1. An extensive aging study of bakelite RPC
Danilo Domenici
University of Rome
“Tor Vergata” and INFN

2. Outlook 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 2
Danilo Domenici

3. Some history…
In 1998 RPCs were proposed for LHCb Muon Detector, in the regions with a maximum flux density of 750 Hz/cm2
In RPCs prototypes were built with identical characteristics:
bakelite electrodes with bulk resisistivity ρ ≈ 1010 Ωcm
50 x 50 cm2 sensitive area
The rate capability of these detector was initially measured to be > 3 kHz/cm2 (NIM A 456 (2000) 95)
In 2001 an extensive test started at GIF to study aging effects on rate capability
Pisa Meeting – May 27, 2003 3
Danilo Domenici

4. Test setup
Aging test 2001: one detector (A) in pos 1, very close to the γ source other detector (B) in pos 2, far from the source, as a reference
Rate capability measures: both detectors in pos 3, at ~1.5m from the source, on the X5 μ beam
Aging test : both detectors in pos 2
Source characteristics:
662 KeV 
from 740 GBq 137Cs
Counting rate:
13 kHz/cm2 at
1 m from the source
The irradiation is measured by the accumulated charge per cm2 (Qint)
We have shown that bakelite resistivity is not affected by pure irradiation up to doses of 20 kGy
Since the rate capability is inversely proportional to ρ, this has been the monitored quantity
Pisa Meeting – May 27, 2003 4
Danilo Domenici

5. Measure of resistivity
We have developed a simple method to measure ρ continuously at the GIF
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
It requires the detector to be exposed to large flux of radiation
R depends exponentially on the temperature
a = 0.12 was measured and found in agreement with other measurements
Pisa Meeting – May 27, 2003 5
Danilo Domenici

6. Measure 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
Pisa Meeting – May 27, 2003 6
Danilo Domenici

7. Current vs time 7 months of irradiation 0.4 C/cm2 acc. charge (RPC A)
Current drawn at V
corrected by temperature
7 months of irradiation
0.4 C/cm2 acc. charge (RPC A)
I decrease by a factor 6
ρ increase by the same factor
In the same period RPC B was used as reference and accumulated only 0,05 C/cm2
Qint=0.4 C/cm2
jan 01
aug 01
Pisa Meeting – May 27, 2003 7
Danilo Domenici

8. ρ vs time 1999-2001 RPC A RPC B Date Qint (C/cm2) R20 (MΩ)
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
irradiating
Remarks:
Large increase for RPC A
Evidence of increase not related to irradiation for both
Pisa Meeting – May 27, 2003 8
Danilo Domenici

9. ρ vs time 2002
Test: 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
Hypothesis: drying up of bakelite
Pisa Meeting – May 27, 2003 9
Danilo Domenici

10. ρ vs time in 2003: humid gas no flow 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 ρ
on RPC A the effect is much reduced
ρ rapidly restores to old values when dry gas flow is restored
Pisa Meeting – May 27, 2003 10
Danilo Domenici

11. Effects on rate capability
We define a RPC detector “capable” to stand a given rate if:
efficiency > 95% (trigger requirement)
at least 400 V plateau (safety requirement)
HV < (streamer limitation)
The detectors were tested at GIF by means of X5 muon beam
Pisa Meeting – May 27, 2003 11
Danilo Domenici

12. GIF test August 2001 RPC A Resistivity at 20 ºC ρA = 39 x 1010 Ωcm
Rate capability at 20ºC
~ 640 Hz/cm2
T = 25 °C
Pisa Meeting – May 27, 2003 12
Danilo Domenici

13. GIF test July 2002 Resistivity at 20 ºC ρA = 110 x 1010 Ωcm
Rate capability at 20ºC
~ 200 Hz/cm2
T = 24.5 °C
Pisa Meeting – May 27, 2003 13
Danilo Domenici

14. ρ vs time 1999-2001 RPC A RPC B Date Temp (°C) ρ (1010 Ωcm)
rate cap (Hz/cm2)
oct 99 23
<1
>3000 ~2
aug 01
25.1 20
1150 -
jul 02
24.5 65
350 45
380
Except for the 1999 data (affected by large uncertainties) is confirmed the 1/ρ dependance of rate capability
Pisa Meeting – May 27, 2003 14
Danilo Domenici

15. 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 1010Ωcm
After 2 years operation ρ had already increased to ~100 x 1010Ωcm reaching the value of ~200 x 1010Ωcm 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
Rate capability dropped from few kHz/cm to about 200Hz/cm2  no RPCs in LHCb!
Flow of humid gas doesn’t appear to be a practical method to restore detector performances
Pisa Meeting – May 27, 2003 15
Danilo Domenici