Experiments with thermal neutrons are most useful in the study of lattice vibrations. The advantage stems from the basic nature of the neutron: The De Broglie wavelength of a thermal neutron is of the same order of magnitude as the interatomic distances in crystals, and the energy of a thermal neutron is of the same order as the energy of a lattice excitation. Table 6.1 gives some characteristical values and physical constants of thermal neutrons. In a scattering process, the neutron matches the lattice vibrations with respect to energy and wave vector (Fig. 1.1), and there exist pronounced interference phenomena. Coherent neutron scattering allows direct measurement of the dispersion relation, i.e., of the frequencies for distinctive polarizations and wave vectors in the Brillouin-zone, and hence gives the best proof for the existence of phonons.