The scattering of slow neutrons by diatomic molecular fluids

Abstract

In this paper we develop useful expansions for the differential cross section in terms of the frequency moments of Van Hove's scattering function, i.e. the Placzek [1] and Wick [3] series. Using only the density independent terms of these series we compare their predictions with those obtained from the models described in paper I. It is concluded that for nitrogen, deuterium chloride and hydrogen chloride the single molecule interference term is given to high accuracy by our Placzek series, so that no density dependent terms are required. For the self terms of nitrogen, and chloride in the HCl and DCl molecules Placzek's series is satisfactory over the range of angles used in most experiments although Wick's series may be needed for angles near 180°. For the self term of D in DCl Wick's series is good over a moderate range of angles, but a realistic model for the rotational behaviour is needed over a wider range and also for all cases involving H in HCl.

The results presented here may be used for the interpretation of a wide range of neutron diffraction experiments, and are shown to be accurate for many cases to ∼1 per cent.