Detection of high energy cosmic rays with the resonant gravitational wave detectors NAUTILUS and EXPLORER and comparison with the direct measurements with an aluminum superconductive bar

The cryogenic resonant gravitational wave detectors NAUTILUS and EXPLORER, made of an aluminum alloy bar, can detect cosmic ray showers. At temperatures above 1 K, when the material is in the normal conducting state, the measured signals are in good agreement with the values expected based on the cosmic rays data and on the thermo-acoustic model. When NAUTILUS was operated at the temperature of 0.14 K, in superconductive state, large signals produced by cosmic ray interactions, more energetic than expected, were recorded. The NAUTILUS data in this case are in agreement with the measurements done by a dedicated experiment on a particle beam. The largest event detected up to now has an energy in the first longitudinal mode of ∼ 670 K corresponding to ∼ 360 TeV in the bar.