Introduction

The theory of the scattering of thermal neutrons by liquids is complicated, mainly because the liquid state itself is complicated. For a crystalline solid we have a relatively simple model, namely a perfect crystal with harmonic forces, which serves as a zero-order approximation for more refined calculations. For a gas we have the perfect gas model – point particles in uncorrelated motion. For a liquid neither of these extreme situations applies.

Coherent neutron scattering gives information about the relative positions and motions of different particles in the liquid. From this scattering we may determine what is known as the structure of the liquid, which is, in effect, the static pair-correlation function g(r). Measurements of the coherent scattering at low values of momentum transfer also show effects due to excitations of cooperative modes in the liquid. Incoherent scattering depends on the motion of a single particle and is therefore easier to interpret.

In the present chapter we give some of the basic results of theory and experiment. We restrict the discussion to classical monatomic liquids, and moreover confine the treatment of coherent scattering to that part that gives information on the equilibrium, as opposed to the dynamic, properties of liquids. Readers who wish to pursue some of the topics omitted here will find an excellent series of articles in Reports on Progress in Physics – by Allen and Higgins (1973) on neutron studies of molecular motion, by Woods and Cowley (1973) on liquid helium, and by Copley and Lovesey (1975) on the dynamic properties of monatomic liquids.