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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 magnitudeThe 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.^[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.^[ref]