Nuclear Instruments and Methods in Physics Research A 581 (2007) 206–208 Glass resistive plate chambers in the OPERA experiment A. Candela a , E. Carrara b , M. D’Incecco a , A. Di Giovanni a,d,Ã , S. Dusini b , C. Gustavino a , A. Lindozzi a , A. Longhin b , S. Micanovic c , D. Orlandi a , M. Stipcevic c , E. Tatananni a , I. Zamboni c a Laboratori Nazionali del Gran Sasso dell’INFN, Assergi (AQ), Italy b University and INFN Padova, Via Marzolo 8, Padova, Italy c Rudjer Boskovic Institute (IRB), Zagreb, Croatia d L’Aquila University, department of Physics, Via Vetoio 10 Coppito (AQ), Italy Available online 31 July 2007 Abstract OPERA is an underground neutrino oscillation experiment to search for n t appearance from a pure n m beam produced at CERN. To flag the events due to the neutrino interactions with the rock surrounding the OPERA detector, a large VETO system, based on the use of Glass Resistive Plate Chambers (GRPC) has been realized. We describe the detectors, the tests performed before the installation in the underground laboratories and the monitor system for the water pollution in the GRPC gas mixture. r 2007 Elsevier B.V. All rights reserved. 1. Introduction The CNGS (CERN Neutrinos to Gran Sasso) long baseline project is focused on the appearance of n t in a n m beam to explicitly prove the n m ! n t nature of the atmospheric oscillation. The OPERA experiment is placed in the Gran Sasso underground laboratory, 732 km from CERN. The CNGS beam is a wide band neutrino beam optimized for n t appearance with a mean neutrino energy of 17 GeV. The OPERA experiment [1] is based on the direct observation of the n t decay topology. The basic target unit is a brick of 10:2  12:7  7:5 cm 3 made of 56 lead plates(1 mm thick) and 57 emulsion films (about 200 000 bricks in total). The bricks are arranged in two modules, each one composed of 31 vertical walls. Planes of scintillator strips allow to localize the neutrino interactions and select the corresponding bricks. To identify muons and measure their momentum and charge, each target block is followed by a spectrometer equipped with 22 planes of Resistive Plate Chambers (RPC) with bakelite electrodes [2,3]. The measurement of the muon momentum is complemented by six sections of drift tubes for precision tracking through the magnetic field. A large number of CNGS neutrinos interact with rocks and concrete around the OPERA detector; part of secondaries issued from these interactions enter the detector, and may induce false triggers leading to a large amount of erroneous brick extractions and subsequent scanning. For this reason a VETO system based on the use of glass RPCs (GRPC), has been installed upstream of the OPERA apparatus. 2. The VETO system of the OPERA experiment Resistive Plate Chambers are gas detectors composed of two parallel plates generating a uniform electric field. When a particle ionizes the gas between the electrodes, an avalanche process occurs. If the electric field is intense enough, the avalanche reaches the critical size and generates the streamer, a plasma filament between the electrodes, producing an intense current pulse. As shown in Fig. 1, the RPCs of the VETO have glass electrodes 3 mm thick, with a volume resistivity of about 5  10 12 O cm at 20  C. The high voltage on the electrodes is distributed by means of a resistive coating (surface resistivity 400 kO=&). The distance between the spacers has been chosen taking into account the requirement of a minimal dead area and the effect of the deformation produced by the electric field and the gas pressure. In the ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.07.065 Ã Corresponding author. E-mail address: digiovanni@lngs.infn.it (A. Di Giovanni).