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Muon Detection in BaBar with Limited Streamer Tubes

Muon Detection in BaBar with Limited Streamer Tubes

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Muon Detection in BaBar with Limited Streamer Tubes

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  1. Muon Detection in BaBar with Limited Streamer Tubes Bryan Fulsom University of British Columbia April 27, 2005 SLAC Graduate Student Seminar

  2. Outline • Physics of muon detection • Focus on streamer mode detectors • Resistive Plate Chambers in BaBar • What they are • Why they failed • Limited Streamer Tubes in BaBar • Hardware components • Quality control and research • Early Run 5 LST subsystem performance • Future Possibilities / Conclusions Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  3. Muon Basics • Massive long-lived charged leptons • Energy loss mainly due to ionization (up to ~100 GeV) • No significant EM shower component • Deeply penetrating • Generally detected by ionization tracks at a large distance • m detection vital to many BaBar analysis topcis: semi- / leptonic decays, J/yY(2S) decays, lepton tag for CP, etc. Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  4. Muon Detector Types • Scintillator detectors • Scintillator material (plastic/crystal), PMTs • Expensive to produce on a large size scale • Cerenkov detectors • High energy / cosmic ray muons • Gas/wire detectors • Proportional mode (drift chamber) • Streamer mode (RPCs, LSTs) Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  5. Proportional Mode Gas Detectors • m ionize the gas, producing electron / ion pairs • Electric field causes electrons to drift toward anode • Accelerated e- gain energy, cause further ionization • Charge multiplication near the anode wire, proportional to the number of primary electrons (called gas amplification or Townsend effect) • Avalanche is collected at anode and detected as signal Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  6. Streamer Mode Gas Detectors • Avalanche begins like proportional mode • Higher voltage yields larger multiplication • Secondary e-/ion pairs produce a field, of the order of applied field • External field cancelled, avalanche becomes saturated • Results in field between avalanche tip and anode wire • New avalanches form from photoionized electrons • Recombination occurs with positive ions • Streamer propagates to anode wire, produces signal Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  7. Gas Selection • Choose and fine tune a gas mixture depending upon detector needs (i.e.: high gain, fast drift, operation mode, flammability/toxicity, etc.) • Typically require high gas gain, but also need to control the avalanches produced to prevent continuous discharge • Due to their simple molecular form, Nobel gases (Ar, Xe) offer the best gain per applied electric field (ionization is the main energy dissipation mechanism) • Excited Nobel gases emit UV photons that can trigger secondary avalanches, so they also need to be controlled by a quench gas • Quench gases are typically diatomic/organic (isobutane, methane), which can absorb UV photon energy in many ways other than ionization (vibration, rotation, dissociation) • Electronegative quench gases (carbon dioxide, freon) have an added bonus of being able to mop up excess electrons to prevent spurious avalanches Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  8. Signal Detection Segmented strips adjacent to detector pick up induced charge signal from the streamer Anode wire signal can be read out directly from on top of HV This yields two position coordinates. The third is gained by installing multiple layers of streamer detectors. Combined, these allow full tracking. Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  9. BaBar IFR Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  10. BaBar RPCs • Resistive bakelite plate electrodes, coated with linseed oil • High resistivity prevents spread of streamer, localizes “dead area” on plate due to charge deposition (i.e.: current does not spread out into bakelite) • Linseed oil creates a smooth surface, reduces noise/other discharge • Muon ionizes gas in plate gap, streamer moves to plates • Charge induced in pick-up strips for both x and y position, layer position of RPC determines z Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  11. RPC Characteristics • Pros • Cheap and simple to construct • Thought to be a mature / robust technology (see below) • Large active area coverage possible, very little dead space • Small gap / fast streamer allows good time resolution (~ ns) • Streamers produce much larger signal than proportional mode • Cons • Require high voltage (7000+ V) for operation • Linseed oil and bakelite is a “black art”, perhaps not so well understood… Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  12. BaBar RPC Performance Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  13. What happened? • Linseed Oil • Large temperature increase in BaBar (up to 34oC) over short time • Increase of dark current/noise (~20%/0.5oC), reached current limits and required disconnection • Test stand found linseed oil improperly cured, pooling into droplets and leaving other areas thinly covered • Oil can “bridge the gap”, leading to field reduction below streamer mode or outright shorting between the plates • Roughness and defects in exposed bakelite also affect the field • Graphite problems? • Studies (ATLAS, BaBar) find graphite electrode affected by aging Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  14. Attempted Solutions • RPCs were cooled to control temperature • Some efficiency was recovered, but never to original levels • Upgrade to high voltage and electronics • Replacement of endcap RPCs (2002) • Produced using strict quality control (clean bakelite surfaces, oil purification / application, final inspection of a sample of RPCs) • These new RPCs show steady efficiency • However…the RPCs continue to fail at a constant rate, and muon efficiency continues to drop • Decision was made to remove and completely replace the IFR barrel RPCs with a different technology Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  15. BaBar LSTs • Tubes consist of 7 or 8 cells • Cell dimensions: 15 x 17mm x ~4m • Consists of silver-plated anode wire and graphite-painted PVC walls (cathode) • Enclosed in PVC sleeve • Endcaps include HV / gas connections Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  16. LST Construction • Tubes manufactured in Italy by Pol.Hi.Tech. • Phi grounding plane made at SLAC • Sent to Princeton and OSU for Q/C and assembly into modules • Transported to SLAC for final Q/C and staging in CEH Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  17. BaBar z-planes • Used to read-out z-coordinate of signal • 96 strips of copper tape, 