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

The OPERA experiment

• OPERA (CNGS) is a dedicated experiment for the detection of through appearance (baseline=730 km)• selection based upon topological criteria (decay vertex reconstruction with m precision using emulsion layers alternated to 1 mm thick Pb sheets (target section)

Pb

Emulsion layers

1 mm

CERNGran Sasso

kink

I.p.

• 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

The detector

beam

Muon spectrometer( P measurement from deflection through 24 magnetised iron slabs)

Target section

Sensitivity

signal

(m2 = 1.9 x 10-3 eV2)

signal

(m2 = 2.4 x 10-3 eV2)

signal

(m2 = 3.0 x 10-3 eV2)Background

OPERA

(1.8 kton)8.0 12.8 19.9 0.8

5 years @ 4.5x1019 pot / year (full mixing)

On schedule to start on summer 2006 with 1 supermodule !!

RPCs in the OPERA muon spectrometersBakelite 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

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)

RPCs installation completed since march 2005(A. Longhin talk)

RPCs in the OPERA muon spectrometers

Type B (with grooves to fit the skrews holding together the magnet)

1 layer = 21 RPCs of size (2.9*1.1) m²1 spectrometer = 462 RPCs for ~1500 m2 of detection area

8.75 meters

8 m

ete

rs

Type A (without grooves)

OPERA RPCs characteristics:1. Same technology developed for

LHC experiments, Babar and Argo2. 2 mm gas gap ensured by a lattice

of spacers with 10 cm pace3. High resistivity electrodes

cm4. Streamer operation regime (high

amplitude signals)

B type RPC

Experimental set-up of the long term test

Dedicated facility at the Gran Sasso external laboratory 1. Real size (2.9*1.1 m2) prototypes under test2. RPCs 1,2,3,4 equipped with 3 cm orthogonal strip panels 3. Gas flushed at 5 refills per day4. Copper piping one half of the external humidity in the gas5. Current monitoring with 0.1 A precision (power supply)6. Counting rate (300 Hz/m2) 10 times greater than underground rate

Long term operation test summary

# tested RPCs

Gas mixture

Ar/C2H2F4/i-C4H10

Time

(days)

Description HV (kV)

3 38/60/2 450 Old RPCs 7.7

76/20/4 + 0.7% SF6 130 5.9

3 38/60/2 365 OPERA-like RPCs 7.7

76/20/4 + 0.7% SF6 130 (improved oil coating) 5.9

6 76/20/4 + 0.7% SF6 240 Special test on local noise 5.7

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

First test results

Old RPCs with anomalous currents

OPERA preproduction RPC

Ar/TFE/i-But=38/60/2

Some improvement on one damaged RPC by:1. Doubling the gas flow2. Lowering the voltage (-200 V)3. Using a gas mixture with a lower streamer charge (SF6 addition)

T=30oC

Final test on RPC1 (A-type)

Some efficiency loss

Noise map (1 pixel = 3.5*3.5 cm2) Hot spot observed by self-triggering the detector

Single rate = 2.2 kHz/m2 (while 300 Hz/m2 for good RPCs)Ohmic current = 95 nA/kV (10 times lower for good RPCs)

350 Hz/pixel

Efficiency map (1 pixel = 10*10 cm2)

Final test on RPC2 (B-type)

Good efficiency Hot spots observed by self-

triggering the RPC

Both chambers with cm, the lowest among tested RPCsAging not due to Grooves

grooves

Single rate = 615 Hz/m2

Ohmic current = 14 nA/kV

60 Hz/pixel

Noise map (3.5*3.5 cm2 pixels)

RPC1 Autopsy

S. Dusini

10 cm

Spacer

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

RPC2 Autopsy

S. Dusini

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

Chemical analysis of damaged electrodes

Yellow wide zones contains 40 times more F than outside

Campione E (mV)A 129.1 3.32E-06 3A 103.3 9.25E-06 9A 115.9 5.60E-06 5B 121.8 4.43E-06 4B 104.0 8.99E-06 9B 149.5 1.47E-06 1C 116.5 5.47E-06 5C 123.4 4.16E-06 4C 120.2 4.72E-06 4D 22.4 2.31E-04 219D 26.9 1.93E-04 183D 22.5 2.30E-04 218E 18.3 2.71E-04 258E 18.1 2.74E-04 260E 20.2 2.52E-04 239

dev st

252

2.8

3.6

0.6

20

11

5

5

207

6

µg F- nel campione

Conc. F-

(mol/l)media

Chemical technique:10 cm2 samples scraped from the electrode surface, then solution inside water/TISAB

S. Dusini & A. Garfagnini

Anode and cathode yellow zones

Anode and cathode normal zones

Chemical analysis of damaged electrodes

White spots also contain F (SEM analysis on mm2 samples)

White spot

Outside white spot

7% weight

HF and surface resistivity

Surface resistivity measured on three samples cut from one RPC electrode, before and after different treatments

Sample n. Treatment i (G■) f (G■) fi

1 No treatment 35.1 ± 0.4 39.2 ± 0.5 1.12

2 Water 61 ± 1 77 ± 2 1.26

3 Water/HF=60/40 40.5 ± 0.4 11.56 ± .05 0.285

HF seems to lower the electrode surface resistivity and explain:1. The high ohmic current of damaged RPC12. The increase of the local discharge rate ?

