Figure 3. Coherent and incoherent neutron scattering.

Coherent and incoherent neutron cross-sections of the ‘biological’ elements (σ_c, displayed as circles) and their respective neutron scattering lengths (b_c, 10^-12 cm; where σ_c = 4πb_c²). ¹H has a negative coherent scattering length (represented as a black circle) compared to deuterium and the other commonly occurring biological isotopes. Coherent scattering arising from correlated distances within a particle’s volume produce a scattering profile from which structural information can be extracted. Conversely, incoherent neutron scattering cannot be used to extract shape/structural information and contributes to a SANS profile as ‘noise’ across all angles. ¹H has a considerable incoherent scattering length, the effect of which is demonstrated by the SANS scattering from lysozyme in 100% v/v ¹H₂O (left) which is considerably noisier than the same sample collected in 100% v/v ²H₂O (right.) SANS data were collected on the Quokka-SANS instrument at ANSTO⁹⁵ using the same neutron wavelength, exposure times, detector distances, instrument geometry, sample path length and protein concentration. Neutron scattering lengths are taken from Sears, 1992⁵⁰.