An extensive aging study of bakelite RPC

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An extensive aging study of bakelite RPC

• Danilo Domenici
• University of Rome
• Tor Vergata and INFN

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Overview

• Historical review of detector characteristics
  and performances
• Test setup at Gamma Irradiation Facility (CERN)
• Measuring the bakelite resistivity
• Effects of aging on resistivity
• Flowing of humid gas
• Effects of aging on rate capability

Pisa Meeting May 27, 2003
Danilo Domenici
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Some history

• In 1998 RPCs were proposed for LHCb Muon
  Detector, in the regions with a maximum flux
  density of 750 Hz/cm2
• In 1999 2 RPCs prototypes were built with
  identical characteristics
• bakelite electrodes with bulk resisistivity ?
  1010 Ocm
• 50 x 50 cm2 sensitive area
• The rate capability of these detector was
  initially measured to be gt 3
  kHz/cm2 (NIM A 456 (2000) 95)
• In 2001 an extensive test started at GIF to
  study aging effects on rate
  capability

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Test setup at GIF
Source characteristics 662 KeV ? from
740 GBq 137Cs Counting rate 1?3 kHz/cm2 at
1 m from the source
2001 A in pos 1, B in pos 2002-2003 both in
pos 1
1 aging position 2 reference position 3
testbeam position

• The irradiation is measured by the accumulated
  charge per cm2 (Qacc) we had previously shown
  that the resistivity of bakelite slabs is not
  affected by pure irradiation up to doses of 20
  kGy
• Since the rate capability is inversely
  proportional to ?, this has been the monitored
  quantity

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Measurement of resistivity

• We have developed a simple method to measure ?
  continuously at the GIF
• It requires the detector to be exposed to large
  flux of radiation
• The model is described in G. Carboni et al. NIM
  A 498 (2003) 135
• It is based on the hypothesis that all the
  physical properties of an RPC
  must depend on

Vgap V0 - RI
S
d

• R depends exponentially on the temperature
• For bakelite we measured the value a 0.12
  0.01

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Measurement of resistivity
Current saturation with flux
Current linearity with HV

• Prediction of the model

• For fixed V0 the current depends exponentially
  on the
  temperature through R

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Current vs time
Current drawn at 10800 V corrected by temperature
8 months of irradiation 0.4 C/cm2 acc. charge
(RPC A) In the same period RPC B was used as
reference and accumulated only 0.05 C/cm2

• I decrease by a factor 6
• ? increase by the same factor

Qacc0.4 C/cm2
jan 01
aug 01
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? vs time 1999-2001
irradiating

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

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? vs time 2002

• Test both detectors now installed close to the
  source to measure ?
  continuously
• Only 0.05 C/cm2 accumulated charge (large ? ?
  low current)
• Both detectors show a steady increase of ? with
  time
• Hypothesis drying up of bakelite (see also I.
  Crotty et al., NIM A 346, 1994)

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? vs time 2003 humid gas
no flow
humid flow

• 1.2 of vapor H2O added to the usual gas mixture
  
• Clear effect of humid gas, (see also ALICE 2002
  results) but
• on RPC B there is a sharp decrease of ?
• on RPC A the effect is much reduced
• ? rapidly resume to old values when dry gas flow
  is restored

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Effects on rate capability

• We define 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)

The detectors were tested at GIF by means of X5
muon beam
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GIF test August 2001
Resistivity at 20 ºC ?A 39 x 1010 Ocm Rate
capability at 20ºC 640 Hz/cm2
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GIF test July 2002
Resistivity at 20 ºC ?A 110 x 1010 Ocm Rate
capability at 20ºC 200 Hz/cm2
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Testbeam summary

• Except for the 1999 data (affected by large
  uncertainties) the values are in agreement with
  the 1/? dependence of rate capability

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Conclusions

• A method has been developed and applied to
  measure RPC bakelite resistivity on-line, during
  chamber operation
• Aging effects on bakelite RPCs have been
  extensively studied for 3 years on two
  identical detectors with building resistivities
  around 1010Ocm
• After 2 years operation ? had already increased
  to 100 x 1010Ocm reaching the value of 200 x
  1010Ocm at the end of the third year
• Although irradiation contributes to the
  resistivity increase, we
  believe 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, whereas in
  RPC A the effect is very much
  reduced
• Restoring dry gas flow has resulted again in
  fast resistivity increase
• Flow of humid gas doesnt appear to be a
  practical method to recover detector
  performances
• Rate capability dropped from few kHz/cm2
  to
  about 200 Hz/cm2 ? no RPCs in LHCb!

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