9.1 X-ray Spectra X-rays are generated when high energy electrons strike a solid target e.g. the metal anode in a “cathode-ray” tube. The spectrum of the X-rays generated in this way consists firstly of a continuous range of wavelengths down to a limiting value corresponding to the maximum energy of the incident electrons. These X-rays are the result of the deceleration of the charged particles and are known as “bremsstrahlung” or “braking radiation”. Intensity Wavelength 0.1 nm Bremsstrahlung Threshold energy Maximum energy of incident electrons Intensity Wavelength 0.1 nm Characteristic X-rays Threshold energy Figure 36: X-ray spectra consist of bremsstrahling and characteristic lines from inner-shell transitions. When the energy of the electrons is increased above a certain value, for a given target material, sharp peaks i.e. discrete lines, appear superimposed on the continuous “bremsstrahlung”. The spectrum of these discrete lines are characteristic of the target element. These characteristic X-rays have the following properties: • The wavelengths fit a simple series formula. • All the lines of a particular series appear together once the incident electrons exceed a particular threshold energy. • The threshold energy for a particular series just exceeds the energy of the shortest wavelength in the series. • Above a certain energy, no new series appear. These observations are explained by the following process: The incident energy (from electron impact) is transferred to an inner-shell electron. If this energy is sufficient the inner-shell electron is raised to a vacant energy level. Now the vacancy energy levels are those lying between the atoms ground state energy and the ionization limit. This is a range of only 10eV or so. If the incident energy is of the order of 10 3 eV, or greater, then the most likely result is to ionize the atom i.e. the inner-shell electron escapes with a kinetic energy equal to the 56
Atomic Physics, P. Ewart 9 X-Rays: transitions involving inner shell electrons incident impact energy less the binding energy of the inner shell. An electron from a higher inner shell may “fall” into the vacancy. As a result of this transition an X-ray photon is emitted with an energy corresponding to the difference in binding energy of the two shells. For example, creation of a vacancy in the n = 3 shell allows electrons from n = 4, or higher shells to “fall” into the n = 3 vacancy. (Higher energy impacts may eject electrons from deeper shells n = 2 and n = 1) These transitions are the source of the discrete, characteristic, X-ray lines. X-ray spectroscopy developed its own nomenclature and it is still (unfortunately) used, so we have to live with a further set of labels. In the context of X-rays the n = 1 , 2 , 3 shells are knows as K, L, M etc. respectively.
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