Calibration of the Resonant Gravitational Wave Detectors Explorer and Nautilus in 2003 and 2004 Using Cosmic Rays

The cryogenic resonant gravitational wave detectors EXPLORER and NAUTILUS are able to detect cosmic ray showers. The experimental result leads to classify the responses in two categories: many small signals, in most cases with small multiplicity, obeying the thermo-acoustic model, and few large signals, usually associated to large multiplicity, which exceed the thermo-acoustic model by orders of magnitude, and whose understanding is still under investigation. Using the low multiplicity showers, we make a relative calibration of the apparatuses, comparing the response of EXPLORER with that of NAUTILUS. This comparison turns out useful when searching for coincidences between gravitational waves detectors.