NAUTILUS detects cosmic rays
Jan 7, 2000
Thirty years ago two physicists predicted that cosmic rays could excite mechanical vibrations in a metallic cylinder and therefore provide accidental background noise for experiments on gravitational waves. Now, the NAUTILUS gravitational wave detector at the Frascati Laboratory in Italy has seen such an effect for the first time. The detector consists of a 2300 kg aluminium cylinder cooled to 100 milliKelvin and is surrounded by cosmic-ray detectors. As the cosmic rays pass through the aluminium, they lose energy as they hit atoms in the metal. This energy is absorbed by the cylinder causing thermal expansion, which in turn creates mechanical vibrations in the detector (Phys. Rev. Lett. 84 14).
NAUTILUS was designed to observe the gravitational waves produced by collisions between objects such as black holes or neutron stars. According to the Frascati's group calculations, signals from such events could be masked by cosmic-ray showers. To take this effect into account they surround the cylinder with 116 cosmic ray detectors. When the aluminium cylinder vibrates from the effects of the gravitational waves, a transducer converts the mechanical signal into a electrical signal, which is in turn amplified and recorded. The group compared the signal from the cylinder with that from the cosmic-ray detectors over a three month period, and found that both the cosmic-ray detectors and the gravitational-wave detector registered events simultaneously on several occasions. The mechanical vibration of the cylinder - as small as 10-18 metres - corresponds to an energy deposit of 10-6 eV. According to the paper, future detectors operating near the quantum noise limit (i.e. cooled to 10-7 Kelvin) would have to be put underground to limit this cosmic interference.