2. The ALTAIR detector
The ALTAIR detector (Bonifazi et al. 1993) consists of a cylindrical bar equipped with a capacitive resonant transducer and a d.c.SQUID amplifier. ALTAIR is located at IFSI-CNR laboratories in Frascati (Rome). The antenna is an Al-6061 alloy cylinder, long, in diameter, with mass , resonant frequency (first longitudinal mode) . The antenna is equipped with a capacitive resonant transducer, consisting of a small disk, which vibrates at a frequency . Because of the resonant transducer there are two resonant modes at frequencies about and . The read-out system, very similar to that used with the Explorer (Astone et al. 1993) and Nautilus (Astone et al. 1994a, 1997a) detectors, has been described elsewhere (Bassan et al. 1990).
The signal from the SQUID amplifier is sent to lock-in amplifiers (with integration time of ) that extract the signal components at the frequencies of interest: at the two mode frequencies and and at the frequency of a reference signal (), used to monitor the gain of the SQUID. The data, sampled at , are processed and recorded on a Digital MicroVax 3300, using the DAGA2 acquisition system (Frasca 1993).
We report the results of our analysis only for the mode at the frequency of , at , where sensitivity was best. Extra-noise, probably due to mechanical resonances in the suspensions, driven by the Helium boil-off, affected the measurement at the other mode at frequency in normal operation. Results at the mode are instead reported for the antenna operating at , in superfluid helium regime.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999