K-alpha

Characteristic X-ray is a line spectrum of X-ray radiation, which is produced during transitions between energy levels of the inner shell electrons and is characteristic of the particular element. It was discovered by Charles Glover Barkla, who in 1917 received the Nobel Prize for Physics for this.

Formation

The characteristic lines of the X-ray spectrum (, , ...) created in the image of the Bohr model of the atom as follows:

• A free, high-energy electron strikes out a bound electron from an inner shell of its atom. At least the energy must be transferred to the electron bumped that to a still unoccupied shell is necessary for excitation. Usually it is higher than the previous binding energy of the electron, and the atom becomes ionized.
• The resulting gap is closed by an electron from an outer shell. Since the electrons have on the outer shells higher energies, they must give the difference of the energy in their change to a further to the interior shell.
• This is due to the typically lying in the range 1-100 keV energy difference between the electron cloud in the two conditions (lack of electrons in the inner shell and outer shell ) in the form of X-rays. Thus the radiation has the energy difference between a higher (for example, L-) and lower ( for example, K - ) shell. Since this energy difference is element- specific, is called the X-ray radiation " characteristic X-rays ."
• Is the wavelength, and thus the energy of the emitted radiation may be calculated with the law Moseley.

Identification of the spectral lines

To designate the X-ray lines are added initially the inner shell to, in which the electron has been transferred at the emission, such as K, L, M, etc. A Greek character as an index is the difference from the principal quantum number n of the outer shell where was coming from the electron. For example corresponds

• An index of an alpha- 1, i.e., the next higher tray ( for the K - series that is the L- cup)
• An index of a beta 2 (for the K- series is the M- shell ), etc.

In the L- and M- series as well as for atoms with higher atomic number, this assignment is no longer as clear. Here the fine structure splitting plays a role. In addition to the Greek index then a numerical index to distinguish the lines is still being used.

Occurrence of several spectral lines by an electron excitation

Atoms with higher atomic numbers have more outer shells that can deliver an electron to fill the hole in the inner shell. Also the hole in various inner shells can occur. Accordingly, these atoms can also emit X-rays of different energies.

• After an electron is dropped to the K-shell, in turn, for example, the L shell is staffed. Another electron from a higher shell still falls down through the emission of another photon. This second photon of lower energy is, in this example, to the L line.
• In addition to the X-ray emission forms - especially in light atoms with atomic numbers - the transfer of energy to electrons located further out another way to compensate for the difference in energy (see Auger effect ).

Generation in the X-ray tube

In a X-ray tube high-energy electrons hit an anode, where they produce a characteristic x-ray radiation side, on the other hand also the bremsstrahlung is generated. In the graphical plot of the spectrum lines of the characteristic X-rays appear as high elevations, while the background is formed by the bremsstrahlung.

Application

The characteristic X-rays is evaluated with the detectors determine the energy or the wavelength of the X-ray quanta. From the spectrum can be qualitatively concluded that the elemental composition of the sample by a ZAF correction and a quantitative analysis is also possible. This principle is applied in X-ray fluorescence analysis and X-ray energy dispersive spectroscopy (EDX / EDS) and wavelength dispersive X-ray spectroscopy ( WDX / WDS).