Guinier plot

A diagrammatic representation of data on scattering from large particles, obtained at different angles but at the same concentration, constructed by plotting $log 10 Δ R θ$ or $log 10 P θ$ versus $( sin ⁡ θ 2 ) 2$ or $q 2$ and (usually) used for the evaluation of the radius of gyration. $Δ R θ$ is the excess Rayleigh ratio, $P θ$ the particle scattering function, $θ$ the scattering angle and $q$ the length of the scattering vector.

Source:

Purple Book, p. 67