Effective atomic number in the Rayleigh to Compton scattering ratio

Detection and counting X-ray photons scattered by the Rayleigh and Compton processes enable matter to be characterized locally. A theoretical relation was first established which simulates the result of a Rayleigh to Compton ratio measurement. It can thus be shown that a correct choice of scattering angle and photon energy enables a result to be obtained which is almost independent of X-ray attenuation inside the sample. With this condition, the Rayleigh to Compton scattering ratio depends only on the mixture under study and provides a local measurement of certain complicated functions of the atomic number Z and of the weight percentage of the different elements which constitute the compound. This function is usually called the "effective atomic number", Z(eff). Different methods of calculation of Z(eff) are found in the literature, four of them, those used most frequently, were tested. There is no unique relation between the computed Z(eff) and 80 experimental results performed on aqueous solutions with different concentrations of eight elements, having Z values ranging from 13 to 64. This observation led us to the conclusion that any effective atomic number is valid only for given experimental conditions. Finally, a new method of calculating Z(eff) was developed for the Rayleigh to Compton scattering ratio, which is applicable for any material, scattering angle or photon energy. (C) 1999 Elsevier Science B.V. All rights reserved.