Matter is made of elementary particles consisting of a nucleus composed of protons and neutrons surrounded by electrons. The protons are hydrogen nuclei with a positive charge and their number determines the atomic number. The neutrons, which have no electric charge, form with the protons the nucleons which determine the atomic mass. Electrons are negative elementary particles gravitating around the nucleus on concentric orbits with a liaison energy which is higher as the orbit nears the nucleus. The absorption of X-ray depends on the energy of the beam. Schematically, an incident-photon can be totally absorbed and drive out an electron : this is the photo-electric interaction. The interaction between the incident-photon and the electron results in the emission of a diffused photon as the incident electron is shifted. This is the Compton interaction.