Quality Factors of Bare and Metal-Coated Quartz and Fused Silica Torsion Fibers C.A. Hagedorn, S. Schlamminger and J.H. Gundlach Department of Physics, Center for Experimental Nuclear Physics and Astrophysics, Box 354290, University of Washington, Seattle, Washington 98195-4290 Abstract. The thermal noise of a torsion balance is determined by the mechanical quality factor Q of its fiber, where higher-Q fibers are less noisy. We report here on the fabrication and character- ization of fused quartz and fused silica torsion fibers with Qs as high as 3.6 × 10 5 at frequencies below 0.1 Hz. Fibers as small as 10 μ m in diameter were made conducting with thin metal coatings while retaining Qs larger than 10 4 . Pressure-dependent damping was discerned at pressures lower than 10 -5 Pa, suggesting residual gas damping may limit experiments using these higher-Q fibers. Keywords: torsion fiber, mechanical quality factor, fused quartz, fused silica, gas damping, LISA PACS: 07.10.Pz, 04.80.Cc, 61.43.Fs, 62.40.+i INTRODUCTION The University of Washington LISA torsion balance experiment [1] is designed to in- vestigate weak physical phenomena that may influence the LISA Gravitational Refer- ence Sensors. At present, our experimental sensitivity is limited by the thermal noise of Brownian motion in the torsion fiber. The thermal torque noise at low frequency is τ ( f )=  2k B T κ /(π Qf ), where κ is the fiber’s torsion constant, Q its quality factor, f a frequency of interest, T temperature, and k B Boltzmann’s constant [2]. To decrease the experiment’s thermal noise, while keeping it at room temperature, we have renewed our search for fibers with a superior κ /Q ratio and high tensile strength. Such fibers will not only improve our LISA test; they will benefit many other small force measurements on the ground and in space. Fused quartz fibers have been known to have very high Qs and tensile strength since before the 1930s [3, 4]. Recently, there has been strong interest in characterizing the properties of quartz fibers from the ground-based gravitational wave detection commu- nity. Reports of high (10 6-7 ) [5, 6, 7, 8] and exceptionally high (10 8 ) [9] vibrational Qs in fused silica, and results that suggest thin fibers [10] at low frequencies [3, 4] can have good Qs (10 5-6 ) solidified our interest in quartz as a fiber material. For comparison, tungsten torsion fibers typically have Q ∼ 4500, with greater κ and tensile strength than quartz fibers of equal diameter. EXPERIMENT We measure the Q of a torsion fiber by observing the amplitude decay of a torsion pendulum. A light beam reflected from a mirror on the pendulum passes over a detector