How it works
Neutron reflectometry examines the way in which surfaces or interfaces reflect neutrons in order to provide information on their organization, thickness, composition and roughness. Surface stacking can be studied on a depth of 1 to 300 nm (5 to 1500 atomic layers).
If the incident angle is lower than the critical angle, the material acts like a mirror and reflects all neutrons. If it is wider, only a part of neutrons are reflected. Measuring the proportion of reflected neutrons for a reflection angle equal to the incident angle makes it possible to explore the sample in a direction perpendicular to its surface. If measurement is made for different incident and reflecting angles, the sample is explored in its surface plane.
Reflection on a plane surface (the interface between two materials)
To make the illustration clearer, angles are greatly exagerated from their actual values for cold neutrons.
Neutron reflectometry examines the way in which surfaces or interfaces reflect neutrons in order to provide information on their organization, thickness, composition and roughness. Surface stacking can be studied on a depth of 1 to 300 nm (5 to 1500 atomic layers).
If the incident angle is lower than the critical angle, the material acts like a mirror and reflects all neutrons. If it is wider, only a part of neutrons are reflected. Measuring the proportion of reflected neutrons for a reflection angle equal to the incident angle makes it possible to explore the sample in a direction perpendicular to its surface. If measurement is made for different incident and reflecting angles, the sample is explored in its surface plane.
Determining layer thickness
Neutrons are no longer reflected from a single surface but from two semi-transparent surfaces. The reflectivity curve still has a total reflection plateau but the descending portion shows the presence of oscillations resulting from interference between beams reflected by the two surfaces. The size of these oscillations is related to the layer thickness.
The actual situation is generally much more complex (multi-layers, rough or diffuse surfaces, etc.) and interpretation of observations is realised by model-based computer simulations.