The cosmic ray veto system of the Mario Schenberg gravitational wave detector

The Mario Schenberg gravitational wave antenna is a spherical cryogenic resonant mass detector located at IFUSP, São Paulo. It is well known that cosmic rays interact with cryogenic resonant mass detectors generating acoustic signals. Depending on the shower energy, they could provide a substantial background noise which should be vetoed to reduce the false alarm rate. For this purpose, in December 2011, we have installed a cosmic ray veto system which is, since then, acquiring data. The cosmic ray veto system is composed of three particle detectors containing each one a scintillator, a photomultiplier and a tension divider. As the shower number of particles is used to define a threshold for the veto, it is important that the cosmic ray veto provides a linear response to high-energy cosmic ray events. The veto setup response was optimized and allows measurements up to 23,000 equivalent muon charge particles per square meter. We present here the experimental setup, its calibration and performance. Finally, to confirm the linearity of the data acquisition we show the measured particle multiplicity