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Final results from an extensive aging test on
bakelite Resistive Plate Chambers Stefano de
Capua University of Rome II Tor Vergata and
INFN
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Outlook

• Rate Capability vs. bakelite resistivity
• Aging studies at the Gamma Irradiation
  Facility (CERN)
• Measurements of the bakelite resistivity
• Effects of humidity on bakelite resistivity
• Conclusions

VII RPC Workshop - October 21, 2003 2/15

Stefano de Capua
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Historical review

• Resistive Plate Chambers were proposed in the
  LHCb Muon Detector, in the regions with a
  maximum flux density of 750 Hz/cm2.
• Two RPC prototypes (A and B) were built with
  identical characteristics
• - bakelite electrodes with bulk resitivity r
  1010 Wcm
• - sensitive area 50x50 cm2
• The rate capability of these detectors was
  initially measured to be Rcap gt 3 kHz/cm2
  (NIM A 456 (2000) 95)
• An extensive test started at the Gamma
  Irradiation Facility to study aging effects
  on the Rate Capability.

VII RPC Workshop - October 21, 2003 3/15

Stefano de Capua
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Rate Capability vs. bakelite resistivity
We defined a RPC detector capable to stand a
given rate if - efficiency gt 95 (trigger
requirement) - at least 400 V
plateau (safety requirement) - HV lt 11000
(streamer limitation)
Testbeam measurements (X5 muons beam)
T25.0 oC
T24.5 oC
July 2002 ?A 110 x 1010 Ocm _at_ 20 oC Rcap
200 Hz/cm2 _at_ 20 oC
August 2001 ?A 39 x 1010 Ocm _at_ 20 oC Rcap
640 Hz/cm2 _at_ 20 oC
VII RPC Workshop - October 21, 2003 4/15

Stefano de Capua
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Monitoring the RPC resistivity during irradiation

• We used a simple method to measure the bakelite
  resistivity in the detector continuosly
  during the GIF test (G. Carboni et al., NIM A 498
  (2003) 135)
• The model is based on the hypothesis that
  all the physical properties of an RPC must
  depend on the quantity
• Vgap V0 RI
• and it requires the detector to be exposed
  at a large flux of radiation.

radiation
S
d
If F ? ? then R DV/DI
r is strongly affected by temperature r20 r
ea(T-20)
Current saturation with F
Current linearity with HV
VII RPC Workshop - October 21, 2003 5/15

Stefano de Capua
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Aging test in 2001

• The gas mixture was C2H2F4/i-C4H10/SF6 (96/4/1)
  _at_ 1l/h (steel tubes).
• During the first seven months the irradiated
  detector (RPC A) accumulated a charge Qint
  0.4 C/cm2 (3.5 LHCb years).
• The reference dectector (RPC B) accumulated
  only 0.05 C/cm2.

irradiation

• Large resistance increase for RPC A
• Evidence of increase not related to
  irradiation for both

• The temperature coefficient a was measured
  (a0.12) and found in agreement with our
  other measurements performed with different
  bakelite samples. This result confirm the
  validity of the model.

VII RPC Workshop - October 21, 2003 6/15

Stefano de Capua
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Aging test in 2002

• Both detectors now installed close to the
  source to measure ? continuously
• Only 0.05 C/cm2 accumulated charge
• Both detectors show a steady increase of ? with
  time

RPC A
RPC B
? Hypothesis resistivity increasing due to
drying up of bakelite
VII RPC Workshop - October 21, 2003 7/15

Stefano de Capua
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Aging test in 2003 effects of humid gas
RPC A
RPC B

• humid flow 1.2 of vapor H2O added to the
  usual gas mixture
• Clear effect of humid gas, but on RPC B there
  is a sharp decrease of resitivity on
  RPC A the effect is much reduced
• But ? rapidly restores to old values when dry
  gas is flowed

VII RPC Workshop - October 21, 2003 8/15

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Study of bakelite resistivity
In 2003 a test on bakelite resistivity was
performed. The test started in August and went on
60 days.
A thermostatic chamber was used to control the
temperature
Two bakelite samples 10x10 cm2 were tested
A vacuum pump was used to keep samples in a
vacuum bell.
Courtesy of G. Passaleva
VII RPC Workshop - October 21, 2003 9/15

Stefano de Capua
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Temperature coefficient (1)
Sample A
Sample B
r(GWcm)
r(GWcm)
T(oC)
T(oC)
r20(GWcm)
r20(GWcm)
days
days
The temperature was increased from 15 oC up to
30 oC in not conditioned enviroment
(humidity50). a was measured and we found a
consistent value with values previously measured.
a?0.14?0.02
a?0.12 ?0.02
VII RPC Workshop - October 21, 2003
10/15
Stefano de Capua
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Temperature coefficient (2)
Sample A
Sample B
r(GWcm)
r20(GWcm)
T(oC)
T(oC)
r(GWcm)
r20(GWcm)

• The temperature was increased from 15 oC up to
  40 oC in 100 humid air enviroment.
• was measured and we found a different value.

a?0.10?0.02
a?0.09 ?0.02
VII RPC Workshop - October 21, 2003
11/15
Stefano de Capua
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Temperature correction
Temperature was increased from 15 oC up to 40 oC.
Each step lasted about 1 day.100 humid air
enviroment.
Resistivity corrected using aA 0.14 Still
depending on temperature
Resistivity corrected using aA 0.10 Clear
dependance of the temperature coefficient on
humidity percentage(see also Arnaldi et al., NIM
A 456 (2000) 142)
Sample A
35 less in 2 days
?
Hypothesis since the dependance on T was
corrected, the resistivity decrease depended on
the progressive water absorption.
VII RPC Workshop - October 21, 2003
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Stefano de Capua
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Effects of humidity
Sample A
Sample B
Sample A left in the thermostatic chamber with
100 humid airand constant T ? r decreased
rapidly
But, when the thermostatic chamber was opened,
the previous r value was restored
VII RPC Workshop - October 21, 2003
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Stefano de Capua
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Drying up in vacuum
Sample A
Sample B
When the vacuum was created,r increased rapidly
.A silica gel was used to absorb humidity.
The bakelite resistivity depends strongly on the
percentage of water in its enviroment.
VII RPC Workshop - October 21, 2003
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Stefano de Capua
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Conclusions

• Aging effects on bakelite RPCs have been
  extensively studied for 3 years on two
  identical detectors with resistivities around
  1010Ocm.
• After 2 years operation, ? increased to 100 x
  1010Ocm reaching the value of 200 x 1010Ocm
  at the end of the third year.
• Although irradiation (0.4 C/cm2) contributes to
  the resistivity increase, the effect is
  mainly related to dry gas flow.
• Humid gas has been flowed with different
  response -
  RPC B shows a sharp decrease of resistivity -
  RPC A shows an effect very much reduced
• Restoring dry gas flow has resulted again in a
  fast resistivity increase.
• A study of resistivity was performed on two
  bakelite samples.
• The temperature coefficient showed a clear
  dependance on humidity percentage.
• The measurements confirmed the strong
  dependance on the enviroment conditions of
  the bakelite resistivity, especially on the
  humidity percentage.
• Flow of humid gas does not appear to be a
  practical method to recover detector
  performances. A better solution could be a
  continous flow of humid gas.

VII RPC Workshop - October 21, 2003
15/15
Stefano de Capua