If the projectile electron interacts with an inner-shell electron of the target atom, rather than an outer-shell electron, characteristic x-radiation can be produced. Characteristic x-radiation results when the interaction is sufficiently violent to ionize the target atom, by the total removal of the inner-shell electron. Excitation of a inner (K)-shell electron does not immediately produce characteristic x-radiation.
When the projectile electron ionizes a target atom by removal of a K-shell electron, a temporary electron hole is produced in the K shell. This is a highly unnatural state for the target atom, and is corrected by an outer-shell electron falling into the hole in the K shell. The transition of an orbital electron from an outer shell to an inner shell is accompanied by the emission of an x-ray photon. Photons of this sort have energies that are, of course, characteristic of the anode material, and are emitted from the atom with equal probability in all directions. This x-ray has energy equal to the difference in the binding energies of the orbital electrons involved (K-L).
A K-shell elctron is removed from a tungsten atom and is repleced by an L shell electron. What is the energy of the characteristic x-ray that is emitted?
For tungsten, K electrons have binding energies of 69.5 keV, and L electrons are bound by 12.1 keV. Therefore, the characteristic x-ray emitted has energy of: 69.5 - 12.1 = 57.4 keV
In summary, characteristic x-rays are produced by transitions of orbital electrons from outer to inner shells. Since the electron binding energy for every element is different, the characteristic x-rays produced in the various elements are also different. This type of x-radiation is called characteristic radiation, because it is characteristic of the target element. The effective energy characteristic x-rays increases with increasing atomic number of the target element.