Design and Optimization of a Neutron Polariser for ESS Imaging Instrument ODIN

Sammanfattning

Neutrons have a magnetic moment that interacts with magnetic fields. This has been put to use to study magnetic materials. Polarised neutron imaging (PNI) is one such application that allows the observation of, for instance, magnetic domains and domain walls inside a sample. To facilitate a PNI experiment, the neutron beam from a neutron source needs to be polarised, meaning that the magnetic moments of all neutrons are aligned with an applied magnetic field. The device used to achieve this is called a “Polariser”. The focus of this thesis is the design of a polariser for the ODIN imaging instrument that is being constructed at the European Spallation Source (ESS). Among the different choices of polarisers, a “v-cavity” geometry will be used, which works on the principle of the polarisation-dependent reflection of neutrons from a surface structure called “polarising supermirror”. Here, Monte Carlo ray-tracing simulations are used to study the ODIN beam characteristics and subsequently to evaluate and optimise the polariser design. In addition, the design of a magnetic housing for the polariser is presented. This is needed, because the polarising supermirrors require a magnetic field to function. Using finite element method to compute the magnetic field, a magnetic housing is designed that would satisfy the field strength and field alignment requirements of the polariser.