X-ray spectra - continuous and characteristic X-ray spectra

X-ray spectra - continuous and characteristic X-ray spectra.

The spectrum from an X-ray tube contains two distinct parts :

(i) Continuous X-ray spectra

It consists of radiations of all possible wavelengths, from a certain lower limit to higher values continuously, as in the case of visible light.

Origin - Continuous X-ray spectra

X-rays are produced, when high velocity electrons strike the target material of high atomic number. It has also been mentioned in the production of X-rays, that most of the energy of the electrons goes into the heating of the target material.

A few fast moving electrons penetrate deep into the interior of the atoms of the target material and are attracted towards the nuclei by the attractive forces of their nuclei. Due to these forces, the electrons get deflected from their original path. As a result of this, the electrons are decelerated, and hence energy of the electron decreases continuously. This loss of energy during retardation is given off in the form of X-rays of continuously varying wavelength. The X - rays consist of continuous range of frequencies upto a maximum frequency í_max or minimum wave length λ_min. This is called as continuous X - rays. The minimum wave length depends on the anode voltage. If V is the potential difference between the anode and the cathode

eV = hν_max = hc / λ_min

The minimum wavelength of the given radiation is,

λ_min = hc /eV

where h is Planck's constant, c is the velocity of light and e, the charge of the electron. Substituting the known values in the above equation.

λ_min  = 12400/V A⁰

For the given operating voltage, the minimum wave length is same for all metals.

(ii) Characteristic X-ray spectra

It consists of definite, well defined wavelengths superimposed on the continuous spectrum. These spectral lines generally occur in the form of small groups and are characteristic of the material of the target.

Origin - Characteristic X-ray spectra

Few of the fast moving electrons having velocity of about (1/10)^th of the velocity of light may penetrate the surface atoms of the target materials and knock out the tightly bound electrons even from the inner most shells (like K, L shells) of the atom. Fig 6.22a shows the case, when the fast moving electrons knock off one electron from K-Shell and the vacancy is filled by the nearby electron from the L shell. During this transition, the energy difference is radiated in the form of X-rays of very small wave length. This corresponds to K_α - line of the series. The frequency ν₁ of this line is given by the relation (E_K - E_L) = hν₁. Suppose, the electron from M shell jumps to the K shell, it gives out K_β line and so on. If an electron jumps from the M-Shell to the vacant state in L-Shell, it contributes L_α line and if the vacancy in L-Shell is filled up by an electron of N shell, it contributes L_β and so on (Fig 6.22b). The frequency of radiation depends upon the target material. The X-ray spectra consists of sharp lines and is the characteristic of target material. Hence this spectra is known as characteristic spectra.