When the particle beam interacts with atoms of the target, may occur the ionization of an electron from one of the inner layers, usually on the layers K or L. The filling of this space (called gap) is taken by an electron from an outer layer. The excess energy, due to the electron being in an outermost layer, is released in the form of an X-ray with characteristic energy, i.e., unique for each atom in nature. The detection of the characteristic X-ray allows to identify the atomic composition of the target. As the minimum energy for detection of X-rays is about 1 keV, the smallest element that can be detected by their K or L line is sodium (Z = 11).