Neutron reflectometry

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Science with neutrons
Foundations
Neutron temperature Flux, Radiation, Transport Absorption, Activation
Neutron scattering
Neutron diffraction Small-angle neutron scattering GISANS Reflectometry Inelastic neutron scattering Triple-axis spectrometer Time-of-flight spectrometer Backscattering spectrometer Spin-echo spectrometer
Other applications
Neutron tomography Activation analysis, Prompt gamma activation analysis Fundamental research with neutrons: Ultracold neutrons, Interferometry Fast neutron therapy Neutron capture therapy
Infrastructure
Neutron sources: Research reactor, Spallation, Neutron moderator Neutron optics: Supermirror Detection
Neutron facilities
America: HFIR, LANSCE, NIST CNR -SNS Australia: OPAL Asia: J-PARC, HANARO Europe: BER II, FRM II, ILL, ISIS Neutron and Muon Source, JINR, SINQ Historic: IPNS, HFBR Under construction: ESS
v t e

Neutron reflectometry is a

, structure of thin film magnetic systems, biological membranes, etc.

History

Neutron reflectometery emerged as a new field in the 1980s, after the discovery of giant magnetoresistance in antiferromagnetically-coupled multilayered films.^[1]

Technique

The technique involves shining a highly

onto an extremely flat surface and measuring the intensity of reflected radiation as a function of angle or neutron wavelength. The exact shape of the reflectivity profile provides detailed information about the structure of the surface, including the thickness, density, and roughness of any thin films layered on the substrate.

Neutron reflectometry is most often made in

, denoted ${\displaystyle q_{z}}$, which describes the change in momentum of a neutron after reflecting from the material. Conventionally the ${\displaystyle z}$ direction is defined to be the direction normal to the surface, and for specular reflection, the scattering vector has only a ${\displaystyle z}$-component. A typical neutron reflectometry plot displays the reflected intensity (relative to the incident beam) as a function of the scattering vector:

${\displaystyle q_{z}={\frac {4\pi }{\lambda }}\sin(\theta )}$

where ${\displaystyle \lambda }$ is the neutron wavelength, and ${\displaystyle \theta }$ is the angle of incidence. The

or the Parratt recursion can be used to calculate the specular signal arising from the interface.

Off-specular reflectometry gives rise to diffuse scattering and involves momentum transfer within the layer, and is used to determine lateral correlations within the layers, such as those arising from magnetic domains or in-plane correlated roughness.

The wavelength of the neutrons used for reflectivity are typically on the order of 0.2 to 1

if the atomic composition is known.

Comparison to other reflectometry techniques

Although other reflectivity techniques (in particular optical reflectivity, x-ray reflectometry) operate using the same general principles, neutron measurements are advantageous in a few significant ways. Most notably, since the technique probes nuclear contrast, rather than electron density, it is more sensitive for measuring some elements, especially lighter elements (

) for a uniform layer density.

Disadvantages of neutron reflectometry include the higher cost of the required infrastructure, the fact that some materials may become

radioactive

upon exposure to the beam, and insensitivity to the chemical state of constituent atoms. Moreover, the relatively lower flux and higher background of the technique (when compared to x-ray reflectivity) limit the maximum value of ${\displaystyle q_{z}}$ that can be probed (and hence the measurement resolution).

References

External links

• Catalogue of neutron reflectometers at www.reflectometry.net
• Catalogue of data analysis software at www.reflectometry.net