Development and Characterization of a Water-Based Muon Veto for the V-Angra Experiment

The nu-Angra experiment has developed an antineutrino detector intended to operate at sea level, a few dozen meters from the core of a nuclear reactor. The operating principle of the detector is based on the water Cherenkov light detection to be in compliance with the safety rules of the power plant operator. The detector is exposed to a high rate of background events, mainly due to cosmic rays, making its veto system a key element for efficient background rejection. The nu-Angra Collaboration has designed a veto detector composed of three layers filled with water for both passive shielding and active detection of crossing particles. The veto surrounds a volume of 1.50 x 1.05 x 1.39 m(3), in which a target detector is placed, where the search for anti-neutrino events occurs. Any external particle hitting the detector must cross at least 25 cm of water before reaching this internal volume, which is protected by the veto structure, thus providing an important barrier against neutrons and low energy particles. In addition, the veto system has been instrumented with eight 8-inch photomultiplier tubes to detect external particles that may eventually reach the target detector. By making use of a trigger system based on scintillating paddles, energy distribution and detection efficiency of the veto system were assessed for different detector positions. This paper, therefore, presents the achieved results related to the characterization of such system concerning the detection of cosmic-ray muon particles passing through it.