An extensive aging study of bakelite RPC Danilo Domenici University of Rome “Tor Vergata” and INFN
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
Some history… In 1998 RPCs were proposed for LHCb Muon Detector, in the regions with a maximum flux density of 750 Hz/cm2 In 1999 2 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
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 2002-2003: both detectors in pos 2 Source characteristics: 662 KeV from 740 GBq 137Cs Counting rate: 13 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
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
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
Current vs time 7 months of irradiation 0.4 C/cm2 acc. charge (RPC A) Current drawn at 10800 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
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
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
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
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 < 11000 (streamer limitation) The detectors were tested at GIF by means of X5 muon beam Pisa Meeting – May 27, 2003 11 Danilo Domenici
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
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
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
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/cm2 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