Optical properties 63
the fibre-optic tip is sampled, the technique can be used for
concentrated dispersions.Small angle neutron scatteringNeutrons from reactors or accelerators are slowed down in a
moderator to kinetic energies corresponding to room temperature
(thermal neutrons) or less (cold neutrons), for which the de Broglie
wavelengths (A = hlmv) are of the order of 10~^10 to 10~^9 m,
Therefore, a/\ *> 1. The theory of light scattering by large particles,
discussed on pages 59-61, applies equally to neutron scattering,
except that the dependence on the relative values of the refractive
indices of particles and dispersion medium is replaced by a
corresponding dependence on neutron scattering length densities. In
contrast to conventional light scattering, neutron scattering is very
weak and, consequently, it can be exploited to study concentrated
dispersions.
Neutron scattering is also useful for the study of adsorbed material
on hydrosol particles. The neutron scattering length densities of
hydrogen and deuterium differ considerably. By preparing a hydrosol
in an appropriate H 2 O/D 2 O mixture, it is possible to match the
neutron scattering length densities of the dispersion medium and the
core particles. The neutron beam thus 'sees' only the adsorbed layer,
the thickness of which can be estimated. Alternatively, the dispersion
medium can be matched to the adsorbed layer to permit estimation of
the core-particle size.