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

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.
Neutrons can be used as a versatile tool to study material properties. Due to neutron having a magnetic moment, it is especially useful for studying magnetic materials. The magnetic moment is associated with an intrinsic property call “spin”. The spins of the neutrons need to be aligned in those studies. In this condition, the neutron beam is called “polarised”. One of the techniques that utilizes a polarised neutron beam is “polarised neutron imaging”. Using this technique, for instance, the magnetic domains inside a metallic sample can be directly seen and their behavior analyzed. At the European Spallation Source, a new large sale facility for the scientific use of neutrons, the imaging instrument ODIN is currently under construction. Polarised neutron imaging will be one of ODIN’s unique capabilities. The device to create a polarised beam is called “polariser”. It is based on a special magnetic layered structured called “polarising supermirrors” which reflects only neutrons with a selected spin-orientation, creating a spin-aligned neutron beam. This work focused on designing a polariser for ODIN by adapting conventional arrangement of polarising supermirrors for nonfocusing beam to a focusing polariser that will match ODIN’s strongly focused beam. A key design constraint is that ODIN will use neutrons in a wide energy range and reflection by supermirror is highly energy-dependent. Using “McStas”, a simulation tool for designing neutron instrument components, the performances over a range of polariser design parameters were analyzed. We succeeded in finding a geometry that satisfied all design requirements, primarily the degree of polarisation achieved for the full range of neutron energies to be used on ODIN. The polariser requires a magnetic field to function. Finite element calculations were done using COMSOL software to verify a magnetic housing design to have exceeded the design requirements.