The atomic number, Z, is a ubiquitous parameter in atomic and nuclear physics where it occurs in almost any formula. In Composites Materials for photon interactions, the atomic number cannot be represented uniquely across the entire energy region, as in the case of elements, by a single number. For CM this number is called "effective atomic number" – Zeff, and it varies with energy (Igelli, Erzeneoglu et al. 2005; Manohara, Hanagodimath et al. 2008). Zeff has a physical meaning and allows many characteristics of a material to be visualized with a number (Singh, Kaur et al. 1996). This number is a very useful parameter for technology, and in many fields of scientific applications. In such applications the concept of effective atomic numbers Zeff is introduced to describe the properties of these composite materials in terms of an equivalent element (Igelli, Erzeneoglu et al. 2005). The Zeff is a convenient parameter for representing the attenuation of y-rays in a complex medium and particularly in CM (Han, Demir et al. 2009). Higher values (Zeff >20) correspond to inorganic compounds and metals, and lower values (Zeff < 10) correspond to organic substances. Analysis of Zeff is of a great importance for geological studies of ore and mineral composition, searching for new sources of fossil fuels, structure monitoring of composite materials, separate diagnostics of soft and bone tissues, determination of calcium content in tests for osteoporosis, etc. Study of Zeff provides conclusive information about the target with which the radiation interacts (El-Kateb, Rizk et al. 2000). Zeff can be determined also from the plots of atomic cross-sections versus atomic numbers of the individual elements (Mudahar, Singh et al. 1991; Parthasaradhi, Esposito et al. 1992).
The effective atomic number of a composite sample is equal (Duvauchelle, Peix et al. 1999) with the sum of equation (6). It should be mentioned, however, that the theoretical determination of Zf does not always coincide with the experimental results (Manninen, Pitkanen et al. 1984; Duvauchelle, Peix et al. 1999; Manohara, Hanagodimath et al. 2008).
= Xafz. (6)