Neutron interferometry is based on the wave nature of neutrons. If neutrons are isolated from the atomic nucleus, for example in the fission process of a research reactor, they behave like waves. This is usually refereed the as wave-particle duality. Inside the interferometer partial wave-function of the sub-beams in the individual interferometer paths are created and recombined coherently afterwards, resulting in interference effects. The first fully operating neutron interferometer was produced and demonstrated successfully in 1974 by Prof. Rauch, Treimer and Bonse at the rather small (250 kW) TRIGA reactor at the Atominstitut Vienna, Austria – a sensational find, which years later continued to bring Prof. Rauch and the Atominstitute world fame in the entire scientific community.
Compared to the geometrical theory, the dynamical theory of diffraction takes the spatial periodicity of the interaction potential in perfect single crystals into account. Thereby the Schrödinger equation has to be solved within this potential. When neutrons illuminate a perfect crystal under near-Bragg orientation conditions, the dynamical theory of diffraction predicts a coherent splitting of the incident wave into four components, with two traveling wave components passing within the crystal in the Bragg direction and two components in the forward (incident) direction. This splitting results in a periodic beating of radiation density traveling in either the Bragg or forward direction at different depths in the crystal, this feature being described as a Pendellösung structure. Using this theoretical framework all wavefunctions occurring inside the interferometer can be calculated.