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Evolution of Muon Detector Technologies in LHCb

This presentation discusses the changes in Muon Detector technologies in the LHCb experiment, including studies done with different detector types, ageing studies, and the conclusions drawn. The presentation was given by Burkhard Schmidt from CERN.

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Evolution of Muon Detector Technologies in LHCb

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  1. LHCb Muon System TechnologyWhy has it changed ?Burkhard Schmidt, CERN Outline: • Overview of the Muon Detector Technologies • Studies done with MRPCs • Ageing studies of bakelite RPCs (talk PD2-2, G. Carboni) • Ageing studies of MWPCs • Conclusions IEEE Symposium 2003, Portland

  2. M1 M2 M3 M4 M5 The LHCb Muon Detector • 5 Muon stations, M1 in front and M2-M5 behind the calorimeters -> large variations in particle flux:0.2kHz < Φ < 500kHz • 435m2 of detector area have to be equipped Muon System is primarily used for L0 muon triggering -> RPC-type technology seemed good choice for many regions -> Industrial production appealing However, particle fluxes are higher than in ATLAS and CMS -> Pushing the limits ... IEEE Symposium 2003, Portland

  3. TDR: 05/2001 Addendum: 01/2003 MWPC 51% (up to 100kHz/cm2) RPC: 48% (up to 1kHz/cm2) ? : <1% MWPC 99.9% GEM ?: 0.13% Muon System Technology Evolution TP: 02/1998 CPC 14% MRPC: 86% up to 5kHz/cm2

  4. Multi-gap RPC: V0 d’ ΔV = ρ·d’·Φ·q g’ ¾ V0 Veff g’ ½ V0 ΔV=0 g’ ¼ V0 g’ d’ ΔV = ρ·d’·Φ·q 0 V On intermediate plates: Q=const. ΔV = 0 Veff = V0 – 2·ρ·d’·Φ·q Comparison RPC - MRPC Single-gap RPC: V0 d ΔV g Veff ΔV 0 V R= ρ·d/A I=Φ·A·q -> ΔV = ρ·d·Φ·q Veff = V0 – 2·ρ·d·Φ·q • Veff in single and multi-gap RPCs is the same (if d=d’) • Multiple small avalanches in MRPCs have no advantage in terms of efficiency because the signal in the far gaps is reduced due to the weighting field. IEEE Symposium 2003, Portland

  5. Comparison RPC - MRPC Rate Capability: SRPC MRPC 1.8kHz/cm2 1.9kHz/cm2 ρ≈1010cm ρ≈ 1012cm • Rate capability is determined by the resistivity • We could not get thin melamine plates of ρ < 1011cm IEEE Symposium 2003, Portland

  6. Experience with MRPC Problems encountered when operating MRPCs with dry gas: (resistive plates are of melamine-phenolic) • It changes the plate resistivity (from 6 x 1011cm -> 6 x 1012cm) • It causes large deformations: Melamine sheets (0.8mm) • These problems could be reduced when flushing 50% of the gas mixture (C2H2F4/ iso-C4H10/SF6 95/4/1) through water -> MRPC have been dropped quickly in LHCb, since they offered no advantages IEEE Symposium 2003, Portland

  7. Aging Studies done with RPCs • 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 IEEE Symposium 2003, Portland

  8. Test setup at GIF Source characteristics: 662 KeV  from 740 GBq 137Cs Counting rate: 13 kHz/cm2 at 1 m from the source 2001: A in pos 1, B in pos 2002-2003: both in pos 1 1: aging position 2: reference position 3: testbeam position • The irradiation is measured by the accumulated charge per cm2 (Qacc) we had previously shown that the resistivity of bakelite slabs 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 IEEE Symposium 2003, Portland

  9. Measurement of resistivity • A simple method has been developed to measure ρ continuously at GIF • It requires the detector to be exposed to large flux of radiation • 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 • R depends exponentially on the temperature • For bakelite we measured the value a = 0.12 ± 0.01 IEEE Symposium 2003, Portland

  10. Measurement 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 IEEE Symposium 2003, Portland

  11. Current vs time Current drawn at 10800 V corrected by temperature 8 months of irradiation 0.4 C/cm2 acc. charge (RPC A) In the same period RPC B was used as reference and accumulated only 0.05 C/cm2 • I decrease by a factor 6 • ρ increase by the same factor Qacc=0.4 C/cm2 jan 01 aug 01 IEEE Symposium 2003, Portland

  12. ρ vs time 1999-2001 irradiating Remarks: • Large increase for RPC A • Evidence of increase not related to irradiation for both IEEE Symposium 2003, Portland

  13. ρ vs time 2002 • Test: both detectors now installed close to the source to measure ρ continuously • Only ~0.05 C/cm2 accumulated charge (large ρ low current) • Both detectors show a steady increase of ρ with time -> Drying up of bakelite (see also J.Va’vra’s talk yesterday DA2-1) IEEE Symposium 2003, Portland

  14. ρ vs time 2003: humid gas no flow humid flow • 1.2% of vapor H2O added to the usual gas mixture • Clear effect of humid gas, (see also ALICE 2002 results) but: on RPC B there is a sharp decrease of ρ on RPC A the effect is much reduced • ρ rapidly resume to old values when dry gas flow is restored IEEE Symposium 2003, Portland

