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Characteristic X-rays are emitted from the atomic shells, when electrons jump from the shells at higher energy levels (with Iower binding energy) to the vacancies in the shells at lower energy levels (with higher binding energies).
The binding energy of the K-shell electron is the largest in an atom (for example, it is 13.6eV for H and up to 115keV for U)
The energy of the emitted X-ray is determined by
where E m is the upper energy level and En is the lower energy level.
In addition to loosing its kinetic energy in collisions with the atomic electrons causing ionization or excitation of the atoms along its path, a charged particle (in our case an electron) gives up its kinetic energy by a photon emission as it is deflected (or accelerated) in the electric field of nuclei.
The emitted EM radiation has a continuous energy spectrum from 0 to E k , where E k is the kinetic energy of a charged particle.
For E k < 100 keV, radiation is emitted at 900 to the direction of the charged particle. For higher E k the direction of the emitted radiation shifts toward the forward-peaked direction.