  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 biggest recorded event was in
EXPLORER and excited the first longitudinal mode to a vibrational energy of
about 670 K, corresponding to about 360 TeV absorbed in the bar. Cosmic rays
can be an important background in future acoustic detectors of improved
sensitivity. At present, they represent a useful tool to verify the
gravitational wave antenna performance.
