The NAUTILUS detector consists of an Al5056 cylindrical bar of length L = 3 m, diameter of 0.6 m and mass of 2270 kg, resonating in its first longitudinal mode of vibration at a frequency of about 910 Hz at liquid-helium temperature. It is located at the INFN Frascati National Laboratories (12.4E, 41.5N) and has been oriented approximately parallel to the other resonant GW detectors. The layout of the NAUTILUS apparatus is shown in Figs. 1 and 2. The relevant feature of the cryostat is its central section, which is shorter than the cylindrical. The central section contains:
• Two aluminum alloy shields cooled by helium gas.
• A stainless steel liquid helium (LHe) reservoir (2000 liters capacity).
• Three massive copper rings.
• A special ³He-⁴He dilution refrigerator, through the top central access.
End caps are fastened to each stage of the cryostat to complete the seven shields system surrounding the bar. The shields are suspended one from the other, forming a cascade of low-pass mechanical filters. The overall mechanical vibration isolation at the bar resonant frequency (about 900 Hz) is about -220 dB. In order to eliminate the acoustic noise from the boiling liquid helium, the helium bath is kept in a superfluid regime, at a pressure of about 20 mbar.
The bar suspension and the thermal link to the refrigerator were defined after experimental tests on various solutions. We adopted a U-shaped (Weber type) oxygen-free high-conductivity (OFHC) copper cable, wrapped around the bar central section and hung from the central ring of the innermost shield of the cryostat. This cable suspension also provides the necessary thermal path between the refrigerator thermal contacts and the bar.
The internal copper shield hangs from the intermediate copper shield by means of two U-shaped titanium rods (alloy Ti64). The middle shield is suspended from the external shield by four Ti64 cables with intermediate lead masses. The liquid helium vessel and the two external shields cooled by the helium gas are suspended by four Ti64 cables, hung at room temperature to a stack of 8 rubber and steel disks.
The external copper shield is thermally anchored to the 1Kpot of the refrigerator. The intermediate shield is in thermal contact with a step heat exchanger of the dilution refrigerator; the inner shield is in thermal contact with the mixing chamber. Here the thermal path is provided by OFHC copper streaps, properly annealed in order to minimize the transmission of mechanical vibration to the bar and increase the thermal conductivity.
The thermal behavior of the cryostat is monitored by a thermometry system consisting in GaAs diodes and Ge thermometers.
The overall LHe evaporation rate at regime is about 90 liters/day (40 from the helium reservoir and 50 from the 1Kpot).
In order to operate the detector, the pressure of the residual gas in the antenna vacuum chamber must be kept below 10^-5 mbar; otherwise, the capacitive transducer cannot be biased with the necessary dc voltage. The bar has two stable equilibrium temperatures: about 100 mK, if the refrigerator is fully operating, and about 1.3 K, if just a very low mixture flow is kept in the refrigerator, which then acts as a good thermal link to the 1 K pot.

The vibrations of the bar are converted into electrical signals by a capacitive transducer resonating at the antenna frequency in order to improve the energy transfer from the bar to the electronics. The signals are applied to the input coil of a dc SQUID amplifier by means of a superconducting transformer, which provides the required impedance matching.
The capacitive transducer bolted to one end of the antenna consists of a vibrating disk with mass Mt=0.32 kg and of a fixed plate with a gap of d= 10 µm and total capacitance about 10 nF. The transducer is biased with a voltage Vp through the polarization resistors Rp. The transducer and the bar form a system of two well-matched high-Q coupled oscillators, with normal mode frequencies f_-=905 Hz f₊=920 Hz.
The high-impedance transducer is connected to the low-inductance (≈1 µH) input coil of the SQUID through a decoupling capacitor (Cd = 200 nF) and a superconducting transformer with a high turn ratio (≈2000), primary inductance of about 2 H, secondary inductance of about 1 µH, and coupling factor k≈0.8. The electrical circuit exhibits a resonance around 1700 Hz, so we have an electrical mode, which is, however, only very weakly coupled to the mechanical modes.
All the read-out parameters are chosen in order to increase the antenna sensitivity.
The detector can be calibrated by two devices: a) a second capacitive transducer (calibrator), mounted at the opposite end of the bar; b) a piezoelectric ceramic glued on the bar surface near the central section.

NAUTILUS has been equipped with a cosmic-ray detector consisting of layers of streamer tubes placed above and below the cryostat. A cosmic ray detector is necessary because extensive air showers (EAS) or energetic single particles (muons or hadrons) interacting with the bar may produce detectable signals whose rate increases with a better sensitivity. of the antenna. For instance, with a sensitivity of 1 mK, about two cosmic-ray events per day are expected, but the rate increases to about 5 10³ if the quantum limit sensitivity is reached.

Recently, the passage of cosmic rays (link verso results) has been observed to excite the antenna. A very significant correlation (more than 10 standard deviations) has been observed.

The experimental apparatus includes others vetoes, such as vibration sensors (accelerometers) which monitor the laboratory environment.