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

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 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, 2003 3/15 Stefano de Capua

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 August 2001 ρA = 39 x 1010 Ωcm @ 20 oC Rcap ~ 640 Hz/cm2 @ 20 oC July 2002 ρA = 110 x 1010 Ωcm @ 20 oC Rcap ~ 200 Hz/cm2 @ 20 oC VII RPC Workshop - October 21, 2003 4/15 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. 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, 2003 5/15 Stefano de Capua

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 RPC B Date Qint (C/cm2) R20 (MΩ) ρ20 (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, 2003 6/15 Stefano de Capua

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, 2003 7/15 Stefano de Capua

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, 2003 8/15 Stefano de Capua

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, 2003 9/15 Stefano de Capua

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, 2003 10/15 Stefano de Capua

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, 2003 11/15 Stefano de Capua

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, 2003 12/15 Stefano de Capua

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, 2003 13/15 Stefano de Capua

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, 2003 14/15 Stefano de Capua

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, 2003 15/15 Stefano de Capua