Characteristics X-rays Line Spectrum

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When the target of an X-ray tube is stuck by energetic electrons, it emits two kinds of X-ray radiation. One of then has a continuous  spectrum whose intensity distribution in  wavelength depends entirely on the tube voltage. The other consists of distinct spectral lines whose frequencies are characteristics of the material of the target.

Hence this line spectrum is called the X-ray spectrum of the  material of the target. The radiation forming the line spectrum is called as characteristics X-rays. The frequencies of the characteristics X-rays are in general about 1000 times higher than the optical frequencies.


Kossel explained the origin of the characteristic X-ray line spectrum on the basis of the shell structure of the atom. An atom is built up of a central positively-charged nucleus with discrete aggregates of electrons known as K shell, L shell, M shell, N shell ect. The maximum number of electrons which the K,L,M,N….. Shells can hold are 2, 8, 18, 32……. the electron on the K-shell are attracted by the positive nucleus with the greatest force, and to eject them from the atom maximum energy is required. Lesser energy is required to eject an electron from the shell still lesser from M-shell and so on.

Now in an X-rays tube an electron emitted from the cathode strikes the target with a tremendous velocity so that it penetrates well inside the atom of the target. If it ejects an electron from the K-shell of the atom, a vacancy is created in the K-shell. Immidiately an electron from one of the outer shells, say L-shell jumps to the K-shell, emitting an X-ray photon of energy equal to the energy difference between the two shells.

Similarly, if an electron from the M-shell jumps to the K-shell, X-ray photon of higher energy is emitted. The X-ray photons emitted due to the jump of electron from the L.M.N-shells to the K-shell give Ka , Kb,Kg lines of the K-series of the spectrum.


If the electron striking the target atom, an electron from the L-shell of the target ejects an electron from the M , N……. shells jumps to the –shell so that X-rays photons of lesser energy are emitted these photons from the L-series are smaller than those of the K-series. In a similar ways the formation of M-series, N-series etc. may be explained. The K ,L ,M….Shells correspond to quantum numbers n = 1 ,2 , 3