Photon Scattering by Compounds: an Assessment of the Validity of the Independent Atomic Model Approximation

Abstract

The validity of the independent atomic model (IAM) for x-ray scattering, which assumes that each atom in the sample scatters independently of the others, was examined for materials and irradiation geometries where interatomic and intermolecular cooperative effects are expected to provide a non-negligible contribution. Samples of water, fat, polymers and powder were investigated by means of a simple powder diffractometer in the interval of momentum transfer 0.017⩽χ⩽0.67 {χ=E[sin(θ/2)]/12.4, E being the incident photon energy and θ the scattering angle}. Accurate evaluation of self-absorption, air scattering and other contamination was carried out by means of a Monte Carlo code. Simulation of the incident photon spectra permitted an assessment of non-monochromaticity effect. This allowed the accurate evaluation of the scattering intensities in terms of fundamental impact parameters. All the materials investigated showed marked discrepancies from the corresponding IAM theoretical results, even for intervals of momentum transfer in which this model is usually claimed to be valid. However, this work shows that the errors introduced by corrections are relatively low, making it possible to validate the use of simple diffractometry to evaluate the scattering factors in real samples without having to resort to sophisticated irradiation facilities. © 1997 John Wiley & Sons, Ltd.