1. 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 VII RPC Workshop - October 21, 2003 2/15 Stefano de Capua 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

3. Historical review VII RPC Workshop - October 21, 2003 3/15 Stefano de Capua 1998. Resistive Plate Chambers were proposed in the LHCb Muon Detector, in the regions with a maximum flux density of 750 Hz/cm 2. 1999. Two RPC prototypes (A and B) were built with identical characteristics: - bakelite electrodes with bulk resitivity  ~ 10 10  cm - sensitive area ~ 50x50 cm 2 2000. The rate capability of these detectors was initially measured to be R cap > 3 kHz/cm 2 (NIM A 456 (2000) 95) 2001. An extensive test started at the Gamma Irradiation Facility to study aging effects on the Rate Capability.

4. Rate Capability vs. bakelite resistivity VII RPC Workshop - October 21, 2003 4/15 Stefano de Capua 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) August 2001 ρ A = 39 x 10 10 Ωcm @ 20 o C R cap ~ 640 Hz/cm 2 @ 20 o C July 2002 ρ A = 110 x 10 10 Ωcm @ 20 o C R cap ~ 200 Hz/cm 2 @ 20 o C Testbeam measurements (X5 muons beam) T=25.0 o CT=24.5 o C

5. Monitoring the RPC resistivity during irradiation VII RPC Workshop - October 21, 2003 5/15 Stefano de Capua 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 V gap = V 0 – RI and it requires the detector to be exposed at a large flux of radiation. d S Current saturation with  Current linearity with HV If    then R =  V/  I  is strongly affected by temperature:  20 =  e (T-20) radiation

6. Aging test in 2001 VII RPC Workshop - October 21, 2003 6/15 Stefano de Capua The gas mixture was C 2 H 2 F 4 /i-C 4 H 10 /SF 6 (96/4/1) @ 1l/h (steel tubes). During the first seven months the irradiated detector (RPC A) accumulated a charge Q int = 0.4 C/cm 2 (~3.5 LHCb years). The reference dectector (RPC B) accumulated only 0.05 C/cm 2. RPC ARPC B DateQ int (C/cm 2 ) R 20 (MΩ) ρ 20 (10 10 Ωcm) Q int (C/cm 2 ) R 20 (MΩ) ρ 20 (10 10 Ωcm) oct 990<3<20 ~5~3 jan 010.07610.66.6--- mar 010.1113.68.5--- jul 010.36142.026--- aug 010.4262.6390,052013 dec 010.4211169--- irradiation Large resistance increase for RPC A Evidence of increase not related to irradiation for both The temperature coefficient  was measured (~0.12) and found in agreement with our other measurements performed with different bakelite samples. This result confirm the validity of the model.

7. Aging test in 2002 VII RPC Workshop - October 21, 2003 7/15 Stefano de Capua Both detectors now installed close to the source to measure ρ continuously Only ~0.05 C/cm 2 accumulated charge Both detectors show a steady increase of ρ with time RPC ARPC B  Hypothesis: resistivity increasing due to drying up of bakelite

8. Aging test in 2003: effects of humid gas VII RPC Workshop - October 21, 2003 8/15 Stefano de Capua RPC ARPC B humid flow: 1.2% of vapor H 2 O 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

9. Study of bakelite resistivity VII RPC Workshop - October 21, 2003 9/15 Stefano de Capua 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 A vacuum pump was used to keep samples in a vacuum bell. Courtesy of G. Passaleva Two bakelite samples 10x10 cm 2 were tested

10. Temperature coefficient (1) VII RPC Workshop - October 21, 2003 10/15 Stefano de Capua   0.140.02   0.12 0.02   (Gcm) T ( o C)  (Gcm)   (Gcm) T ( o C)  (Gcm) Sample ASample B The temperature was increased from 15 o C up to 30 o C in not conditioned enviroment (humidity~50%).  was measured and we found a consistent value with values previously measured. days

11. Temperature coefficient (2) VII RPC Workshop - October 21, 2003 11/15 Stefano de Capua The temperature was increased from 15 o C up to 40 o C in 100% humid air enviroment.  was measured and we found a different value. Sample A  (Gcm) T ( o C)   (Gcm) T ( o C)  (Gcm) Sample B   0.100.02   0.09 0.02

12. Temperature correction VII RPC Workshop - October 21, 2003 12/15 Stefano de Capua Temperature was increased from 15 o C up to 40 o C. Each step lasted about 1 day. 100% humid air enviroment. Resistivity corrected using:  A = 0.14 Still depending on temperature Resistivity corrected using:  A = 0.10 Clear dependance of the temperature coefficient on humidity percentage (see also Arnaldi et al., NIM A 456 (2000) 142) Hypothesis: since the dependance on T was corrected, the resistivity decrease depended on the progressive water absorption. Sample A ~35% less in 2 days 

13. Effects of humidity VII RPC Workshop - October 21, 2003 13/15 Stefano de Capua Sample A left in the thermostatic chamber with 100% humid air and constant T   decreased rapidly But, when the thermostatic chamber was opened, the previous  value was restored Sample ASample B

14. Drying up in vacuum VII RPC Workshop - October 21, 2003 14/15 Stefano de Capua When the vacuum was created,  increased rapidly. A silica gel was used to absorb humidity. The bakelite resistivity depends strongly on the percentage of water in its enviroment. Sample ASample B

15. Conclusions VII RPC Workshop - October 21, 2003 15/15 Stefano de Capua Aging effects on bakelite RPCs have been extensively studied for 3 years on two identical detectors with resistivities around 10 10 Ωcm. After 2 years operation, ρ increased to ~100 x 10 10 Ωcm reaching the value of ~200 x 10 10 Ωcm at the end of the third year. Although irradiation (0.4 C/cm 2 ) 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.