Neutron and X-ray reflectivity

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Introduction

The techniques of neutron and X-ray reflectivity are powerful, non-destructive, methods for probing the structures of thin films. The wave-like character of X-rays and neutrons means that they may be reflected or refracted in much the same way as light (in fact X-ray photons differ from optical photons only in having a very much shorter wavelengths.) The wavelengths of the particles used in the methods allow sub-nanometre resolution to be achieved. The following discuusion will concentrate mainly on neutrons, but the under lying principles are the same for X-ray reflectivity.

At the interface between two media with different neutron refractive indices a beam of neutrons may be totally reflected when the angle of incidence is less than some critical angle. In the region of the critical angle the reflectivity falls off rapidly with increasing angle. The way this reflectivity falls off with angle provides important information about the nature of the interface, particularly its roughness. This is illustrated in figure 1.

Example of reflectivity

Figure 1. Simulation of neutron reflectivity from the interface between air and a polished silicon wafer.
The red dots correspond to the reflectivity from a perfectly smooth interface. The blue dots are from
an interface with a 10 Angstrom roughness.

Data analysis

Brief notes about the Pro Fit plugins for neutron analysis can be found in the Data_analysis pages