Slow-neutron scattering by molecules

The experimental scattering cross-section for homonuclear diatomic molecular perfect gases has been explicitly calculated using virtually exact expressions for the scattering law. These recoil and detector corrections are illustrated for a series of molecules at various temperatures on various instruments. It is shown that the conventional Placzek corrections are quite inadequate for vibrating molecules and a simple recipe is proposed which leads to readily calculable expressions which give good agreement with the 'exact' results even for nuclei as light as deuterium. This recipe can be generalized for use for almost any nucleus in any molecule and may be expected materially to improve the accuracy of structure determination in molecular gases and liquids. In particular it is found that the Debye-Waller coefficient is very dependent on the experimental parameters. The generalization to time-of-flight experiments is given in the following paper. (author)