1. Water vapour contamination in the gas mixtures plays the main role in the GRPC ageing [5], and must be kept at the level of hundred ppm or below. To control the contamination, a humidity monitoring system has been developed. It consists of 32 hygrometers (organized as shown in the figure above). The sensors have a sensitivity of about 10 ppm. Their calibration is performed with the setup shown in the figure. The plot shows the response of all the 32 sensors after the insertion of the plastic tube: it is apparent how the humidity “cloud” propagates from the first to the last sensor. Water Vapour Monitoring System 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 VETO GRPC the spacers are 20 cm apart (0,25% dead area, maximal glass sagitta ~10 microns) [2]. performed by automated microscopes, charged tracks will be reconstructed allowing for vertex and decay kink finding. Interspersed between the target bricks stacked in vertical walls, planes of scintillator strips [1] allow to localize on line neutrino interactions and select the corresponding bricks. There are two target blocks of 31 brick planes interleaved with 31 scintillator planes. To identify muons and measure their momentum and charge to fight charm background, each target block is followed by a spectrometer, composed of a dipolar magnet. It is equipped with 22 planes of Resistive Plate Chambers (RPC) with bakelite electrodes. The measurement of muon momentum is complemented by 6 sections of drift tubes for precision tracking through the magnetic field. During data taking, the bricks tagged by electronic detectors will be daily extracted by two automated manipulators. After film development, scanning stations will operate quasi on-line to fully complete data acquisition with the vertex location and the search of τ decay topology. Glass Resistive Plate Chambers in the OPERA experiment Resistive Plate Chamber are gas detector composed of two parallel plates generating a uniform electric field. When a particle ionizes the gas between the electrodes, an avalanche process occurs. When 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. Spacer Glass electrode Resistive coating HV insulator Read-out copper strip The VETO system is installed to reduce the extraction and analysis of bricks, in which fake events are generated by the interactions of neutrinos with the rock and the concrete around the OPERA detector. The VETO is based on the use of glass RPCs (GRPC). It consists of two 1004X923 cm 2 GRPC layers, each one made of 32 chambers organized into 8 raws. Each raw has 4 chambers, three of them are 2.60 m long and 1.14 m high, while the fourth is of 2.40 × 1.14 m 2. The VETO planes are equipped with horizontal and vertical copper strips, 3 cm wide, to pick-up the The CNGS (CERN Neutrinos to Gran Sasso) long baseline project is focused on the appearance of ν τ in a ν μ beam. Its objective is to prove explicitly the ν μ −ν τ nature of the atmospheric oscillation. The OPERA experiment is placed in the Gran Sasso underground laboratory, 730 km from CERN. The CNGS beam is a wide band neutrino beam optimized for ν τ appearance with a mean neutrino energy of 17 GeV. The OPERA experiment is based on the direct observation of the ν τ decay topology. The basic target unit is a brick of 10.2 x 12.7 x 7.5 cm made of 56 lead plates (1 mm thick) and 57 emulsion films (about 200 000 bricks in total). The emulsion films are made of a plastic base with two emulsion layers of 45 μm. After emulsion film scanning Oscillation Project with Emulsion tRacking Apparatus: an hybrid detector Test on Glass RPC: radiography of the chambers In the VETO RPC the electrodes are layers of float glass 3 mm thick, with a bulk resistivity of about 5 x 10^12 Ωcm @ 20˚C. The HV is supplied to the electrodes by means of a new resistive coating (surface resistivity ~ 400 kΩ/ □ ). Total charge is a function of the voltage. A large increase of the charge is observed when C 2 H 2 F 4 <48%, Iso-C 4 H 10 <4% because of the larger number of the afterpulses. Best performances are reached for Argon/iso-C 4 H 10 / C 2 H 2 F 4 /SF 6 = 47/4/48/1% [3]. NB: to easily compare the behaviour of different gas mixtures, the voltages are rescaled in such a way the efficiency curves can be superimposed with V knee =±3.9 kV. [3] C. Gustavino et al., Performance of glass RPC operated in streamer mode with four-fold gas mixtures containing SF 6, NIM A 517 (2004) 101-108 [4] C. Gustavino et al., The OPERA cosmic ray test facility at the Gran Sasso, NIM A 533 (2004) 221–224 [2] A. Di Giovanni et al., The VETO system of the OPERA experiment, Nucl.Phys.Proc.Suppl.158:40-43,2006 [1] Y. Gornushkin et al., The target tracker detector for OPERA experiment, Nuclear Science Symp. 2004 IEEE, 126 - 129 Vol. 1 Relief bubbler Exhaust bubbler hygrometer Flow resistor GRPC The VETO gas system is in stainless steel and tygon r2075 pipeline to minimize the water vapour content in the gas flowing through the detectors. Each wall has a distribution system with 8-fold parallel channels, each one equalized by a flow resistor (needle). Its impedance is about 300 times larger than that of four GRPC string. The relief bubbler protects the detectors from overpressure. At the beginning and at the end of each line, hygrometers (HiH-4602-C) are read out by a field point system. [5] A. Candela et al., Ageing and recovering of glass RPC, NIM A 533 (2004) 116-120 Polycarbonate frame The quality controls on the GRPCs of the OPERA experiment are made in a dedicated external hall at the LNGS [4]. Mechanical properties are first investigated in a fully automatized station, measuring the gas leakage and checking the gluing of the spacers. Then the response uniformity is tested, measuring the efficiency on the entire detector with cosmic rays. The test program is divided into three steps: - Average efficiency (1) and counting rate (2) as a function of gap voltage; - local RPC efficiency at the nominal working voltage (3): the position of the spacers is apparent. The Glass RPC construction The glue is applied by digital dispenser Spacer positioning on the glass The glue is placed on the glass The glue is applied over the spacers Superimposition of the glass layers The gap of the detector before the sealing “Vacuum gluing” (24 hours) Gas inlet and polycarbonate frame for detector sealing The detector is ready! (1) (2)  g (  m)  1/tg(α)  (%)  g(rms )  40  m (  g/g  2 %)  (rms)= 20% (3) (4) A.Candela a, M.D’Incecco a, A.Di Giovanni a, N.Di Marco a, C.Gustavino a, M.Lindozzi a, S.Micanovic b, D.Orlandi a, M. Stipcevic b, E.Tatananni a, I. Zamboni b a) Laboratori Nazionali del Gran Sasso-INFN,Assergi (L’Aquila),67010 Italy b) Rudjer Boskovic Institute (IRB), Zagreb, Croatia Mail to: adriano.digiovanni@lngs.infn.it signals produced by crossing particles. The Front End Boards (FEB) collect and transmit the signals to the data acquisition system. References Resistive Plate Chambers or RPC The VETO system of the OPERA experiment As the electric field is inversely proportional to the gap (E=V 0 /g), it is possible to estimate the gap uniformity by measuring the local efficiency well before the working point (4). As shown in the figure, the uniformity of the gap is limited by the float glass technology, at the level of 2%.