X-ray Fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the incident X-ray photons (synchrotron beam) and subsequently release X-ray photon when the system relaxes, that is when the electrons transition from the higher energy levels of the atom to the vacant inner shell. The beauty of this process is that each secondary X-ray photon (sometimes called characteristic radiation) emitted from the sample has a specific energy which is a fingerprint of the atom from which it has originated. By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample at the point where the X-ray beam hits the sample. Typically a special type of detector called energy-dispersive detector is used to precisely measure the energy of each photon. The plot of the number of photon counts versus their energy, the X-ray spectrum, typicallly shows a number of peaks which are directly associated with specific elements, so by just glancing at the spectrum it is possible to quickly deduce which elements are present in the sample.
By use of calibrated standards it is possible to make synchrotron XRF a quantitative technique.
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