ar X iv : g r - qc / 0 20 60 79 v 1 2 6 Ju n 20 02 Effect of cosmic rays on the resonant gravitational wave detector NAUTILUS at temperature T = 1 . 5

The interaction between cosmic rays and the gravitational wave bar detector NAUTILUS is experimentally studied with the aluminum bar at temperature of T=1.5 K. The results are compared with those obtained in the previous runs when the bar was at T=0.14 K. The results of the run at T = 1.5 K are in agreement with the thermo-acoustic model; no large signals at unexpected rate are noticed, unlike the data taken in the run at T = 0.14 K. The observations suggest a larger efficiency in the mechanism of conversion of the particle energy into vibrational mode energy when the aluminum bar is in the superconductive status. The gravitational wave (GW) detector NAUTILUS recently recorded signals due to the passage of cosmic rays (CRs) [1, 2, 3]. Several authors [4]-[11] estimated the possible acoustic effects due to the passage of particles in a metallic bar. The mechanism adopted assumes that the mechanical vibrations originate from local thermal expansion caused by warming up due to the energy lost by the particles crossing the material. It was predicted that for the vibrational energy in the longitudinal fundamental mode of a metallic bar the following formula would hold: E = 4 9π γ 2 ρLv 2 dW dx 2 sin πz 0 L sin(πl 0 cos(θ 0)/2L) πRcos(θ 0)/L 2 (1) where L is the bar length, R the bar radius, l 0 the length of the particle's track inside the bar, z 0 the distance of the track midpoint from one end of the bar, θ 0 the angle between the particle track and the axis of the bar, E the energy of the excited vibration mode, dW/dx the energy loss of the particle in the bar, ρ the density, v the sound velocity in the material and γ is the Grüneisen coefficient (depending on the ratio of the material thermal expansion coefficient to the specific heat) which is considered constant with temperature. The resonant-mass GW detector NAUTILUS [12], operating at the INFN Frascati Laboratory, consists of an aluminum alloy 2300-kg bar which can be cooled to very low temperatures, of the order of 0.1 K, below the superconducting transition temperature of this alloy, T C = 0.92 K [13]. The bar is equipped with a capacitive resonant transducer, providing the read-out. Bar and transducer form a