Multilayer reflectivity calculations applied to the Energy Dispersive Method

The specular reflectivity of periodic multilayers illuminated by a white parallel X-ray beam is calculated versus photon energies over a range of 4 keV to 40 keV for fixed grazing incidence angles. The influence of the anomalous dispersion and absorption terms in the atomic scattering factors, respectively Delta f' and Delta f '' on the reflectivity profiles is discussed, Calculations show that the use of an X-ray tube as source and a Si(Li) diode as detector for peak reflectivity, band pass and overlapping orders measurements is justified. By choosing the incidence angle, the Bragg peaks can be set at any energy, in particular close to an absorption edge in order to observe the step reflectivity change due to abrupt index variations. Spiller's rules are invoked to explain these variations.