A gentle nodal suspension for measurements of the acoustic attenuation in materials

Loss angle measurements in ultralow mechanical loss materials are normally affected by a large systematic error due to the excess losses introduced by the suspension system used to hold the samples. Low loss materials are of great interest for ground-based interferometric detcors for gravitational wave observations. Crystals such as sapphire and silicon or amorphous materials such as fused silica can have intrinsic loss angles in the range of 10^-10 ÷ 10^-6; these materials are of extreme interest in the detection of small displacements, due to a very low achievable level of thermal noise. An innovative measurement system of the acoustic attenuation in materials is proposed. GeNS (Gentle Nodal Suspension), where a disk-shaped sample is suspended in equilibrium on the top of a sphere, touching in one of the nodal points of vibration, has been developed in INFN Virgo laboratory in Florence and then upgraded to a cryogenic stage in the Laboratoire des Materiaux Avances in Lyon. The advantage of this system, as compared to others used so far, is twofold: first, one surface only of the sample is touched and the contact surface is minimized because of the absence of applied forces; secondly, some relevant parameters of the suspension can be measured and eventually varied, giving the experimentalist the possibility to identify whether the measured loss is limited by the suspension system in use or it is an intrinsic property of the sample under investigation. State of the art and crucial results at room temperature will be presented.