= Small-angle neutron scattering
<Small-angle neutron scattering> probes structure using a <neutron> beam deflected through small <scattering angles>. For elastic scattering at <wavelength> $\lambda$, if $\theta$ is the full angle between incident and scattered beams, the transferred <wave vector> has magnitude
$$
q=\frac{4\pi}{\lambda}\sin\frac\theta2.
$$
The corresponding structural length scale is roughly $2\pi/q$, so a small angle accesses larger structures than an atomic-scale diffraction angle at the same <wavelength>.<Small Angle Neutron Scattering by National Institute of Standards and Technology>[Overview, relation between scattering angle, wavelength and probed size]{ref}
A collimated <neutron> beam with selected <wavelength> passes through the sample; a distant position-sensitive detector records the angular distribution. Comparing different sample-to-detector distances accesses different $q$ ranges. The measured intensity depends on <neutron scattering length density> contrast, shape and spatial correlations, rather than giving a direct photograph. Subtracting background and specifying the measured $q$ range are essential to interpreting a fitted size or correlation length.<Small Angle Neutron Scattering by National Institute of Standards and Technology>[SANS measurement principle and instrument geometry]{ref}
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