W0249

Near Surface Small Angle Neutron Scattering: Techniques and Applications. Paul Butler¹, William Hamilton², ¹NIST Center for Neutron Research, Gaithersburg, MD, ²Oak Ridge National Laboratory, Oak Ridge, TN

Grazing incidence X-ray scattering has been used successfully now for several years. In this technique the X-ray beam is brought into the sample below the critical angle for total reflection. There is nonetheless an evanescent wave which penetrates the first several nanometers of the system under investigation, producing a scattering pattern from the very near surface structures. Comparatively few neutron evanescent wave diffraction measurements (and to our knowledge no small angle measurements) have been reported due to the relatively much lower intensities involved.

However, by going slightly above the critical angle, which is on the order of 0.23 degrees for a quartz/D2O interface and 5A neutrons, one can probe the structures in the “near” surface region as well. In this case, the depth which is “seen” in the scattering is governed NOT by the depth of the evanescent wave, but by the absorption cross-section of the system, which limits the total path length that can be traversed by the neutron beam. Generally this “near” surface region is actually much larger than that viewed by an evanescent wave. While the geometry of such a setup IS a grazing incidence one, the term grazing incidence scattering has been associated mostly with evanescent wave scattering. In order to distinguish this technique from evanescent wave scattering we usually refer to it as “near-surface SANS” or, sometimes, “reflection geometry SANS”.

An overview of the technique will be presented here. This will be followed by a discussion of results from several surfactant systems, from rods to sponges. As will be made clear from these examples, the technique is particularly useful in conjunction with bulk SANS and/or reflectometry. Finally a perspective on future directions will be given.