Neutron Reflectometry

Neutron Reflectometry is a laboratory technique for measuring the structure of thin films, similar to the often complementary techniques of X-ray reflectivity and ellipsometry. The technique provides valuable information over a wide variety of scientific and technological applications including chemical aggregation, polymer and surfactant adsorption, structure of thin film magnetic systems, biological membranes, etc.

Technical details

The technique involves shining a highly collimated beam of neutrons 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 a specular reflection technique, where the angle of the incident beam is equal to the angle of the reflected beam. The reflection is usually described in terms of a momentum transfer vector, denoted $q\_z$, which describes the change in momentum of a neutron after reflecting from the material. Conventionally the $z$ direction is defined to be the film normal direction, and for specular reflection, the scattering vector has only a $z$-component. A typical neutron reflectometry plot displays the reflected intensity (relative to the incident beam) as a function of the scattering vector:

$q\_z\; =\; frac\{4pi\}\{lambda\}sin\; (\; heta\; )$

where $lambda$ is the neutron wavelength, and $heta$ is the angle of incidence. The Abeles matrix formalism or the Parratt recursion can be used to describe the specular signal arising from the interface.

The wavelength of the neutrons used for reflectivity are typically on the order of 0.2 to 1 nm (2 to 10 Å). This technique requires a neutron source, which may be either a research reactor or a spallation source (based on a particle accelerator). Like all neutron scattering techniques, neutron reflectometry is sensitive to contrast arising from different nuclei (as compared to electron density, which is measured in x-ray scattering). This allows the technique to differentiate between various isotopes of elements. Neutron reflectometry measures the neutron "scattering length density" (SLD) and can be used to accurately calculate material density 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 (hydrogen, carbon, nitrogen, oxygen, etc.). Sensitivity to isotopes also allows contrast to be greatly (and selectively) enhanced for some systems of interest using isotopic substitution, and multiple experiments that differ only by isotopic substitution can be used to resolve the phase problem that is general to scattering techniques. Finally, neutrons are highly penetrating and typically non-perturbing: which allows for great flexibility in sample environments, and the use of delicate sample materials (e.g., biological specimens). By contrast x-ray exposure may damage some materials, and laser light can modify some materials (e.g. photoresists). Also, optical techniques may include ambiguity due to optical anisotropy (birefringence), which complementary neutron measurements can resolve.

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 $q\_z$ that can be probed (and hence the measurement resolution).

Partial list of neutron reflectometers

* [http://lansce.lanl.gov/lujan/instruments/SPEAR/index.html SPEAR] at [http://lansce.lanl.gov/ LANSCE] in Los Alamos National Laboratory

* [http://www.ansto.gov.au/bragg/facilities/instruments/platypus.html Platypus] at ANSTO in Sydney, Australia

* [http://neutron.nrc-cnrc.gc.ca/c5gen_e.html C5 spectrometer] at NRC Canada Chalk River Labs in Chalk River, Canada. "Note: a new dedicated reflectometer (D3) was commissioned in 2006."

* [http://neutron.nrc-cnrc.gc.ca/d3gen_e.html D3 reflectometer] at NRC Canada Chalk River Labs in Chalk River, Canada.

* [http://www.ill.fr/d17/ D17] , [http://www.ill.eu/adam/home-adam/ ADAM] and [http://www.ill.eu/figaro/home-figaro/ Figaro] at the Institut Laue-Langevin ( [http://www.ill.eu/ ILL] ) in Grenoble, France
* [http://www-llb.cea.fr/spectros/pdf/eros-llb.pdf EROS] and [http://www-llb.cea.fr/spectros/pdf/prism-llb.pdf PRISM] ( [http://www-llb.cea.fr/prism/PRISM.html alternate] ) at CEA Laboratoire Léon Brillouin ( [http://www-llb.cea.fr/index_e.html LLB] ) in Saclay, France

* [http://www.gkss.de/central_departments/genf/instruments/003142/index_0003142.html.en NeRo] and [http://www.gkss.de/central_departments/genf/instruments/003138/index_0003138.html.de PNR] at the GKSS Research Centre ( [http://www.gkss.de GKSS] ) in Geesthacht, Germany
* [http://www.hmi.de/bensc/instrumentation/instrumente/v6/v6_en.htm V6 reflectometer] at Hahn-Meitner-Institut ( [http://www.hmi.de/index_en.html HMI] ) in Berlin, Germany
* [http://www.fz-juelich.de/iff/wns_hadas/ HADAS] at Jülich Research Centre in Jülich, Germany

* PNR at the Dhruva reactor, Bhabha Atomic Research Centre in Mumbai, India

* [http://flnp.jinr.ru/140 REFLEX] and [http://flnp.jinr.ru/139/ REMUR] at Joint Institute for Nuclear Research IBR-2 in Dubna, Russia

* [http://kur.web.psi.ch/amor/ AMOR] at the Paul Scherrer Institute ( [http://www.psi.ch PSI] ) in Villigen, Switzerland

* [http://www.isis.rl.ac.uk/largescale/surf/surf.htm SURF] and [http://www.isis.rl.ac.uk/largescale/surf/crisp.htm CRISP] at ISIS neutron source ( [http://www.isis.rl.ac.uk/ ISIS] ) in Oxfordshire, United Kingdom

* [http://www.ncnr.nist.gov/instruments/ng1refl/ NG1] , [http://www.ncnr.nist.gov/instruments/ng7refl/ NG7] and [http://www.ncnr.nist.gov/programs/reflect/ANDR/ AND/R] at the NIST Center for Neutron Research ( [http://www.ncnr.nist.gov NCNR] ) in Gaithersburg, Maryland, United States
* [http://neutrons.ornl.gov/instrument_systems/beamline_04b_lr/index.shtml Liquids] and [http://neutrons.ornl.gov/instrument_systems/beamline_04a_mr/index.shtml Magnetic] at the Spallation Neutron Source ( [http://neutrons.ornl.gov/ ORNL] ) in Oak Ridge, Tennessee, United States

Partial list of neutron reflectometry software

* [http://www.ncnr.nist.gov/programs/reflect/data_reduction/software/reflred.html Reflred] and [http://www.ncnr.nist.gov/programs/reflect/data_reduction/software/reflfit.html reflfit] ( [http://www.ncnr.nist.gov/programs/reflect/data_reduction/software/index.html NIST Center for Neutron Research] )
* [http://motofit.sourceforge.net Motofit]
* [http://www.hmi.de/bensc/instrumentation/instrumente/v6/refl/parratt_en.htm Parratt32] (HMI)
* [http://www.physics.brocku.ca/~tharroun/yanera Yanera] (Brock University)
* [http://stochfit.sourceforge.net StochFit]

ee also

* Neutron scattering