Geometrical aspects of QENS experiments in case of anisotropic samples

Diffusion in anisotropic samples is often studied by quasi-elastic neutron scattering at time-of-flight (t-o-f) instruments. During the data analysis the measured dynamic structure factor S(Q,E) is fitted by a model function, which - reflecting the anisotropy of the sample - depends on the sample orientation. The error introduced in this way will be discussed. Correct measurements can be performed for such cases at a triple axis spectrometer, where energy scan can be performed for a certain Q vector fixed to the sample orientation. Clays show 2D anisotropy in the structure, which is supposed to be visible also in the diffusion of water located between the clay sheets. A first experiment performed on TASP (SINQ) will be shown.