AURIGA: a detector overview

 

The AURIGA detector is a  INFN project aiming at a direct detection of gravitational waves (g.w.) of astrophysical origin.

Due to the small cross section of the interaction of gravitational radiation with matter, the detector has to be carefully designed in order to maximize the signal-to-noise ratio (SNR) and increase detection chances. 

The AURIGA detector is a resonant detector: this means that the best sensitivity is peaked on a detector mechanical resonance, where the energy absorption from the passing g.w. is maximum.

The detector is schematized in the picture below.

The detector is based on a resonant body, called bar: this is a 2.3 tons heavy, 3 m long aluminum cylinder. The resonant mode of interest is the first longitudinal one.

With AURIGA g.w. would be detected as they squeeze and stretch the bar. 

To read out the bar end faces vibrations, which occur at about 1000 Hz, another mass of about 1 kilogram is elastically fixed on one of them. As it is constructed to vibrate at the same frequency and is so much lighter than the bar, the second mass picks up resonantly the bar vibrations with a much larger amplitude. This second mechanical resonator is called resonant transducer.

Then as it is part of a capacitor, charged up at high voltage, the vibrations are transformed into current oscillations in an electrical circuit, amplified, digitized and fed to a computing center to be analyzed.

Even from the strongest sources in the cosmos, as inspiralling black holes, the bar vibrations excited by the g.w. are extremely tiny, of the order of 10-20 m: this means that they are of the same order of the quantum mechanical uncertainty of the bar position in its ground state!

Therefore the g.w. induced vibration would easily be blurred out by the vibrations induced by mechanical ambient noise and by the ubiquitous spontaneous thermal motion of matter. To reduce both these noises, the bar is suspended in vacuum and cooled to ultra low temperatures, a fraction of a degree close to the absolute zero. In these conditions the detector runs for years, waiting for a strong enough signal to be separated from the noise.

 


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