Inner- Shell Transitions and Characteristic X-Rays:
Hydrogen atom has only one shell in ground state. Its energy levels are of the order of few electron volts (ev) ; En = – 13.6 ev/n2. Therefore its spectrum has only lines in ultraviolet, visible and infrared region. But other heavy atoms have K, L, M, N…. shells. So their spectrum also has lines in x-ray region.
Suppose a heavy target material is bombarded with a beam of electrons that has been accelerated by applying high potential difference across the electrodes (several kev). Some of these electrons collide with inner-shell electrons of target material and knock them out of their respective atom.
Further suppose that a K-shell electron is knocked out from an atom creating a vacancy in K-shell. Then an electron from either L, M or N shell will quickly jump down to fill the vacancy in the K-shell emitting energy as x-ray photon. An x-ray photon due to transition from L-shell to vacancy in the K-shell is called Kα characteristic x-ray. The transition from M and N-shells to the K-shell give rise to Kβ and Kγ characteristic x-ray respectively. Similarly L, M and N series characteristic x-rays, relatively of low energy, are produced due to the ejection of electrons from L, M and N-shells respectively.