35mm width, ~2mm gap • Strips soldered to cables running the length of z-plane to output pins • Bottom acts as a ground plane • Laminated with mylar • Z-plane width varies with layer width • Dimensions: ~4.0 x 3.8m (length x width) • Constructed at SLAC Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  18. LST High Voltage • HV power supplies custom built at OSU • Designed for up to 6100V, usually operated at 5500V • Each supply can provide output for • 80 modules (320 wire channels) • Connected to modules via two cables: a “short haul” cable from the LST to just outside of the IFR region, connecting to a “long haul” to the power supplies above the electronics hut • Cables made by CSU Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  19. LST Electronics • Produced in Italy (Ferrara, Genoa) • Negative signal from anode wire follows phi plane to transition electronics, z-strip signals leave pigtail cables at the centre of the plane • Connected to a FEC daughterboard, including an amplifier and signal discriminator (thresholds: 700mV for wires, 900mV z-strips) • Integrates signal pulses above threshold to determine “hits” • Housed in crates located above BaBar and in the platform Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  20. LST Gas System • Choice of LST gas mixture: CO2 (89%), Isobutane (8%), Argon (3%) • Require ~20lbs. of CO2 per day, ~2lbs. of isobutane • Supply located outside of BaBar, mixing through the gas shack • Isobutane at this level is non-flammable, mix is closely monitored • Flows down to IR2, distributed through LST layers, then vented off • RPC: Ar 57%, Freon 59%, Isobutane 4% Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  21. Quality Control • Rigorous quality control procedures implemented to ensure excellent tube performance (and to avoid repeating the RPC problems!) • Clean HV contacts • Measure resistance / capacitance between wires and ground • Nominally infinite resistance, ~75pF capacitance for proper wires • HV Conditioning • Ease the tubes through a “burn-in” period • Slowly increase the LST HV up to 6000V • Monitor the current drawn for self-sustaining discharge • Check for gas leaks in modules • Radioactive Source Scan • Singles Rate Plateau Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  22. QC – Gas Leak Check • Fill tube with gas or air to a pressure of 4” of water • Seal all connections, monitor pressure over a period of 10 minutes • Calculate decay rate / lifetime in hours • Search for leaks with gas detector / soap bubbles • Repair with epoxy Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  23. QC – Radioactive Source Scan • Self-propelled cart with radioactive source scans the length of the tube • High radiation dose creates streamers, but they will be self-sustaining discharges in a region containing defects Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  24. QC – Singles Rate Plateau • Characteristic of LST operation is a wide plateau of voltage producing limited streamer mode operation • Cosmic ray background is a readily available source of muons • Plot cosmic rate versus applied voltage • Bad tubes will have short / non-existent plateau operating regions Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  25. QC – Repairing Tubes • Investigated extensively Princeton • Three common causes of self-sustaining discharge: • Uneven cathode surface • Graphite paint “bald spots” • Dirt/debris on the anode wire • Tube opened under clean room conditions, contamination (paint chips, dirt) can be removed with pressurized air and ethanol/acetone • Non-invasive remediation using negative high voltage treatment (negative corona discharge action) • Ions migrate to the wire • Collisions with coating on surface generate UV photons, creating more avalanches • Eventually breaks apart wire dirt, swept away with gas flow Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  26. Summer Installation 2004 • 18 layers of RPCs removed from each of the top and bottom sextants, replaced by 12 layers of LSTs and 6 layers of brass per sextant • Corresponding to 188 LST modules, arranged for maximum coverage • 2284 z-strips in 24 z-planes • 1552 channels in 7 HV supplies Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  27. Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  28. Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  29. LST Performance in BaBar • Initially tested in BaBar with cosmic ray muons • Monitoring hardware performance • Monthly plateau measurements with cosmics • Operations with beam • FastMon plots to monitor phi and z strip hits • Layer-by-layer efficiency • Minor operational issues • Still to come … extracted reconstruction efficiency Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  30. Sample Cosmic Ray Track Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  31. Monthly Plateaus Layer 18 Top Sextant Consistent plateaus Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  32. Phi (Wire) Performance • All LST wires operational, RPC sector coverage patchy at best Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  33. Z Strip Performance • Z strips performing well, very small number of failures (<0.7%) Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  34. Coincident Efficiency by Layer • Given a hit in Layer N+1 and N-1, how often is a hit recorded in N? • LSTs over 90%, RPCs offer too few coincident hits to measure Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  35. The Future • Continue monitoring performance as Run 5 continues • Beam conditions offer new quirks and features of the LST subsystem to investigate and understand • Understand small amount of z-strip noise • As luminosity increases, monitor current draw • Full upgrade during 2006 shutdown • Only 2/6 sextants of the barrel have replaced • Large batch of tubes here at SLAC still to be tested, prepared for installation, and operated under long-term evaluation • Final shipment of cables, power supplies and LSTs due soon • The future for RPCs • Research into avalanche operation mode of RPCs • RPCs at LHC … how will they fare? Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  36. Conclusions • RPCs offer cheap, large area muon detection with good resolution …provided they work…! • Many technical challenges and issues to still be understood • LSTs are another mature, easy to construct techonology for muon detection, still relevant to current particle physics experiments • Early indications of LST performance have shown them to be far superior to the RPCs they replaced • Due in large part to diligent research and development and quality control by the entire BaBar LST team • Complete barrel upgrade scheduled for 2006 to bring BaBar’s muon detection system back up to par • Stay tuned for more results as Run 5 unfolds! Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005

