Ageing tests and gas analysis

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VII Workshop on Resistive Plate Chambers and
Related Detectors Clermont-Ferrand 20-22 October
2003
Ageing tests and gas analysis of Resistive
Plate Chambers for the trigger of the ALICE
dimuon arm Giovanna Travaglia,
INFN-Torino ALICE Collaboration
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ALICE detector
A Large Ion Collider Experiment
LHC-CERN 2007 p-p,Ar-Ar,Pb-Pb 5.4-14
TeV per nucleon pair
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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The Dimuon Trigger System
Trigger system has to identify muon pair from J/?
and ? decay
Trigger detectors requirements

• large detection area
• muon detection efficiency ?95
• low sensibility to ? and neutrons
• time resolution 1-2 ns
• good granularity (210-3 hits/cm2)
• rate capability ?100 Hz/cm2 (max 20 Hz/cm2
  Ar-Ar)
• spatial resolution better than 1 cm

6.5 m
Single-gap Resistive Plate Chambers (General
Tecnica) Four planes, 72 RPCs in all Each module
is 70x280 cm2, 2 mm gas gap
5.5 m
See B. Forestier talk
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Results of performance studies

• Streamer mode
• Low-resistivity bakelite (PANPLA) ? ? 109 ??cm
• Low-gain gas mixture 49Ar-40C4H10-7C2H2F4-4SF
  6

R.Arnaldi et al., Nucl. Instr. And Meth. 451
(2000) 462 R.Arnaldi et al., Nucl. Instr. And
Meth. 457 (2001) 117 R.Arnaldi et al., Nucl.
Instr. And Meth. 490 (2002) 51
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Ageing tests at GIF
Ageing proportional to integrated current
All chambers 50x50 cm2

• ?-induced counting rate 100 Hz/cm2, operating
  voltage 200 V above knee
• Current, rate and efficiency continuously
  monitored with source on
• periodical
  data-taking with source off

RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Early ageing studies
We observe an increase of dark ( source off)
current and rate Noticeable improvement
with Double linseed oil coating Reduced content
of SF6 (and of operating voltage)
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Comparison among four chambers

• Double oiled, dry mix wet mix at 70 mC/cm2 and
  at 330 mC/cm2
• Triple oiled-1, wet mix dry mix since 95 mC/cm2
• Triple oiled-2, wet mix dry mix since 82 mC/cm2
• Re-oiled, dry mix
• (first life 17 mC/cm2 with 4 SF6 dry mix and 55
  mC/cm2 with 1 SF6 dry mix,
• at the end, the dark current was 250 ?A and the
  single rate 4 Hz/cm2)

• 50.5Ar-41.3C4H10-7.2C2H2F4-1SF6
• gas flow at 1.8 l/h (3.6 volumes/h)
• Gamma-induced counting rate between 70 and 100
  Hz/cm2
• Operating voltage 200 V above efficiency knee
  (8000-8100 V)

RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Dark current
Dark current (?A)
In all prototypes, current is not justified by
counting rate always lower than 5 Hz/cm2
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Efficiency
Efficiency
Efficiency
Efficiency stable up to about 50 mC/cm2 for all
chambers.
Ttriple oiled chamber-1 HV 8100 V
Ttriple oiled chamber-2 HV 8000 V
Integrated current
Integrated current
For values of the integrated current ? 50
mC/cm2, some chambers show unstable
performances and decrease of the efficiency.
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Dark current origin
The hypothesis is that the increase of the dark
current is due to a chemical deterioration of the
inner electrode surface due to polluting
molecules formation caused by discharges. We
investigated the behaviour of the double-oiled
chamber, after the prolonged ageing, 330 mC/cm2,
in lab. Current drawn by the chamber with the 1
SF6 streamer mixture at 8200 V reaches 150 ?A
and counting rates are of the order of few
Hz/cm2. Current passes through the gas chamber
operated with a 5 SF6 avalanche gas mixture
draws 30 ?A at 12000 V and 30 ?A at 8200
V. Current is not due to underthreshold
discharges discrimination thresholds at 80 mV or
at 5 mV give rates not different enough to
justify the phenomenon.
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Local efficiency decrease
On the huge aged chamber, we measured the
efficiency for cosmic rays in lab all active
surface mapped (tracking in 2X2 cm2 cells)
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Electrode surface analysis

• We opened a single-oiled chamber operated during
  early ageing test.
• It has integrated 30 mC/cm2.
• Inner face of the electrodes, in simmetric
  positions indipendently of polarity,
• are deteriorated.
• EST-SM team from CERN performed chemical analysis
  of the electrodes
• Carbon, Oxigen and Silicon are present on the
  examined samples
• Fluorine and Sulphur, not present on a oiled
  bakelite plate,
• contaminate all the surface of the electrodes

RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Exhaust gas analysis

• We took samples of exhaust gas from 2 chambers
• a chamber already aged (dark rate 1 Hz/cm2,
  dark current ? 100 ?A)
• a new chamber (dark rate 0.05 Hz/cm2, dark
  current ? 1 ?A)
• with different dry gas mixture (3.6 volumes/h)
• 50.5Ar-41.3C4H10-7.2C2H2F4-1SF6
• 50.91Ar-41.56C4H10-7.27C2H2F4-0.26SF6
• both with source on and source off.

EST-SM/CP team at CERN measured HF content and
impurity percentage with respect to reference
mixture taken at the entrance of the chambers.
See C. Petitjean poster
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Gas analysis results New chamber
Impurity production with the source off is almost
zero. Impurity percentage divided by counting
rate has a linear dependence with the operating
voltage. This may indicate that the beneficial
effect of SF6 reduction comes mainly from the
associated H.V. reduction.
Source on
Impurities /rate
0.26 SF6 1 SF6
High Voltage
HF is present in sample taken with the source on
tens of ?g per liter of gas. Measurement
precision is lower than the one obtained with gas
chromatograms.
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Gas analysis results Aged chamber
Impurity production by current under irradiation
is about 3 times impurity production by dark
current.
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Life-time requirements at ALICE
Possible scenario for the first 4 years at LHC
Max rate is only in the central part of the
detector.
RPC2003, Clermont-Fd 20-22 October
G.Travaglia
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Conclusions

• All chambers work satisfactorily up to 50 mC/cm2
  (ALICE ion program)
• After 50 mC/cm2 we observe unstable performance,
  with important increase of the dark current and
  small increase of counting rate
• The dark current passes through the gas
• Polluting substances in exhaust gas (HF ???)
  with a percentage
• that depends on the counting rate and on the
  operating voltage
• After ageing, inner electrode surface is damaged
  and contaminated by
• Sulphur and Fluorine
• Life-time improvement with a double linseed oil
  layer and with reduced SF6 content
• For very huge ageing, we noted a local decrease
  of the efficiency, connected to linseed oil
  remotion

RPC2003, Clermont-Fd 20-22 October
G.Travaglia