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 in 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 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.