High energy photons emitted by a X-ray tube interact with metal & materials. The high energy photon is absorbed by an electron
of the atom. This electron is accelerated and forced to leave the atom. The so created hole in the structure of the electron shell is filled up by an electron of higher energy. The difference is energy between the leaving electrons position and the filling up electron
may leave the atom as a photon of defined energy or as an electron. In the case of a leaving photon this process is called X-ray fluorescent and the energy of the leaving photon is characteristic for this atom therefore for the element. The electron shells of an atom are called K-, L-, M-shell. Filling up a hole in the K-shell creates, K-radiation (Ka if the filling up electron has is origin in the L-shell, Ka if the filling up electron has is origin in the M-shell). Filling up a hole in the L-shell creates L-radiation and so on. Only K and L radiation is on interest because the energy of K and L radiation is in the region which can be detected with standard detectors.