Development and test at T=42K of a capacitive resonant transducer for cryogenic gravitational-wave antennas

The characteristics of a new capacitive resonant transducer developed and tested on a small (M=11.2 kg) cryogenic gravitational-wave antenna at the liquid helium temperature are presented. The resonator frequency can be tuned within 0.1 Hz of the antenna frequency. The system has a mechanical merit factor Q approximately=5*10/sup 5/ and a ratio between the electrical energy in the transducer and the energy in the antenna beta =3*10/sup -3/ at T=4.2K. With these parameters, the transducer allows one to reach an effective noise temperature T /sub eff/ approximately=60 mK using a cooled FET preamplifier, and, if coupled to a RF-SQUID, allows one to reach T/sub eff/ approximately=10 mK for the 5000 kg cryogenic gravitational-wave antenna of the Roma group at CERN (Geneva).