30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site1 MWPC Aging Studies Outline: Overview of aging tests done Results in terms of gain variations G/G and Malter current Visible inspection of the chamber Conclusions and next steps 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site2 Overview Expected accumulated charges in 10 LHCb years: -> up to 1.4 C/cm2 on cathodes and 0.3 C/cm on wires (M2R1) Two long term aging tests have been performed: 1st test at GIF in 2001, where charges of 0.25 C/cm have been accumulated in 6 months. Open loop gas system has been used Gas mixture: Ar / CO2 / CF4 (40%, 50%, 10%) 2 nd test at ENEA Casaccia in June 2003, where charges of 0.5 C/cm have been accumulated in 1 month (using the 25kCi (!) Co source). 2 chambers tested, using final chamber materials/components 1 chamber has been in open loop, 1 in closed loop Test used to validate the gas system as well Gas mixture: Ar / CO2 / CF4 (40%, 40%, 20%) 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site3 MWPC Aging studies Setup during the irradiation in Casaccia: M3R1 chamber was the 2nd chamber in the re-circulation gas system. S/A chamber was the 2nd chamber in the open gas system. -> Both chamber s were flushed with gas from an upstream chambers, imitating well a real experimental situation. Gas flow rate in open loop was ~4.8, in closed loop ~5.2 l/hr, with a fraction of a fresh gas ~ 1.35l/hr (~25% of full flow) Parameters controlled : Gas gain: One gap was used in both chambers as reference gap to avoid corrections for P and T variations. The reference gap was only for very short periods per day under HV. Malter currents: The self-sustaining rest current was measured following the beam off, using current monitors with a resolution of 1 nA. 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site4 Developments of currents with time B1 and C2 have been the reference gaps in the 2 chambers -> No visible deterioration of the gas gain 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site5 Am Source tests: The ratios after/before of the source currents measured before and after irradiation averaged over the area ~360 cm 2 Precision of the measurement: about 10% ->No aging in gaps S1,S2 and A1. GapS1S2A1 Ratio(03/02) P,T corrected Current ratio after/before irradiation GapS1S2 Current ratio P,T corrected Tests at GIF: 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site6 Rest currents in the S/A chamber (in nA): Malter currents Date HV (kV)Gap S1Gap S2HV (kV)Gap A After GIF Rest current: The currents were measured immediately after the source off has been switched off. 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site7 No signs for aging from the measured parameters: Relative currents did not change. Malter currents were not observed. No variations in current ratios after/before Casaccia. HV components inside the gaps with 2mm pitch survived. Summary of Results from Casaccia ChamberGapIC (C/cm) M3R1A,C,D0.43 S/AS10.52 S/AS20.42 S/AA20.38 Accumulated charges: Correspond to ~15LHCb years in the regions under consideration in this review (0.3 C/cm). 30 October Inspection of Surfaces Strong FR4 etching of open surfaces Bubbles under Kapton foil 1st the wires... CF 4 is a fantastic gas ! CF 4 is a terrible gas ! Detaching of the ground grid between the pads (due to FR4 etching) Deformation of the pad border and ground strip due to FR4 erosion 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site9 Cathode of the S/A chamber Black deposit lines Babbles under Kapton foil Etched FR4 surfaces 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site10 Analysis of Deposits on Cathodes Deposit reflecting wiring structure Brownish deposit limited by the field wire Presence of the deposit might provoke Malter current under certain circumstances. Si 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site11 Rotameter glass tube etching F Inner glass surface of a gas rotameter from closed loop is strongly eroded ( besides Si, F was found). O-rings (from NBR) used in valves of the gas system are damaged. Effects in the Gas System 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site12 Conclusions No drop in gas gain was visible after the Casaccia aging test. -> The chamber works, no modification to chamber design/materials required. The materials exposed to CF 4 show strong surface etching. The effect has been stronger in the 2 nd aging test, where the gas mixture contained 20% CF 4, compared to the 1 st test, where only10% CF 4 have been used. -> Reduce CF 4 contents in the mixture to 5%, or consider CF 4 cleaning runs a la ATLAS TRT. The effects are very similar for the chambers in the two gas system used in Casaccia (vented and re-circulation gas-system). -> Conclusions on the gas system difficult to draw at present, since the strong surface etching might hide problems in the closed loop. -> Gas System validation has to continue 30 October 2003V.Souvorov / B.Schmidt - MWPC PRR CERN site13 Next steps Repeat the test with new mixture at GIF in the coming months, where the test takes however 5 times longer than in Casaccia (source strength 25kCi). Validate new gas mixture and gas system Control of O 2 and H 2 O and contamination will be improved.