Observation of Mechanical Nyquist Noise in a Cryogenic Gravitational-Wave Antenna

Abstract
A 680-kg, Weber-type, cylindrical, gravitational-wave detector has been operated at liquid helium temperature. The average vibrational energy in the lowest longitudinal mode at 1315.3 Hz was found to be consistent with the level of thermal noise at the antenna temperature. An effective noise temperature of 0.39 K for pulse excitation was measured. This is a factor of 20 below the lowest published values for room-temperature detectors.