Figure 1. Imaginary component of the atomic form factor for hydrogen, following a variety of models. Henke et al. [32,33] covers a more restricted energy range, and the Sauter formula (e.g., ref. [39]) only becomes a useful approximation at energies above 40 keV. With these two exceptions, all approaches appear very similar across several decades of energy and form factor. The Chantler result is accurate to within approximately 2 % up to 300 keV for an isolated hydrogen atom.