^Characteristic X – rays
If the fast incident electrons collide with the electrons of the inner most level of the heavy target, knock them out. To fill the vacancy created electrons from the higher energy state jump & an X-ray photon of the corresponding energy gap is emitted in accordance with the Bohr’s hypothesis. Such X-rays are called characteristic X-rays.
The frequency (f) of the characteristic X-rays is given by Moseley law:
f = k ( Z – b )2
Here Z is atomic no. & k, b are constants, their values depends on the series transition.
Moseley made an extensive study of the characteristic X-ray spectra of a number of heavy elements. Moseley’s work provided for the first time a way to determine experimentally the atomic number Z of an element. He concluded that the atomic number is more fundamental than atomic weight in the emission of characteristic X-rays & thus the elements in the periodic table must be arranged in the order of increasing atomic number.