(ed.). Gravitational waves (IOP, 2001)(422s).

Discussion and conclusions 101Figure 8.2. Spectral amplitude ˜h of EXPLORER and NAUTILUS versus frequency (Hz).closure of the universe. It has been found, using equation (8.40), that ≥ 60,where a factor of six has been also included for taking into consideration the factthat EXPLORER and NAUTILUS are separated by about 600 km. The value of we have obtained is much larger than the expected value, but we remark thatit is the first measurement of this type made with two cryogenic resonant GWdetectors.Finally, we would like to mention that recently, making use of the detectorNAUTILUS, we have been able to observe the passage of cosmic rays 1 . Theobserved events in the bar have amplitude and frequency of occurrence inagreement with the prediction (event energy of the order of 1 mK), showing thatNAUTILUS is properly working. In particular, the efficiency of the filter aimedat detecting a small signal embedded into noise is well proven.8.4 Discussion and conclusionsIn previous literature the sensitivity for resonant detectors of gravitational waveshas usually been expressed in terms of T eff , the minimum energy delivered bya GW burst that can be detected by the apparatus. However, what the resonantdetectors really measure is essentially the Fourier transform (over a certainfrequency band) of the GW adimensional amplitude.For studying the operation of a resonant antenna as a GW detector ofstochastic GW we had to deal with noise spectrums. This has made us reconsider1 This result has been presented at the School by Dr Evan Mauceli.