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The resonant-mass gravitational wave (GW) detector NAUTILUS, an aluminum 2300 kg cylinder cooled at 100 mK, operated by the ROG Collaboration at the Frascati National Laboratory, has recently recorded signals due to the passage of cosmic rays [P.Astone et al.,Phys.Rev.Lett.84 ,14 (2000)] in detector.
In 1969 Beron and Hofstander had already carried out experiments aiming to detect oscillations of piezoelectric disks excited by a GeV electron beam. The results led the authors to suggest that a very large cosmic-ray (CR) event could excite mechanical vibrations in a metallic cylinder at its resonance frequency and could provide an accidental background for experiments on GW 's [B.L.Beron, R.Hofstander, Phys.Rev.Lett. 23, 184 (1969); B.L.Beron, et al., IEEE Trans.Nucl.Sci.17, 65 (1970)]. Later, a group at the University of Milan [A.M.Grassi et al., J.Appl.Phys.51, 849 (1980)] estimated the possible effects of particles on a small aluminum cylinder and made an experiment which verified the calculations, although with rather large experimental errors.
The mechanical vibrations originate from the local thermal expansion caused by the warming up due to the energy lost by the particles crossing the material. The effect depends on the thermal expansion coefficient and the specific heat of the material. The ratio of these two quantities is the Gruneisen coefficient. It turns out that while both the expansion coefficient and the specific heat vary with temperature, the Gruneisen coefficient practically does not. In the case of aluminum, this is certainly true above 1 K.
Subsequently, more refined calculations were made by several authors. All these models agree in predicting, for the vibrational energy E of the excited fundamental mode of an aluminium cylindrical bar like NAUTILUS, the following formula:
E =7.64 ·10(^-9) W^2 f
where E is expressed in kelvin, W, in GeV, is the energy released by the particle to the bar and f is a geometrical factor of the order of unity. The above formula has recently been verified with an experiment at room temperature [G.D. van Albada et al., Rev Sci Instrum.71, 1345 (2000)], using a small aluminum cylinder and an electron beam.
NAUTILUS is equipped with a CR detector made of Limited Streamer Tubes. The data regarding the vibrational energy of the bar have been correlated with the data obtained by the CR detector. Very large signals, at a rate much greater than expected have been detected [P.Astone et al., Phys.Lett. B 499, 16-22 (2001)].
In order to investigate this matter and account for these anomalous results, the present experiment to measure the effect of the passage of charged particles in a low temperature mechanical oscillator in a controlled environment. The DAFNE electron Beam Test Facility (BTF) at LNF will be used to excite mechanical vibrations in a small cylindrical bar made of the same aluminum alloy as NAUTILUS. The measurements will be performed in both super-conducting and normal regime of the aluminum bar. Moreover, the measurement will be done using energies of the order of 1 TeV. This is the typical energy released in the bar by the extensive air showers producing the anomalous signals . It means an improvement in sensitivity of about 10^4 -10^5 with respect to past experiments.
The results of this experiment will be important to understand the interactions of ionizing particles with bulk superconductors and also to understand the limitations due to CR's to the sensitivity of the future gravitational resonant detectors not shielded against CR's.