Optimization of Fermi chopper in neutron spectrometer IN4C

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School of Engineering | Master's thesis
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This Master's Thesis was done in Grenoble, France at institute Laue-Langevin (ILL), which is one of the leading neutron research facilities in the World. Thesis describes a development project of a Fermi chopper, which is used to select neutrons with specific qualities. Fermi Chopper is a rotating device located inside a neutron spectrometer IN4C, which is a time-of-flight (TOF) neutron spectrometer. In this technique polychromatic beam of neutrons is passed through two background choppers, a monochromator and the Fermi chopper to create monochromatic pulses of neutrons with specific wavelength. These pulses collide with a sample material, penetrate it and continue to fly until they reach the detector bank, which registers the time-of-flight and diffraction angle of the neutrons. Since the distance between the Fermi chopper and sample, and sample and detector is known. neutrons loss or gain in energy can be registered and material properties of the sample can he studied. Fermi chopper is a shaft or a drum with a penetrating aperture in the middle of it. Inside the aperture is mounted a Slit Package, which is a sandwich type assembly of aluminium and gadolinium plates. This part makes the actual selection of neutrons when the shaft is rotating at speeds around 20 000 rpm. At this rotation speed the centrifugal forces induce significant deformations and stress in both, the rotor and the Slit Package. Objective of his development work was to analyze the Fermi chopper with Finite Element software Ansys Workbench and create a new design for the rotor to withstand rotation speeds up to 40 000 rpm. Another requirement for the new geometry was to be able to use it also in other instruments of the ILL that use Fermi choppers. A 3D-model of the complete Fermi chopper was made and rotation was simulated. Also the Slit Package was modelled, which was done for the first time in the history of the ILL. Slit Package is a very complex assembly to simulate with FEM software because of numerous frictionless contacts in it. Some problematic areas were found in the present Fermi chopper and several suggestions were made to improve the performance of the chopper, some of which were implemented on the instrument IN4C before the development work was done. A new geometry of the rotor was designed and simulated. Material was also chanced from Maraging steel used at the moment to aluminium alloy. The new geometry performs significantly better compared to the present one. Radial deformations have been decreased by more than 50% and required rotation speed of 40 000 rpm can now he achieved with deformations and stress within the safety factor of the material values of the aluminium alloy in question.
Aaltonen, Kalevi
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Bazzoli, David
neutron spectroscopy, neutroni spektroskopia, Fermi chopper, roottorin optimointi, rotor optimization, finite element method
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