PhD (PDF) - Universität Wien

Abstract
Scattering and diffraction experiments proved to be extremely useful for materials investigation
and have developed to standard techniques in condensed matter sciences. Depending on the
desired information and the size of the scattering or diffraction objects, quanta are chosen
having an appropriate interaction with the object as well as appropriate momentum. The most
common ones being light, x-rays, electrons and neutrons.
As the characteristics of the scattered or diffracted radiation reflects the correlation between
the quantum field and the structure of the scatterer it is of utmost importance for the correct
interpretation of diffraction and scattering experiments to determine the coherence properties of
the scattering radiation [1]. The coherence of the beam is characterised by its coherence volume,
a quantity which can be measured by interferometry. For thermal neutrons this task was perfor-
med using a perfect silicon-crystal interferometer [2, 3]. In material sciences cold neutrons in a
wavelength range between 0.6 nm and 1.3 nm are frequently employed for investigations of large-
scale structures, e.g. for structure analysis of biological materials (viruses, proteins, enzymes),
in metallurgy (alloys, magnetic and superconducting materials) or for polymer research. The
continuous development of cold neutron sources and the increasing number of applications at
small-angle scattering facilities for cold neutrons illustrate the demand for a device to determine
the coherence properties of a cold neutron beam.
Here we report on the setup and adjustment of a new Mach-Zehnder type interferometer for cold
neutrons based on holographically generated gratings in triple Laue geometry. We employed
the photo-neutronrefractive effect of DMDPE - doped deuterated poly(methylmethacrylate)
to perform the first direct measurement of the coherence function for cold neutrons. This
task is done by continuously increasing the phase difference between two beam paths of the
interferometer through rotation of a phase-flag. The decay of interference fringes is monitored
and thus the coherence function determined experimentally.
Evaluation of the interference pattern allows to determine the coherence length of the neutron
beam, the coherent scattering length density of the phase-flag and the absolute phase of the
third interferometer grating. The experimental determination of the intensity and the phase
of the scattering radiation permits a direct Fourier transform from reciprocal to real space and
thus the solution of the ”phase problem” in neutron scattering.
I