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The GEO 600 gravitational wave detector is currently under construction in Germany. To ensure that the detector sensitivity is not limited by seismic noise above 50 Hz a significant degree of seismic isolation has to be provided for each test mass. To achieve this level of isolation each test mass, which will be made from fused silica (mass ∼14 kg), will be suspended as the final stage of a triple pendulum from an isolation stack consisting of layers of stainless steel masses and graphite loaded silicone rubber. Extending from the stack will be three cantilever springs supporting an upper mass from which a double pendulum stage will be suspended. This double pendulum will incorporate cantilever springs in the upper stage in order to enhance the vertical isolation, and will use fused silica fibers in the lower pendulum stage in order to minimize thermal noise from the pendulum modes. The expected performance from this arrangement of stacks, vertical spring stages, and double pendulums should achieve our goals for seismic isolation.

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This paper describes some investigations into the construction of a monolithic fused silica test mass suspension for use in interferometric gravitational wave detectors. We summarize results showing that the material Q factor of standard fused quartz in the form of ribbons is of a level which makes it suitable for use as a suspension material for the test masses of long baseline gravitational wave detectors and then present measurements of the Q factor of the pendulum mode of a non-conducting mass suspended on cylindrical fibres of fused quartz. Our results show that electrostatic charging of the mass can result in a significant decrease in the pendulum mode Q factor.