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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
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
Historical review
1998. Resistive Plate Chambers were proposed in the LHCb
Muon Detector, in the regions with a maximum flux
density of 750 Hz/cm2.
1999. Two RPC prototypes (A and B) were built with identical
characteristics:
10
- bakelite electrodes with bulk resitivity r ~ 10 Wcm
- sensitive area ~ 50x50 cm2
2000. The rate capability of these detectors was initially
measured to be Rcap > 3 kHz/cm2 (NIM A 456 (2000) 95)
2001. An extensive test started at the Gamma Irradiation
Facility to study aging effects on the Rate Capability.
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Rate Capability vs. bakelite resistivity
We defined a RPC detector capable to stand a given rate if:
- efficiency > 95% (trigger requirement)
- at least 400 V plateau (safety requirement)
- HV < 11000 (streamer limitation)
Testbeam measurements (X5 muons beam)
T=25.0 oC
T=24.5 oC
July 2002
August 2001
ρA
= 39 x 1010 Ωcm
Rcap ~ 640 Hz/cm2
@ 20 oC
ρA
@ 20 oC
Rcap ~ 200 Hz/cm2
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= 110 x 1010 Ωcm
@ 20 oC
@ 20 oC
Stefano de Capua
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.
If
radiation
S
R
d
2d
ρ
S
F
then R = DV/DI
r is strongly affected
by temperature:
r20 = r ea(T-20)
Current saturation with F
VII RPC Workshop - October 21, 2003
Current linearity with HV
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Aging test in 2001
• The gas mixture was C2H2F4/i-C4H10/SF6 (96/4/1) @ 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.
RPC A
Date
Qint
R20
RPC B
ρ20
Qint
R20
ρ20
(C/cm2)
(MΩ)
(1010 Ωcm)
(C/cm2)
(MΩ)
(1010 Ωcm)
oct 99
0
<3
<2
0
~5
~3
jan 01
0.076
10.6
6.6
-
-
-
mar 01
0.11
13.6
8.5
-
-
-
jul 01
0.361
42.0
26
-
-
-
aug 01
0.42
62.6
39
0,05
20
13
dec 01
0.42
111
69
-
-
-
irradiation
• Large resistance
increase for RPC A
• Evidence of increase not
related to irradiation for
both
• The temperature coefficient a was measured (a~0.12) and found in
agreement with our other measurements performed with different
bakelite samples. This result confirm the validity of the model.
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Aging test in 2002
• Both detectors now installed close to the source to measure ρ
continuously
• Only ~0.05 C/cm2 accumulated charge
• Both detectors show a steady increase of ρ with time
RPC A

RPC B
Hypothesis: resistivity increasing due to drying up of bakelite
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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: ρ rapidly restores to old values when dry gas is flowed
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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
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Temperature coefficient (1)
r(GWcm)
Sample A
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
(humidity~50%).
a0.140.02
a was measured and we found
a consistent value with values
previously measured.
a0.12 0.02
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Sample B
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Temperature coefficient (2)
r(GWcm)
Sample A
r20(GWcm)
Sample B
T(oC)
T(oC)
r(GWcm)
r20(GWcm)
a0.100.02
The temperature was increased
from 15 oC up to 40 oC in
100% humid air enviroment.
a was measured and we found a
different value.
a0.09 0.02
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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
Sample A
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)
~35% less in 2 days

Hypothesis: since the dependance on T was
corrected, the resistivity decrease depended
on the progressive water absorption.
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Effects of humidity
Sample A
Sample A left in the thermostatic
chamber with 100% humid air
and constant T
 r decreased rapidly
VII RPC Workshop - October 21, 2003
Sample B
But, when the thermostatic
chamber was opened, the
previous r value was restored
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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.
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Conclusions
• Aging effects on bakelite RPCs have been extensively studied for 3 years on
two identical detectors with resistivities around 1010Ωcm.
• After 2 years operation, ρ increased to ~100 x 1010Ωcm reaching the value of
~200 x 1010Ωcm 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.
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