2 cm

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

Quality Control tests on OPERA RPCs

Every RPC is tested in the Gran Sasso external laboratory before the installation:1. To reject defective RPCs2. To select very good RPCs for the most important layers

Mechanical tests:1. Gas leakage2. Proper gluing (select RPCs with up

to 2 unglued spacers if not adjacent)3. RPC curvature (select RPCs with less

than 8 mm sagitta)

Rejection=8%

Rejection=4%


Electrical tests:

(average bakeliteelectrodes resistivity)

dI/dV at low voltages

V100 (“primer” voltage at 100 nA)

Pure Argon

Moreover, measurement of the operating current and conditioning with the mixture Ar/C2H2F4/i-C4H10 = 76/20/4 +0.5% SF6


Cosmic rays test:1. Efficiency with respect to an external

trigger2. Global counting rate3. Local noisiness (hot spots) with self-

triggered runs of 50000 events

Normal RPC

RPC with hot spot

StripHot spot runs description:• each detector self-triggered• 50000 events runs• local rates scaling to global counting rate

Str

ip

Countings

1 pixel=3.5*3.5 cm2

QC tests results

Hot spot rateR (Hz/(3.5*3.5) cm2)

Reject RPCs with:• dI/dV > 20 nA/kV• I > 750 nA• R > 5 Hz/pixel

Stricter cuts for selecting very good RPCsLooser 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)

Operating current I (nA)

Ohmic currentdI/dV (nA/kV)

QC tests summary

Test A-type B-type

Mechanical tested 276 1077

Mechanical rejected 16 152

Rejection rate 6% 14%

Electrical tested 257 925

Electrical rejected 1 83

Rejection rate <1% 9%

Cosmic tested 215 808

Cosmic rejected 20 170

Rejection rate 9% 21%

B-type RPCs rejection rate higher in all the tests !!!

A. Garfagnini

From QC tests to detector knowledge: correlations with

electrodes resistivity

dI/dV vs dI/d

V (

nA/k

V)

I vs I

(nA

)

cm1. Currents and hot spot rates decreases with 2. Also highest dI/dV observed at low values3. RPCs with 2*1012 cm are hardly rejected by our tests…

R (

Hz/

(3.5

*3.5

) cm

2 )

cm

Hot spot rate vs

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)

Evolution of global detector parameters

RPC cm) Rate (Hz/m2)

1 1.25±0.06 @ 20oC 320 Hz/m2

2 1.64±0.07 @ 20oC 340 Hz/m2

3 0.36±0.01 @ 20oC 470 Hz/m2

4 0.78±0.02 @ 24oC 280 Hz/m2

5 0.40±0.01 @ 23oC 540 Hz/m2

6 1.48±0.11 @ 20oC 220 Hz/m2

All the currents below 1 AGood efficiency for RPC14 (5 and 6 not monitored)

From QC tests

During long term test @ 27oC

The noisiest RPCs are those with the lowest

Technical stop

RPC5

Hot spot evolution

Initial Hot spot rate ~ 5 Hz/pixel

Hot spot rate increases @ 27OC

Increase reversible with T

Strong increase only for RPC3, presenting the lowest !!

RPC1 RPC2

RPC3 RPC4

Hot spot rate (Hz/pixel)

F. Mastropietro

RPC3 results

1. The noisy zone increases with time

2. Noisiest pixel moves inside an area of 4*4 pixels

3. Only RPC with some slight aging effect (see next slide....)

F. Mastropietro

RPC3 results (II)

Average pixel rate on the 4*4 pixels zone

Average pixel rate outside

Seems more than 2 times higher than the beginning

Hot spot summary

1. Very little aging effects observed on OPERA RPCs. damping on hot spot rates (highest rates for lowest RPC)3. Temperature effects observed (reinforcing the previous statement) 4. Hot spot rate increases exponentially with voltage

Voltage decrease more effective on hot spot than on the rest of the detector

Global rate (Hz/m2)

Hot spot rate (Hz/pixel)

ConclusionsGood 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, -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 RPC

• No efficiency loss observed in the test on final OPERA RPCs (even if rejected by QC test)

The OPERA experimentOPERA is part of the CNGS project, and it is dedicated to the observation of oscillations through appearance over a long baseline (L=730 km) in the parameter region suggested by SK data on atmospheric neutrinos.

Lead/emulsions bricks (interaction vertex and decay reconstruction) alternated to scintillator strips (brick finding)

Muon spectrometers(with drift tubes for P measurement from deflection through 24 magnetised iron slabs)

Target mass=1.8 ktonGran Sasso underground laboratoryHall C

Tests on OPERA RPCs

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