X-rays are produced when high-speed electrons strike some material object. However, majority of the electrons that strike a solid target, do nothing spectacular at all. Most of them undergo glancing collisions with the matter particles, lose their energy a little bit at a time and thus merely increase the average kinetic energy of the particles of the target material. It is found that nearly 99.8 percent of the energy of the electron beam goes into heating the target.
But a small number of the bombarding electrons produce X-rays by losing their kinetic energy in the following two ways:
(i) Some of the high-velocity electrons penetrate the interior of the atoms of the target material and are attracted by the positive charge of their nuclei.As an electron passes close to the positive nucleus, it is deflected from its path as shown in figure. The electron experiences de-acceleration during its deflection in the strong field of the nucleus. The energy lost during this de-acceleration is given off in the form of X-rays of continuously varying wavelength(and hence frequency). These X-rays produce continuous spectrum when analysed by Bragg spectrometer. This spectrum has a sharply defined short-wavelength limit λmin (or high-frequency limit fmax) which corresponds to the maximum energy of the incident electron.
If, as shown in figure, the striking electron has its velocity reduced from v to v' during its passage through the atom of the target material, then its loss of energy is = (1/2mv2 - 1/2mv2).
This must equal the energy of the X-ray photons emitted.
∴ 1/2 m(v2 -v'2) = hv
The highest or maximum frequency of the emitted X-rays corresponds to the case when the electron is completely stopped i.e. when v' = 0. In that case
1/2 mv2 = hvmax .....(i)
If the electron is accelerated through a potential of V volts, then
Such X-rays are very aptly called ‘braking’radiations because they are due to braking or slowing down of high-velocity electrons is the positive field of a nucleus. These radiations constitute, as said earlier, the continuous spectrum of the X-rays because they consist of a series of uninterrupted wavelengths having a sharply defined short-wavelength limit λmin. These X-rays are independent of the nature of the target material but are determined by the potential difference between the cathode and anode of the X-ray tube.
(ii) Some of the high-velocity electrons while penetrating the interior of the atoms of the target material, knock off the tightly bound electrons in the innermost shells(like K, L-shells etc) of the atoms. When electrons from outer orbits jump to fill up the vacancy so produce, the energy difference is given out in the form of X-rays of definite wavelength (and frequency). These wavelengths constitute the line spectrum which is characteristic of the material of the target.
Figure (a) shows the case when the high-velocity incident electron knocks off one electron from the K shell. As shown in figure (b), this vacancy in K-shell is filled by a nearby electron in the L-shell. During the jump an X-ray radiation is emitted whose frequency is given by Ek – El = hv
where Ek is the energy required to dislodge an electron form the K-shell and El is that required for L-shell. Since this energy difference is comparatively very large, the X-rays emitted have very large energy content and hence are highly penetrating. If, however, this vacancy in K-shell is filled up by an electron jumping from M-shell, the X-rays emitted would be still more energetic and would consequently possess still higher frequency because ΔE =(Ek –Em) is more than ΔE = (Ek – EL). Such X-rays arising from millions of atoms produce the K-lines as shown in figure. Usually, two lines Kα and Kβ of this series are detected although there are many more.
As stated earlier, these K, Land M series constitute the line spectra of the X-rays which are characteristic of the material used as target in the X-ray tube.
