11B4C containing Ni/Ti neutron multilayer mirrors

Abstract: The work in this thesis covers the design, growth and characterisation of neutron multilayers. The performance of these multilayers is highly dependent on the obtained interface width between the layers, even a modest improvement can offer a substantial increase in reflectivity performance. As multilayers are such an integral component of many neutron optical instruments, any improvement in terms of reflectivity performance has broad implications for all neutron scattering experiments. This project has been carried out with the construction of the European Spallation Source (ESS) in mind, but the principles extend to all neutron scattering sources.Ni/Ti is the conventional material system of choice for neutron optical components due to the high contrast in scattering length density (SLD). The reflected intensity of such components is largely dependent on the interface width, caused by the formation of nanocrystallites, interdiffusion, and/or intermixing. Apart from hampering the reflectivity performance, the finite interface width between the layers also limits the minimum usable layer thickness in the mirror stack. The formation of nanocrystallites has been eliminated by co-depositing of B4C . This has been combined with a modulated ion assistance scheme to smoothen the interfaces. X-ray reflectivity (XRR) measurements show significantly improvements compared to pure Ni/Ti multilayers. This has further been investigated using low neutron-absorbing 11B4C instead. After deposition, the 11B4C containing films have been characterized using neutron reflectometry, X-ray reflectivity, transmission electron microscopy, elastic recoil detection analysis, X-ray photoelectron spectroscopy. A large part of his work has focused on fitting X-ray and neutron reflectivity measurements in order to obtain structural parameters.The fits to the experimental data suggest a significant improvement in interface width for the samples that have been co-deposited with 11B4C using a modulated ion assistance scheme during deposition. Any accumulation of roughness has been eliminated, and the average initial interface width at the first bilayer has been reduced from 6.3 Å to 4.5 Å per bilayer. The respective reflectivity performance for these structural parameters have been simulated for a neutron supermirror (N = 5000) for both materials at a neutron wavelength at λ = 3 Å using the IMD software. The predicted reflectivity performance for the 11B4C containing samples amounts to about 71%, which is a significant increase compared to the pure Ni/Ti samples which have a predicted reflectivity of 62%. This results in a reflectivity increase from 0.84% to 3.3% after a total of 10 reflections, resulting in more than 400% higher neutron flux at experiment.