrons in their proper orbits; the displacement happens when the x-ray beam energy is higher than the binding energy of the electrons with which it interacts. Electrons are fixed at specific energies in their positions in an atom, and this determines their orbits.
Additionally, the spacing between the orbital shells of an atom is unique to the atoms of each element, so an atom of potassium K has different spacing between its electron shells than an atom of gold Au , or silver Ag , etc. Contact Us. When electrons are knocked out of their orbit, they leave behind vacancies, making the atom unstable. The atom must immediately correct the instability by filling the vacancies that the displaced electrons left behind.
Those vacancies can be filled from higher orbits that move down to a lower orbit where a vacancy exits. The X-ray source and primary radiation filter guarantee that each element in the sample is optimally excited. The vacuum seal separates the sample chamber from the goniometer chamber. During loading the seal is closed and the goniometer chamber remains under vacuum. Therefore only the small volume of the sample chamber needs to be evacuated for solids or flushed with helium for liquids.
During the measurement of liquids the vacuum seal stays closed to protect the components in case of spillage, safes helium and enhances the stability. The analyzer crystals play a crucial role. They break down the multiple frequency fluorescence spectrum into the specific wavelengths for the elements. And finally, the detectors: For the detection of light elements a proportional counter and for the heavier elements a scintillation counter is used.
Both detectors are perfectly suited to the respective energy range.
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