Title

Development and Applications of the Nuclear Lighthouse Effect at high energies and at grazing incidence

Universität Rostock, European Synchrotron Radiation Facility

Supervised by: Prof. E. Burkel, Dr. R. Rüffer and Dr. R. Röhlsberger

Abstract

The nuclear lighthouse effect (NLE) is a new technique to examine Mössbauer isotopes with synchrotron radiation (SR). It is closely related to nuclear forward scattering (NFS), which is the time based analogon to traditional, energy based Mössbauer spectroscopy. However, in rotating rapidly the sample, which is basis of the NLE, the time response of a NFS experiment is mapped to an angular scale. Interacting photons are deviated according to their interaction time within the rotating sample. In principle, it is an improvement of nuclear forward scattering (NFS), as it permits to apply the rules of NFS, but does not need a certain time structure of the synchrotron radiation. In this thesis, an experimental setup for NLE measurements was established to investigate two different types of systems. The first system are model 57Fe-layers in a grazing incidence geometry. This was the first angle-resolved measurement of the NLE in this geometry. The second system is 61Ni metal. This Mössbauer isotope has a very high transition energy of 67.4 keV. This energy is twice as high as all other isotopes used in NFS or other coherent SR based nuclear resonant measurements so far. The lack of investigations on transitions with high energies E is of no surprise, as the Lamb-Mössbauer factor describing the probability of nuclear resonant absorption scales with exp(-2π·E/(hc))2. Further, high resolution monochromators become difficult to realise at these energies. The NLE does not depend on a huge degree of monochromatisation of the incident radiation, as the non-interacting photons are not deviated into the detector. This thesis describes the technical developments realised to use the NLE in grazing incidence geometry and at high energies. The latter application depends crucially on an elaborated scheme to cool the rotating sample. The performances of the beamline ID18 at the European Synchrotron Radiation Facility at high energies are presented. Small angle X-ray scattering (SAXS) leads to an undesired background and has to be minimised. Means how to minimise SAXS are presented. The results obtained for the 57Fe-layer systems are presented and their magnetic properties deduced. The main emphasis however is given to the 61Ni resonance. The first spectra ever obtained in coherent nuclear resonant scattering of SR for a Mössbauer isotope with high transition energy is presented. The analysis of these data obtained with the NLE are compared to literature and to results of a further method, synchrotron radiation based perturbed angular correlation (SR-PAC). SR-PAC exploits the incoherent, inelastic decay channel of a nuclear excitation. It does not depend on the Lamb-Mössbauer factor and leads to higher experimental countrates as compared to NFS with a small Lamb-Mössbauer factor. Advantages and drawbacks of these two methods are discussed.

Full version of this thesis