Neutron and X-ray reflectometry as complementary probes Roger Pynn, Shenda M. Baker*, M. R. Fitzsimmons, Jaroslaw Majewski, and G. S. Smith Los Alamos National Laboratory, Los Alamos, NM 87545 *Harvey Mudd College, Dept. of Chemistry, 301 E 12th St., Claremont CA 91711 It is tempting to conclude that the very high brightness of modern synchrotron x-ray sources has made neutron reflectometry redundant. Such a conclusion is, however, premature for a number of reasons, each of which can be traced to the fundamental nature of the interaction between neutrons and matter. Neutrons are scattered by atomic nuclei, so neutron reflectometry has a different sensitivity to atomic species than x-rays. Corefinement of x-ray and neutron reflection data can thus be useful in fitting model density profiles that are less likely to represent spurious minima of the misfit statistic. Isotopic substitution can be used to allow neutrons to distinguish atoms that are chemically identical. This technique has been used, for example, to probe the interdiffusion of polymer layers when one of the layers has its hydrogen atoms replaced by deuterium. The same type of isotopic substitution has been used to examine multibilayers of phospholipids as well as lipid and polymer layers at an air-liquid or a liquid-solid interface. Even simpler is an experiment to investigate the self diffusion of boron atoms which used neutron reflection to examine the evolution of the interface between isotopically distinct boron layers. The result in this case did not support the usual Fickian theory but demonstrated the presence of a barrier potential at the interface between the layers. Because neutrons interact weakly with matter, they can easily penetrate several centimeters of solid material. This has allowed the construction of a shear cell which permits neutrons to probe the density profile of complex fluids flowing close to a solid surface. In one such experiment, the alignment of thread-like micelles and their stacking in an hexagonal array was demonstrated. Finally, the fact that neutrons interact as strongly with unpaired electron moments as they do with nuclei allows neutron reflection to be used as a sensitive probe of magnetism in layered structures. In one such experiment, which required both neutrons, x-rays, and electron microscopy for its full interpretation, the magnetism of a thin iron layer deposited epitaxially on magnesium oxide was studied to reveal both the magnetic and atomic densities close to the two surfaces of the iron layer.