  37. References • RPC References The BABAR Detector, BABAR Collaboration Nucl. Instrum. Meth. A479 (2002) 1-116 Experience with the BaBar Resistive Plate Chambers, Henry R. Band SLAC-PUB-11098 • LST References Limited Streamer Tubes for the BaBar Instrumented Flux Return Upgrade, Changguo Lu, presentation at ICHEP 2004 www.ihep.ac.cn/data/ichep04/c_paper/9-0143.pdf IFR-LST, Wolfgang Menges, presentation at Feb. 2005 Collaboration Meeting www.slac.stanford.edu/BFROOT/www/Organization/CollabMtgs/2005/detFeb05/Fri2/menges.pdf The BaBar Limited Streamer Tube Detector, Q. Wong, presentation at APS 2005 www.physics.ohio-state.edu/~qdaman/APS05.ppt A Barrel IFR Instrumented with Limited Streamer Tubes, The BaBar LST Team www.slac.stanford.edu/BFROOT/www/Detector/LST/documentation/LSTprop-A4-May30-ajss.pdf Plastic Streamer Tubes and Their Applications in High Energy Physics, E. Iarocci Nucl. Instrum. Meth. 217 (1983) 30-42 Muon Detection in BaBar with Limited Streamer Tubes - SLAC Graduate Student Seminar Bryan Fulsom - April 27, 2005