M. Ventura

Design and prototype tests of the RPC system for the OPERA spectrometers

The Inner Tracker system of the spectrometers of the OPERA experiment makes use of Resistive Plate Chambers (RPC) in a large-scale application. We present here the definition of the project and the full design of the Inner Tracker. Specific performances for the OPERA-RPC in the spectrometer are also reported. Particle detection, muon identification and trigger capability are discussed, in particular. Some results from test beam (T9 and T7 lines at the CERN PS) and measurements from laboratory test (CERN, Frascati, Padova, Gran Sasso) with prototype detectors are discussed, as well as specific solutions developed for the final set-up in OPERA. Full Monte Carlo simulations of the experimental set-up have been also developed.

Tests of OPERA RPC detectors

OPERA is part of the CNGS project and it is an experiment dedicated to the observation of long-baseline numu into nutau oscillations through tau appearance. Resistive Plate Chambers (RPCs) with bakelite electrodes are used to instrument the 2 cm gaps between the magnetized iron slabs of the two spectrometers. The RPC installation ended in may 2004 on the first spectrometer and in march 2005 on the second one. Before the installation, every RPC is subjected to a complete test chain in order to reject the poorer quality detectors. The tests are performed in dedicated facilities to ensure the proper RPC gluing, to measure its electrical properties and to verify the response to cosmic rays and the intrinsic noise levels. We have also tested the long term stability of real size OPERA RPC prototypes operated at cosmic ray fluxes for more than one year. On small size prototypes we are performing studies on the gas mixtures in order to reduce the total charge released in the gas for each detector count. The validation of the installed RPCs has been performed with pure nitrogen. A small part of them has been also tested with the gas mixture Ar/C2H2F4/i-C4H10 /SF6=75.4/20/4/0.6

Performance and aging of OPERA bakelite RPCs

OPERA is an experiment dedicated to the observation of νμ into ντ oscillations through τ appearance on the CNGS beam. The experiment is composed by two identical super-modules, each with a target section (made of emulsion/lead bricks alternated to a scintillator Target Tracker) and a muon spectromter (instrumented with bakelite electrodes RPCs and drift tubes). The OPERA RPC system is composed of about 1000 RPCs for an instrumented area of 3000 m2. The RPCs are operated in streamer mode and flushed with the gas mixture Ar/C2H2F4/i−C4H10/SF6 = 75.4/20.0/4.0/0.6 at five refills/day in open flow. The present performance of the RPC system after six years of operation are presented. The aging status of the detector is also described.

The OPERA spectrometers

The OPERA neutrino experiment at the underground Gran Sasso Laboratory was designed to perform the first detection of neutrino oscillations in the appearance mode, through the study of ν<inf>μ</inf> to ν<inf>Τ</inf> oscillations and subsequent measurement of the tau decay products.