An Investigation of Some Mechanical and Optical Properties of Materials for Test Masses in Laser Interferometric Gravitational Wave Detectors

Logan, Jennifer Elizabeth (1993) An Investigation of Some Mechanical and Optical Properties of Materials for Test Masses in Laser Interferometric Gravitational Wave Detectors. PhD thesis, University of Glasgow.

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Gravitational waves are one of the predictions of Einstein's General Theory of Relativity (Einstein, 1916). They are produced when mass accelerates asymmetrically resulting in quadrupole, or higher order, gravitational radiation and the production of a strain in space. This is in principle detectable by measuring the change in distance, AL, between two free test masses a distance L apart. Despite considerable experimental effort which has gone into developing suitable detectors, gravitational waves remain as yet undetected. This is due to their weakness of interaction with matter. Gravitational waves which are emitted from a violent astrophysical event, such as a supernova, are predicted to produce a strain at the earth of at most ~10-21 in the frequency range accessible to terrestrial detectors (greater than approximately 100 Hz), assuming that a reasonable event rate is required. The most promising type of detector currently under development uses laser interferometry to monitor the displacement of freely suspended test masses - a technique which exploits the quadrupole nature of gravitational waves. Construction of large-scale detectors of this type will soon commence in several places around the world. These should have the required sensitivity to detect gravitational waves from astrophysical sources leading to the opening of a new field of astronomy. The ultimate sensitivity of such detectors will be limited by various noise sources. Above about 100 Hz, thermal motion of the test masses is predicted to make the dominant contribution to the detector noise level when searches for continuous sources of gravitational waves are made. The investigation of such thermal motion forms a substantial part of this thesis. To minimise thermal noise, it is important that the test masses are fabricated from a material which has low internal losses, i.e. a high quality factor Q. The dimensions of each test mass should also be such that its lowest resonant frequency is well above the frequency range of interest for the detection of gravitational waves (approximately 100 Hz to a few kilohertz). As it is important to investigate the Q values of possible materials of interest, an experimental method using laser interferometry to measure the Q of samples of material suspended as pendulums, was developed. The effect of coupling between normal modes in samples of materials was studied and it was noted that the Q values of the coupled system were degraded by the more lossy pure mode. The structure of the coupled modes was studied with the aid of a vibration pattern imager. Information gained from this was then used in order to develop an electrical model of the coupling, in order that its effect on thermal motion of a test mass, in the frequency band of interest for the detection of gravitational waves, could be assessed. It was found that depending on the exact nature of the coupling, thermal motion of the mass, at frequencies well below its lowest resonance, could be increased above that for the uncoupled system. Thus as a general guideline, it is wise to choose the dimensions of a test mass such that its resonant frequencies do not lie close together. Columnar silicon, a particular type of polycrystalline silicon, has been found by the author to have a suitably high Q for it to be considered as a possible material from which to form the test masses for a long base-line interferometric gravitational wave detector. It was found however that the measured Q of the fundamental longitudinal mode of a sample of this material varied, apparently randomly, when the mass was re-suspended. After some experimental investigation it was found that variation in measured Q was due to resonances in the suspension wires. An electrical model of the system was developed and this allowed an evaluation to be made of the effect of wire resonances on the thermal motion of the test mass, at frequencies much lower than the lowest resonant frequency of the mass. It was found that if the mass was suspended such that its measured Q was low due to resonances in the suspension wires, thermal motion of the mass, at frequencies of interest for the detection of gravitational waves, was not increased. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Norma Robertson
Keywords: Theoretical physics, Applied physics, Optics
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-74657
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 17:18
Last Modified: 27 Sep 2019 17:18

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