PHOTON SCATTERING BY COMPOUNDS - AN ASSESSMENT OF THE VALIDITY OF THEINDEPENDENT ATOMIC MODEL APPROXIMATION

The validity of the independent atomic model (IAM) for x-ray scatterin g, which assumes that each atom in the sample scatters independently o f the others, was examined for materials and irradiation geometries wh ere interatomic and intermolecular cooperative effects are expected to provide a non-negligible contribution, Samples of mater, fat, polymer s and powder were investigated by means of a simple powder diffractome ter in the interval of momentum transfer 0.017 less than or equal to c hi less than or equal to 0.67 {chi = E[sin(theta/2)]/12.4, E being the incident photon energy and theta the scattering angle}, Accurate eval uation 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 i n terms of fundamental impact parameters. All the materials investigat ed showed marked discrepancies from the corresponding IAM theoretical results, even for intervals of momentum transfer in which this model i s usually claimed to be valid, However, this,work shows that the error s introduced by corrections are relatively low, making it possible to validate the use of simple diffractometry to evaluate the scattering f actors in real samples without having to resort to sophisticated irrad iation facilities. (C) 1997 by John Wiley & Sons, Ltd.