  15. 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 IEEE Symposium 2003, Portland

  16. RPC A T = 25 °C GIF test August 2001 Resistivity at 20 ºC ρA = 39 x 1010 Ωcm Rate capability at 20ºC ~ 640 Hz/cm2 IEEE Symposium 2003, Portland

  17. T = 24.5 °C GIF test July 2002 Resistivity at 20 ºC ρA = 110 x 1010 Ωcm Rate capability at 20ºC ~ 200 Hz/cm2 IEEE Symposium 2003, Portland

  18. Testbeam summary • Except for the 1999 data (affected by large uncertainties) the values are in agreement with the 1/ρ dependence of rate capability IEEE Symposium 2003, Portland

  19. Conclusions on RPCs • Aging effects on bakelite RPCs have been extensively studied for 3 years on two identical detectors with building resistivities around 1010Ωcm • After 2 years of operation ρhad 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 • Flow of humid gas doesn’t appear to be a practical method to recover detector performances • Rate capability dropped from few kHz/cm2 to about 200 Hz/cm2 • LHCbdropped RPCs ! • This decision did not make the LHCb muon system more expensive ! IEEE Symposium 2003, Portland

  20. Aging Studies done with MWPCs • MWPC chamber characteristics • Overview of aging tests done • Results in terms of gain variations ΔG/G and Malter current • Visible inspection of the chamber • Next steps IEEE Symposium 2003, Portland

  21. HV HV HV HV MWPC Characteristics Pitch: 2 mm Gap: 5 mm Wire: 30 µm HV: 2650 V Cathode field: 6.2 kV/cm Wire field: 262 kV/cm Gain: 105 Total avalanche charge: 0.74pC Materials used for the chamber construction: • FR4 (fire resistant fiber glass epoxy) • Gold plated tungsten wire • Ni/Au plated PCBs • Araldite 2011 and Adekit A 145/50 • Natural rubber gasket for chamber closing • Epoxy glued kapton foil. Expected accumulated charges in 10 LHCb years: -> up to 1.6 C/cm2 on cathodes and 0.3 C/cm on wires IEEE Symposium 2003, Portland

  22. MWPC Aging studies 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%) • 2nd test at ENEA Cassacia in June 2003, where charges of 0.5 C/cm have been accumulated in 1 month (using the 25kCi (!) Co source). • One chamber has been in open loop, one in closed loop • Gas mixture: Ar / CO2 / CF4 (40%, 40%, 20%) Parameters controlled: • Relative gas gain: In both tests one gap was used as reference gap to avoid complicated corrections for P and T variations. The reference gap was only for very short periods per day under HV and always flashed with the fresh gas. • Dark currents, including self-sustaining rest current following the beam off. The dark currents were measured with current monitors with a resolution of 1 nA. IEEE Symposium 2003, Portland

  23. Developments of currents with time • B1 and C2 have been the reference gaps in the 2 chambers -> No visible deterioration of the gas gain IEEE Symposium 2003, Portland

  24. Current ratio after/before irradiation Am Source tests: The ratios after/before of the source currents measured before and after irradiation averaged over the area ~360 cm2 Tests at GIF: Precision of the measurement: about 10% -> No aging in gaps S1,S2 and A1. IEEE Symposium 2003, Portland

  25. Rest current behaviour Rest currents in one of the chambers (in nA): • Rest current: The currents were measured immediately after the source off has been switched off. IEEE Symposium 2003, Portland

  26. Summary of Results from Cassacia Accumulated charges: • 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 from measurements with GIF and with an Am-source. IEEE Symposium 2003, Portland

  27. Analysis of Surfaces 1st the wires . . . Strong FR4 etching of open surfaces CF4 is a phantastic gas ! CF4 is a terrible gas ! Bubbles under Kapton foil Detaching of the ground grid between the pads (due to FR4 etching)

  28. Analysis of Deposits on Cathodes Brownish deposit limited by the field wire Si Presence of the deposit can provoke Malter current on the beam due to spikes of intensity. Deposit reflecting wiring structure IEEE Symposium 2003, Portland

  29. Effects in the Gas System Rotameter glass tube etching F • Inner glass surface of a gas rotameter from closed loop is strongly eroded ( Flurine was found). • O-rings (from NBR) used in valves of the gas system are damaged. IEEE Symposium 2003, Portland

  30. Conclusions and next steps Conclusions: • Although no drop in gas gain was visible after the Casaccia aging test, the materials exposed to CF4show strong surface etching. • The effects are very similar for the chambers in the two gas system used in Casaccia (vented and re-circulation gas-system). • The effect has been stronger the 2nd aging test, where the gas mixture contained 20% CF4, than in 1st test, where 10% CF4 have been used. Plans: • Repeat the test at GIF in the coming months, where the test takes however 5 times longer than in Casaccia (source strength 25kCi). • New mixture with less/no CF4 . • Both gas loops will be used. • Control of O2 and H2O and contamination is foreseen. IEEE Symposium 2003, Portland

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