Carlo Gustavino INFN-LNGS On behalf of the OPERA RPC group IX International Workshop on Resistive Plate Chambers and Related Detectors Mumbai, February 2008 OUTLINE: The OPERA experiment at GRAN SASSO The VETO SYstem Glass RPC construction Glass RPC validation Veto Layout The Monitoring system First commissioning results Conclusions The OPERA VETO SYSTEM

The OPERA experiment is focused on the appearance of  in a  beam. It is placed in the Gran Sasso underground laboratory, 730 km from CERN, where the CNGS  Beam is produced. OPERA is based on the direct observation of the  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 150,000 bricks in total). After emulsion film scanning performed by automated microscopes, charged tracks are reconstructed allowing for vertex and decay kink finding. Planes of scintillator strips allow to localize neutrino interactions and select the corresponding bricks. Oscillation Project with Emulsion tRacking Apparatus: an hybrid detector 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,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.

The VETO system is installed in front of the OPERA experiment, 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 2.40x1.14 m 2. The VETO planes are equipped with horizontal and vertical copper strips, 3 cm wide, to pick-up the signals produced by crossing particles. The electronics is the same used for the Specrometer RPC (see DUSINI talk). The VETO system of the OPERA experiment

Glass RPC (GRPC) In the VETO RPC the electrodes are layers of float glass 3 mm thick, with a bulk resistivity of about 5 x 10^12  20˚C. The HV is supplied to the electrodes by means of a new resistive coating (surface resistivity ~ 400 k  √  In the VETO GRPC the spacers are 20 cm apart (maximal glass sagitta ~10 microns, ~50 spacers/GRPC). The gas inlets and the frame are in polycarbonate. The geometrical dead area of a 2.60x1.14 m 2 GRPC is about 2%. Spacer Glass electrode Resistive coating HV insulator Read-out copper strip Polycarbonate frame

The Glass RPC construction Spacer positioning on the glass The glue is applied by digital dispenser 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!

The quality controls were about the same as for spectrometer RPC. 1) Leakage test: the GRPC are filled at 6 mbar. All the chambers that show a pressure decrease > -0.05mbar/min are discarded. The test is able to detect spacers which “pop-up” during the gas filling. 2) Pushing test: to check if all the spacers are properly glued. 3) Single counting rate test: GRPC showing “noisy” spots are discarded. 4) Efficiency test. GRPC with efficiency lower than 95% or with low efficiency spots are discharged. FINAL RESULT GRPC CONSTRUCTED: 114 LEAKAGE TEST: 114 SPACER GLUING: 70 HOT SPOT: 70 EFFICIENCY (GLOBAL AND LOCAL) 65 Almost all the discarded GRPCs were rejected because of gluing problems, in spite of the high nominal and measured strength of the glue. Furthermore, almost all the discarded GRPCs were coming from 2 consecutive batches. The Glass RPC Validation Popped spacers

A special test has been performed on few GRPCs to estimate the gap uniformity. 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 efficiency plateau. The uniformity of the gap seems limited by the float glass technology, at the level of 2%. It is worth to point out that the “float” side of glass electrodes is the external one in our GRPC, to avoid residual of tin inside the gap. GAP uniformity test

Relief bubbler Exhaust bubbler Hygrometer Flow resistor (needle) GRPC Each VETO wall has a distribution system with 8-fold parallel channels, each one equalized by a flow resistor (needle). Its impedance is about 100 times larger than that of four GRPC string. At the beginning and at the end of each line, hygrometers (HiH-4602-C) are read out by a field point system. Flow rate is monitored with a pressure transducer. Nominal flow rate VETO needle VETO raw VETO gas system Pressure transducer

Glass RPC are relatively robust but can be damaged by water vapour in the gas mixtures, that must be kept at the level of hundred ppm or below. For this reason The VETO gas system is in stainless steel and tygon R2075 pipeline to minimize the water vapour content. The amount of water vapour inside the chamber depends on: Environmental humidity Flux intensity Pipe length H 2 0 Flushing (no efficiency loss!) H 2 0 Flushing (efficiency loss) Dry Flushing 8 hours of ammonia Flushing Gas failure FLUX[cc/min]Tygon r2075 Water ppm Tygon r3603 Water ppm Water Vapour Monitoring System 1 m long plastic pipeline mesurements

32 VETO Sensors Sensor Honeywell HiH-4602-C consists of a planar capacitor. It exploits the high dielectric constant of water Calibration set-up Reference sensor MTS5(GE Panametrics) Calibration of humidity sensors

Hygroscopic Plastic tube Reference Hygrometer Exhaust Dry gas Hygrometer Flow rate= 70 cc/min Absolute error<20 ppm Relative error(r.m.s.)~ 2 ppm Linear response for H 2 O>100 ppm Field point system read-out Response of hygrometers after the insertion of a hygroscopic plastic tube Calibration of humidity sensors

VETO humidity monitor GRPC raw output (16 ch.) Field point system read-out is used The humidity level strongly depends on the flow rate. At ~ 60 cc/min/row: ~ 200 ppm (GRPC row input) ~ 800 ppm (GRPC row output) …slowly drying with time N.B. the monitor curves are not yet corrected with the calibration curves GRPC raw input (16 ch.) VETO flow rate Vs time Water pollution Vs time

Time [min] Currents [nA] The current of the OPERA Veto System is continuosly monitored (the 1 nanoAmperometer every 2 GRPC), with the possibility of measuring the current of single chamber. The conditioning is normal : the currents are rapidly going down with time. Presently the currents are about 80 nA/m 2 on the average (HV=5,8 kV, argon/TFE/IsoC 4 H 10 /SF 6 =74,4/20/4/0,6). VETO: monitor of currents First day of underground operation =200 nA/m 2 =140 nA/m 2 (After 24 hours)

VETO commissioning First Internal neutrino detected by OPERA (no VETO hits)

VETO commissioning External neutrino interaction detected by OPERA (VETO Flag)

The VETO system, based on the use of Glass RPC, has been implemented on the OPERA experiment and is presently under commissioning. The preliminary results of the VETO system are very satisfying. As the OPERA spectrometers are instrumented with bakelite RPC, OPERA represent a unique possibility to compare bakelite/glass performance in the same working condition. For the first time, Glass RPCs have been built in an external firm.This is an important step for future large experiment. Conclusions

SPARE SLIDES

No SF6 Single streamer charge Total charge TFE or C 4 H 10 poor single streamer peak Multistreamer peaks The afterpulses increases for TFE<48% and/or Isobutane<4% With SF 6 the peak charge is 1/3 than w/o it.