Tests of OPERA RPC detectors

Abstract

OPERA is part of the CNGS project and it is an experiment dedicated to the observation of long-baseline nu(mu) into nu(tau) 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

Measurement of the atmospheric muon charge ratio with the OPERA detector

“…The magnetic field is 1.53 T, directed vertically transverse to the neutrino beam axis. The RPC planes are inserted between the iron slabs: they provide the tracking inside the magnet and the range measurement for stopping muons [14].…”

The OPERA Spectrometer Slow Control System

“…The OPERA RPCs [5] are "standard" Bakelite RPCs, similar to those used in the LHC experiments: 2 two electrodes, made of 2 mm plastic laminate are kept 2 mm apart by polycarbonate spacers, of 1 cm diameter, in a 10 cm lattice configuration. Since the expected counting rate is very low [6], electrodes with high volume resistivity ( cm), are employed.…”

“…A detailed description of the OPERA RPC detectors, their characterization and long term operation results is given in [5].…”

“…Due to the chosen gas mixture, the RPCs are operated at a voltage of 5.8 kV with currents below 1 A [5]. The high voltage is split symmetrically between the positive and negative electrodes.…”

“…5 We use Linux Debian installations. 6 Electronic System Design, Gmbh, Hannover, Germany 7 CAEN, Costruzioni Apparecchiature Elettroniche Nucleari s.p.a., I-55049, Viareggio, Italy, http://www.caen.it HV box (with passive filters) which joins the three chambers and provides two output wires (positive and negative) which are connected to the power supply; • one SY127 negative channel is fed to a passive distributor (with one input and twenty four outputs) whose outputs are connected to twenty one RPC rows (i.e., three RPC planes); • one SY127 positive channel is fed to a custom ampere meter, the so-called i-meter [12], which distributes one input voltage to three RPC rows and measures the current flowing in each row, separately, with a nA resolution.…”