Abstract
In a variety of biomedical applications it has been demonstrated that elastic photon-atom scattering can be important. These include: solving for the crystal structure of a macromolecule, imaging, and radiation dose calculations. To the extent that scattering is significant, it is important to remember that there are a number of effects which go beyond the form-factor approximation. In this paper, we discuss what is presently known about the validity of form-factor approximations for predictions of elastic photon scattering by atoms. We quantify the uncertainty in form-factor theories for sample atoms of biomedical interest, including: carbon, oxygen, aluminum, calcium, iron, iodine and lead. The importance of effects that go beyond form-factor approximation is illustrated by demonstrating that errors can be introduced in using a simple form-factor-based scaling technique using scattering factors measured at one energy to predict scattering at another energy. These anomalous dispersion have been used extensively in recent years in studying structures of biological macromolecules, either in combination with or as an alternative to isomorphous replacement techniques. We present accurate, high precision anomalous scattering factors for a range of elements commonly used in macro-molecular structure studies, tabulated on a fixed grid interval in the energy range 1- to 100 keV.
