Haughian, Karen Anne
Aspects of materials research for advanced and future generations of gravitational wave detectors.
PhD thesis, University of Glasgow.
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Gravitational waves were predicted by Einstein, in his General Theory of Relativity in 1916. These waves can be thought of as ripples in the curvature of space-time. They have not yet been directly detected but strong indirect evidence of their existence was provided by Hulse and Taylor when they measured the rate of decay of the inspiral motion of a binary system. Research towards direct detection of gravitational radiation from astrophysical sources has been carried out for many years through the design, construction and initial operation of a network of gravitational wave detectors. Direct detection of gravitational waves will provide insights into the astrophysical sources which produce them and will provide a new method of observing events in the Universe.
Gravitational waves are quadrupole in nature and produce strains in spacetime, which have extremely small amplitudes. The largest most violent events
in the Universe are expected to cause strains of approximately 10^-22 at the Earth in the frequency band of a few Hz to a few kHz. Long baseline interferometry
between suspended test masses is currently used to search for the induced strains in space-time, and thus the direct
effects of gravitational waves of astrophysical origin.
There is currently a network of interferometer detectors running worldwide. A 600 m long detector, GEO600, was built near Hannover in Germany by a collaboration between the Institute for Gravitational Research at the University
of Glasgow, the Albert-Einstein-Intitut in Hannover and Golm, the University of Hannover and Cardiff University. There are three detectors in the United States of America forming the LIGO project. Two detectors, one of 4 km arm length and one of 2 km arm length were constructed on a site near Hanford, Washington State, and one detector of 4 km arm length was constructed near Livingston Louisiana. A 3 km detector, Virgo, was built near Cascina, Italy,
by a European collaboration involving France, Italy and more recently the Netherlands. Six data collecting science runs have taken place to date with different combinations of these detectors in operation during the various runs;
no detections have yet been made.
An important noise source in the current operating frequency band of ground-based detectors is the thermal noise of the test mass mirrors in the interferometers, and the mirror suspension elements. The research presented
in this thesis focusses on studies of techniques for quantifying and reducing the mechanical loss associated with the suspended mirrors and thus reducing the associated thermal noise thereby increasing detector sensitivity. In particular, experiments were carried out to study the loss of fused silica and investigate aspects of the hydroxy-catalysis bonding process used to joint elements of the
test mass suspensions. In addition, silicon was investigated as a potential candidate for use as a mirror substrate material for use in future gravitational
In Chapter 1 the nature of gravitational waves is explained and some of the sources which are expected to produce the largest amount of gravitational wave radiation are described. The development of resonant bar detectors and
interferometers is given along with the current status of detectors and that of planned future projects. Noise sources which cause limitations to the detector sensitivity are discussed and an important noise source, thermal noise, is described in Chapter 2.
Thermal noise is an important noise source in the current frequency band of gravitational wave detectors. Reduction of thermal noise is a major challenge but is possible through careful design of the mirrors and their suspension
One technique aimed at minimising the thermal noise of a suspension system involves the creation of a quasi-monolithic suspension system by the use of
hydroxy-catalysis bonding. This is a high precision, high strength method of adjoining suspension elements. In Chapter 3 investigations were made of the strengths of hydroxy-catalysis bonds and on the effect on strength of various parameters associated with the most commonly used version of the bonding procedure, and of putting bonds through different treatments. It is shown that
the average strength of a hydroxy-catalysis bond between silica substrates is ~ 15 MPa and that somewhat elevated temperature treatment (similar to an airbake) can improve on this strength, but that thermal shock conditions can
decrease the strength. These investigations provide information on processes which can be used in the suspension construction to produce the lowest loss, highest strength suspension system.
Chapter 4 details mechanical loss measurements of bulk silica at room temperature. Different types of fused silica are studied and techniques to reduce their mechanical loss are discussed along with the effect which time and heat treatments can have on the mechanical loss of a hydroxy-catalysis bond. It is shown that Suprasil 3001 is an acceptable choice of material for the mirrors in gravitational wave detectors and that the mechanical loss of silica can be reduced through heat treatment.
In Chapter 5 the mechanical loss of bulk silicon is studied, where silicon forms a potential candidate for future generation gravitational wave detectors.
Silicon samples having two different crystal orientations, <100> and <111>, were studied. Both orientations were manufactured and polished by the same vendor and have equivalent doping levels. At room temperature
it is seen that the crystal orientation <111> material yielded mechanical loss values which were slightly lower than the <100> material. It is shown that it is possible to further reduce the loss of the material through heat treatment.
An upper limit of the mechanical loss of a hydroxy-catalysis bond between silicon substrates is determined and found to be within the range of 0.27 to 0.52.
The results presented in this thesis indicate that the mechanical losses of silica suspensions in gravitational wave detectors can be reduced through methods such as heat treatments and, potentially, chemical etching. Silicon is seen to be an interesting candidate for the suspension material in future generation detectors run at cryogenic temperatures.
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