The AURIGA bar


AURIGA is a resonant acoustic detector of g.w.: the mechanical resonator is an aluminum cylinder, called bar, with the following figures:
length 3 m
diameter 60 cm
mass 2.3 tons
The material the bar is made out is a particular aluminum alloy, commonly referred to as Al5056: this alloy is enriched in Magnesium. The material is thus chosen as it guarantees very high mechanical quality factors (Q) when cooled down to cryogenic temperatures: the mechanical quality factor is about 4x106 at 100 mK. This is of outmost importance for AURIGA: the quality factor of the resonance must be as high as possible in order to maximize the sensitivity. 
The bar material and dimensions are chosen as a good compromise between cost and frequency constrain (the expected signals are at about 1kHz). Moreover the mass of the detector, which the signal cross-section is proportional to, cannot be too large as the cooling power of actual refrigerators is limited.
The resonant frequency of the first longitudinal mode, which is that monitored for g.w. detection, is about 1kHz. Actually it does vary with the temperature, as the Young modulus of the material changes: at room temperature the resonance is at 874Hz while at the detector operating temperature of 0.1K the resonance is at 920Hz.