Since the neutron wavelength is significantly larger than the nucleus dimension, in the case of SANS the neutron scattering amplitude results to be isotropic, i.e., independent of the scattering angle. Therefore, equation ( ) can be approximated to 𝑓 ! atomic (𝑞) = 𝑏 ! . The 𝑏 ! constants, which are available in literature , depend on the number of neutrons and protons that constitute the nucleus. Consequently, isotopes of the same element with different number of neutrons, such as hydrogen and deuterium, have different neutron scattering lengths. This feature provides the basis of the contrast variation technique a powerful advantage of neutron scattering over X-rays, which is usually achieved by using mixtures of hydrogenated and deuterated water in varying proportions. To account for this combination in solvent composition, equation ( ) is modified to: