Characteristic Radiation
Each element has electronic orbitals of characteristic energy. When an inner electron removed by an energetic photon provided by a primary radiation source, an electron from an outer shell drops into its place. There are number of ways that this transfer can happen. The main transitions are given names: L→K transition is traditionally called Kα, M→K transition is called Kβ, M→L transition is called Lα, and like wise. 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 the Planck’s Law.
The fluorescent radiation can be analyzed 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.
Primary Radiation
A source of radiation with sufficient energy is required to expel the tightly held inner electrons and excite the atoms. Conventional X-ray generators are more commonly used because their output can readily be “tuned” for the application and because their higher power can be deployed relative to other techniques. However, gamma ray sources can be used without the need for an elaborate power supply allowing an 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. 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”).