Small-angle neutron scattering

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Sophia Magkiriadou, AP225, Fall 2011

Small-angle neutron scattering is a technique for probing the internal structure of a material by measuring how a beam of neutrons scatters from it when incident at small angles with respect to the plane normal to the beam.

There are several scattering techniques which share the same operating principle: X-ray scattering, neutron scattering, and even optical scattering all come down to illuminating a sample with a beam of particles/waves and recording the resulting scattering pattern. Which one is most appropriate depends on the required resolution and the specific type of information required.

Treating all particles (and photons) as waves, the resolution of these techniques depends linearly on the wavelength, which, in turn, depends, for massive particles, on their momentum, according to de Broglie:

<math>\lambda = \frac{h}{p} = \frac{h}{{m}{v}}</math>

where <math>\lambda</math> is the wavelength, <math>h</math> is the Planck constant, <math>m</math> is the particle's mass and <math>v</math> its velocity.

By Bragg's law, the distance d resolvable by radiation of wavelength <math>\lambda</math> incident on a material with refractive index <math>n</math> at an angle <math>\theta</math> with respect to the plane perpendicular to the beam is

<math>d = \frac{{n}{\lambda}}{2\sin\theta}\!</math>.

Typical values for the wavelength of neutrons in a beam are between 1-1000 angstrom; by changing the angle and the energy of the neutron beam, the resolution of this technique can be tuned in the 1-1000nm regime. Small angle scattering is used when larger lengthscales are of interest, for example to study polymer molecules or biological macromolecules.

Neutrons are not electrically charged and so they interact very weakly with matter. This has the advantage that neutron scattering is elastic, which means that the resulting scattering pattern is not convoluted with secondary processes and can be used to extract purely structural information (see structure factor). Moreover, they can penetrate deeply into a material - sometimes as much as several centimeters - and so they can be used to obtain information about its bulk. The scattering cross-section of atoms to neutron radiation is somewhat constant across the periodic table, so even when a complex material comprised of atoms with varying sizes is probed, all of its constituents have more or less the same chance of being identified. Particularly sensitive to neutrons are hydrogen atoms, so SANS is frequently used for the study of materials made of organic molecules, including biological molecules. At the same time, the weakness of the neutron-matter interaction means that high intensity beams are required, which is a technical challenge.

Neutrons mostly interact with the nucleus, but they also have a small magnetic moment which sometimes induces them to interact with electrons at large orbits. This property is sometimes used to study the electronic structure.

Finally, an interesting aspect of neutrons is that the energies at which they are available is of the same order as phonons in atomic crystals, molecular vibrational modes, and diffusive processes (~1μeV - eV). This makes it possible to study these processes in experiments involving energy exchange between neutrons and the sample.

Neutrons can be obtained from nuclear reactors, where they are a byproduct of fission of Uranium-235, or from particle accelerators, as the byproduct of collisions between protons and atoms with heavy nuclei. There are only a few such facilities in the world; the highest-flux neutron beam is maintained at the Institut Laue-Langevin in Grenoble, France, while other SANS facilities are located in Oak Ringe National Lab and Brookhaven National Lab.

Keyword in references:

Photonic Properties of Strongly Correlated Colloidal Liquids


International Atomic energy Agency, Small angle neutron scattering: Report of a coordinated research project 2000–2003, March 2006