Small-angle neutron scattering

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Entry needed - IN PROGRESS by Sofia.


Small-angle neutron scattering is a technique for probing the internal structure of a material by measuring how neutrons scatter from it. Since neutrons are weakly interacting, when a material is subjected to a beam of them all scattering is elastic. In addition, the weak interaction of neutrons with matter allows them to penetrate deep into the substance under study (sometimes even centimeters), thus providing information about the material bulk.

There are several scattering techniques which share the same operating principle for structural studies: 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 of diffraction, 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\lambda2d}{2\sin\theta}\!</math>.

Typical values for the wavelength of neutrons in a beam are ~ angstrom; by changing the angle, the resolution of this technique can be varied in the 1-1000nm regime.

Neutrons are not electrically charged and this 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. Moreover, their neutrality allows neutrons to penetrate deeply into a material - sometimes as much as several centimeters - which means that this technique can provide information about the bulk of a material. At the same time, the weakness of the neutron-matter interaction means that high intensity beams are required, which is a practical 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, thus probing part of the electronic structure.


neutrons come from 1. nuclear reactor, fission of Uranium-235; particle accelerators, protons directed at materials with heavy nuclei. Facilities: Grenoble (most steady-state high flux) ILL,

Sources: intro to SANS web pdf. website that linked here

image showing neutrons scattering


Keyword in references:

Photonic Properties of Strongly Correlated Colloidal Liquids