Mechanical quality factor measurements of monolithically suspended fused silica test masses of the GEO 600 gravitational wave detector

Internal thermal noise is expected to be a limiting noise source in the most sensitive frequency band of the GEO 600 gravitational wave detector. Because thermal noise is directly related to energy dissipation, care has been taken to construct test mass suspensions from low-dissipation materials and to eliminate inter-material rubbing where possible. Recently, the GEO 600 team finished the installation of triple-pendulum suspensions for the optics of the Michelson interferometer. Each of these triple pendulums incorporates a monolithic fused silica pendulum as the lowest stage. We have made internal mode quality factor measurements of three monolithically suspended test masses. Using these measurements we estimate of the level of internal thermal noise in the GEO 600 interferometer.