Accurate calibration technique for a resonant-mass gravitational wave detector

The methodology for accurately calibrating the Niobe resonant-mass gravitat ional wave detector is presented. The transducer is based on a low noise re sonant microwave cavity transducer that converts the displacement of the re sonating mass to microwave energy. The calibration technique consists of a one off measurement of the microwave frequency versus resonant-mass displac ement characteristic. To measure this accurately, known static forces were applied to the resonant mass and the change in the transducer microwave fre quency was recorded. With the aid of finite element analysis and accurate m easurements of the resonant-mass characteristics, the deflection due to the known force was calculated. The calculated deflections were verified coars ely with measurements from a calibrated linear variable differential transf ormer. Typically, the detector operates with a 1 mK noise temperature. A be st noise temperature of 890 muK between 1300 and 2000 Universal Time Coordi nate (UTC) for day 60 in 1997 is reported. The transducer has been upgraded with a new microwave amplifier, which has a measured electronic noise floo r 40 dB lower than the previous amplifier, which is only 10 dB above the qu antum limit. (C) 2000 American Institute of Physics. [S0034-6748(00)00512-8 ].