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Tests on OPERA RPCs

Tests on OPERA RPCs. A. Paoloni (INFN-LNF) On behalf of the OPERA RPC group (Bologna, LNF, LNGS-L’Aquila, Napoli, Padova, Zagreb) VIII Workshop on Resistive Plate Chambers and Related Detectors Seul October 10-12, 2005. 1 mm. t. n. Pb. Emulsion layers. The OPERA experiment.

jeremy-castro
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Tests on OPERA RPCs

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  1. Tests on OPERA RPCs A. Paoloni (INFN-LNF) On behalf of the OPERA RPC group (Bologna, LNF, LNGS-L’Aquila, Napoli, Padova, Zagreb) VIII Workshop on Resistive Plate Chambers and Related Detectors Seul October 10-12, 2005

  2. 1 mm t n Pb Emulsion layers The OPERA experiment • OPERA (CNGS) is a dedicated experiment for the detection of nm nt through t appearance (baseline=730 km) • t selection based upon topological criteria (decay vertex reconstruction with mm precision using emulsion layers alternated to 1 mm thick Pb sheets (target section) kink Gran Sasso CERN I.p.

  3. Target section The detector n beam Muon spectrometer ( mP measurement from deflection through 24 magnetised iron slabs) • 2 Supermodules (target mass=1766 tons) • 2 magnetic spectrometers with drift tubes (PTs) and RPCs (inner tracker) for muon reconstruction (charm background rejection) • 2 x [31 Target Tracker (scintillator strips)/ Target Walls] • 206336 bricks (56 Pb sheets/emulsion layers) • 12 M emulsion sheets

  4. nmntSensitivity 5 years @ 4.5x1019 pot / year (full mixing) On schedule to start on summer 2006 with 1 supermodule !!

  5. RPCs in the OPERA muon spectrometers • Bakelite RPCs inside 2 cm gaps between the slabs (total 11+11 layers) • to trigger the spectrometer • to detect stopping muons • to measure the vertical coordinate • to give t0 to drift tubes RPCs installation completed since march 2005 (A. Longhin talk) Since RPCs in the spectrometers are not accessible during OPERA lifetime, strong efforts on long term operation and quality control tests Glass RPCs used on the VETO detector (A. Di Giovanni talk)

  6. RPCs in the OPERA muon spectrometers • OPERA RPCs characteristics: • Same technology developed for LHC experiments, Babar and Argo • 2 mm gas gap ensured by a lattice of spacers with 10 cm pace • High resistivity electrodes (r > 5*1011 Wcm) • Streamer operation regime (high amplitude signals) B type RPC Type A (without grooves) Type B (with grooves to fit the skrews holding together the magnet) 8 meters 1 layer = 21 RPCs of size (2.9*1.1) m² 1 spectrometer = 462 RPCs for ~1500 m2 of detection area 8.75 meters

  7. Experimental set-up of the long term test • Dedicated facility at the Gran Sasso external laboratory • Real size (2.9*1.1 m2) prototypes under test • RPCs 1,2,3,4 equipped with 3 cm orthogonal strip panels • Gas flushed at 5 refills per day • Copper piping  one half of the external humidity in the gas • Current monitoring with 0.1 mA precision (power supply) • Counting rate (300 Hz/m2) 10 times greater than underground rate

  8. Long term operation test summary Same bkg colour  successive tests • No severe damage (with loss of efficiency) observed • Some anomalous aging effect (increase of current and rate) observed only in 2 of the 3 old RPCs during the first test

  9. First test results T=30oC Old RPCs with anomalous currents Ar/TFE/i-But=38/60/2 OPERA preproduction RPC • Some improvement on one damaged RPC by: • Doubling the gas flow • Lowering the voltage (-200 V) • Using a gas mixture with a lower streamer charge (SF6 addition)

  10. Final test on RPC1 (A-type) Efficiency map (1 pixel = 10*10 cm2) Some efficiency loss Noise map (1 pixel = 3.5*3.5 cm2) Hot spot observed by self-triggering the detector 350 Hz/pixel Single rate = 2.2 kHz/m2 (while 300 Hz/m2 for good RPCs) Ohmic current = 95 nA/kV (10 times lower for good RPCs)

  11. grooves Final test on RPC2 (B-type) Good efficiency Hot spots observed by self-triggering the RPC Noise map (3.5*3.5 cm2 pixels) Single rate = 615 Hz/m2 Ohmic current = 14 nA/kV 60 Hz/pixel Both chambers with r<1012 W cm, the lowest among tested RPCs Aging not due to Grooves

  12. RPC1 Autopsy 10 cm Spacer S. Dusini

  13. RPC1 Autopsy Remarks: • The yellow is present on both anode and cathode • The white spots are only on the anode • In correspondence on the cathode small Linseed oil droplets S. Dusini

  14. RPC2 Autopsy S. Dusini

  15. Autopsy of RPC6 • No yellow regions found • No anomalous current and rate • White spots found on the anode in the correspondence of the “Hot Spots” on the histogram S. Dusini

  16. Chemical analysis of damaged electrodes Chemical technique: 10 cm2 samples scraped from the electrode surface, then solution inside water/TISAB Anode and cathode normal zones Anode and cathode yellow zones Yellow wide zones contains 40 times more F than outside S. Dusini & A. Garfagnini

  17. Chemical analysis of damaged electrodes White spots also contain F (SEM analysis on mm2 samples) 7% weight White spot Outside white spot

  18. HF and surface resistivity 2 cm Surface resistivity measured on three samples cut from one RPC electrode, before and after different treatments • HF seems to lower the electrode surface resistivity and explain: • The high ohmic current of damaged RPC1 • The increase of the local discharge rate ?

