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

2. 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, / Stefano de Capua

3. 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 > 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, / Stefano de Capua

4. 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 < (streamer limitation)
Testbeam measurements (X5 muons beam)
T=25.0 oC
T=24.5 oC
August 2001
ρA = 39 x 1010 Ωcm @ 20 oC
Rcap ~ 640 Hz/cm @ 20 oC
July 2002
ρA = 110 x 1010 Ωcm @ 20 oC
Rcap ~ 200 Hz/cm @ 20 oC
VII RPC Workshop - October 21, / Stefano de Capua

5. 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, / Stefano de Capua

6. Aging test in 2001
The gas mixture was C2H2F4/i-C4H10/SF6 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
RPC B
Date
Qint (C/cm2)
R20 (MΩ)
ρ (1010 Ωcm)
oct 99
<3
<2 ~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
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.
VII RPC Workshop - October 21, / Stefano de Capua

7. 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
VII RPC Workshop - October 21, / Stefano de Capua

8. 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
VII RPC Workshop - October 21, / Stefano de Capua

9. 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, / Stefano de Capua

10. 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 (humidity~50%).
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, / Stefano de Capua

11. 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, / Stefano de Capua

12. 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
Sample A
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.
VII RPC Workshop - October 21, / Stefano de Capua

13. Effects of humidity Sample A Sample B
Sample A left in the thermostatic chamber with 100% humid air and constant T  r decreased rapidly
But, when the thermostatic chamber was opened, the previous r value was restored
VII RPC Workshop - October 21, / Stefano de Capua

14. 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, / Stefano de Capua

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