A neutron interferometer built of holographic gratings

Research project P 14614 A neutron interferometer built of holographic gratings Romano A. RUPP 26.6.2000 The aim of the present proposal is to construct a triple Laue interferometer for cold neutrons. The principle is based on mounting three thick slabs of photosensitised and deuterated poly(methylmethacrylate)=d-PMNIA in a rigid steel construction. Those slabs are illuminated with UV interference fringes resulting in holographic gratings Which exhibit a modulation of the neutron scattering length density. The gratings act as efficient neutron beam splitters or mirrors. The planned interferometer will have a length of about 30 cm. We will construct an efficient, compact and versatile interferometer having a high visibility by improving the quality of the gratings and the overall length of the interferometer with respect to an already existing prototype. The ultimate goal is to create a standard tool for neutron optics, which can easily be run at any beamline providing cold neutrons of sufficient coherence.

Scattering experiments with various types of quantum objects, e.g., light, X-rays, electrons, or neutrons, proved to be extremely useful for investigating and resolving structures. However, such experiments provide only the scattered intensity instead of the scattering amplitude, thus lacking the complete phase information. This problem can be overcome by employing interferometric techniques. In this project we designed and set up the first interferometer for cold neutrons, that is capable to reveal the required phase information. This interferometer is composed of three equally spaced volume gratings, that act as beamsplitters or mirrors for neutrons. These gratings were produced by illuminating a photosensitive sample with light fringes of a holographic recording setup. In such materials (plexiglass) the light pattern is transformed into a refractive index profile, i.e., a grating, for neutrons. The interferometer then was installed at various cold neutron beamlines. With the help of this device the coherence properties of the neutrons were studied. Utilizing the partial coherence of the neutrons, we developed a new method to determine intensity and phase in a small angle diffraction experiment: The measured interference fringes reveal the complex degree of coherence, from which the phase of a scattering object can be extracted. Important tentative applications are the phase determination in small angle neutron scattering experiments, e.g., to resolve the structure of complex, large-scale biological samples, as well as the measurement of the coherence properties of the neutrons at various neutron scattering facilities.