  19. Considerations from the first long term test • Detectors at low rate in streamer  aging induced by intrinsic local defects (we call them hot spots) • HF seems to play some role (but which ?) • Similar damaging observed by Babar • Introduce hot spot test in QC tests • Long term test on 6 OPERA RPCs discarded because of hot spots

  20. Quality Control tests on OPERA RPCs • Every RPC is tested in the Gran Sasso external laboratory before the installation: • To reject defective RPCs • To select very good RPCs for the most important layers • Mechanical tests: • Gas leakage • Proper gluing (select RPCs with up to 2 unglued spacers if not adjacent) • RPC curvature (select RPCs with less than 8 mm sagitta) Rejection=4% Rejection=8%

  21. Quality Control tests on OPERA RPCs Electrical tests: r (average bakeliteelectrodes resistivity) Pure Argon dI/dV at low voltages V100 (“primer” voltage at 100 nA) Moreover, measurement of the operating current and conditioning with the mixture Ar/C2H2F4/i-C4H10 = 76/20/4 +0.5% SF6

  22. Quality Control tests on OPERA RPCs • Cosmic rays test: • Efficiency with respect to an external trigger • Global counting rate • Local noisiness (hot spots) with self-triggered runs of 50000 events 1 pixel=3.5*3.5 cm2 Normal RPC RPC with hot spot Strip • Hot spot runs description: • each detector self-triggered • 50000 events runs • local rates scaling to global counting rate Strip Countings

  23. QC tests results • Reject RPCs with: • dI/dV > 20 nA/kV • I > 750 nA • R > 5 Hz/pixel • Stricter cuts for selecting very good RPCs • Looser cuts on electrical parameters at T>24 oC (to account for T dependence) • Tested ~ 1400 RPCs • Rejection factors: • 13% on A-types (no grooves) • 38% on B-types (with grooves) Ohmic current dI/dV (nA/kV) Operating current I (nA) Hot spot rate R (Hz/(3.5*3.5) cm2)

  24. QC tests summary B-type RPCs rejection rate higher in all the tests !!! A. Garfagnini

  25. From QC tests to detector knowledge: correlations with electrodes resistivity dI/dV (nA/kV) dI/dV vs r Hot spot rate vs r R (Hz/(3.5*3.5) cm2) I (nA) I vs r r (1012 Wcm) r (1012 Wcm) • Currents and hot spot rates decreases with r • Also highest dI/dV observed at low r-values • RPCs with r>2*1012 Wcm are hardly rejected by our tests…

  26. Long term operation test on RPCs with hot spots • 3 old prototypes + 3 preproduction RPCs replaced in the long term test facility • 6 OPERA RPCs rejected by hot spot QC tests under test (240 days) • 4/6 piled up and equipped with orthogonal strips  efficiency and noise map measurements Gas mixture Ar/C2H2F4/i-C4H10=76/20/4 + 0.7% SF6 V=5.7 kV (rescaled to T0=293 K and P0=900 mbar)

  27. Evolution of global detector parameters All the currents below 1 mA Good efficiency for RPC14 (5 and 6 not monitored) RPC5 Technical stop From QC tests During long term test @ 27oC The noisiest RPCs are those with the lowest r

  28. Hot spot evolution Hot spot rate (Hz/pixel) RPC2 RPC1 Initial Hot spot rate ~ 5 Hz/pixel Hot spot rate increases @ 27OC Increase reversible with T Strong increase only for RPC3, presenting the lowest r !! RPC3 RPC4 F. Mastropietro

  29. RPC3 results • The noisy zone increases with time • Noisiest pixel moves inside an area of 4*4 pixels • Only RPC with some slight aging effect (see next slide....) F. Mastropietro

  30. RPC3 results (II) Average pixel rate on the 4*4 pixels zone Seems more than 2 times higher than the beginning Average pixel rate outside

  31. Hot spot summary • Very little aging effects observed on OPERA RPCs • r-damping on hot spot rates (highest rates for lowest r RPC) • Temperature effects observed (reinforcing the previous statement) • Hot spot rate increases exponentially with voltage Global rate (Hz/m2) Voltage decrease more effective on hot spot than on the rest of the detector Hot spot rate (Hz/pixel)

  32. Conclusions Good understanding of aging phenomena from both long term operation and QC tests: • Aging effects observed only on old RPC prototypes and due intrinsic chamber defects (hot spots), with very high local rate and efficiency loss • Found HF on damaged electrodes from old RPC prototypes (Babar-like) • QC tests designed to check the presence of hot spots and ensure the good quality of each installed OPERA RPC • From QC test data on 1400 RPCs, r-damping of currents and hot spot rates • Long term test with hot spot monitoring: local aging effects ? If yes, localized and only on the lowest r RPC • No efficiency loss observed in the test on final OPERA RPCs (even if rejected by QC test)

  33. The OPERA experiment OPERA is part of the CNGS project, and it is dedicated to the observation of nmntoscillations through t appearance over a long baseline (L=730 km) in the parameter region suggested by SK data on atmospheric neutrinos. Target mass=1.8 kton Gran Sasso underground laboratory Hall C Muon spectrometers (with drift tubes for mP measurement from deflection through 24 magnetised iron slabs) Lead/emulsions bricks (interaction vertex and t decay reconstruction) alternated to scintillator strips (brick finding)

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