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Low-Mass Drift Chambers for HADES

Low-Mass Drift Chambers for HADES. C. Müntz, GSI Darmstadt. HADES : H igh A cceptance D i-Electron S pectrometer The major physics goal : Properties of hadrons in hot and dense nuclear matter

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Low-Mass Drift Chambers for HADES

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  1. Low-Mass Drift Chambers for HADES C. Müntz, GSI Darmstadt • HADES: High Acceptance Di-Electron Spectrometer • The major physics goal: Properties of hadrons in hot and dense nuclear matter • The experimental approach: Spectroscopy of rare vector mesons produced in heavy ion collisions via their decay in penetrating electron-positron pairs • Outline: • Design constraints for the HADES tracking system • Choice of materials • The HADES planar Drift chambers • In-beam performance SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  2. Present HADES Setup @ GSI SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  3. HADES: The Heavy Ion Case Simulation: Central 1 AGeV Au+Au reaction, all (200) charged particles per event: Hadron-blind RICH: Electron pair from w decay: • projected in-spill rates [Hz]: 108 projectiles, 106 reactions (1% target), 105 central reactions • rarer,w vector mesons: branching & production cross section: …0.5 Hz • Electrons from conversion and p0 decay: combinatorial background SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  4. Hades Gross Properties • Electron ID: • RICH • META: Shower / ToF •  Efficient & selective • multi-stage trigger • Momentum measurement: • Magnet, 6 coils • Supercond. toroid (0.7T) • 2p in f, 18o < J < 85o • Tracking system • High acceptance ( 40% for pairs) • High invariant mass resolution (ca. 1% in the r mass region) • This translates in: • Maximum efficiency for MIPs • High resolution tracking • (intrinsic spatial cell resolution <140 mm) • Use of low-mass materials to minimize multiple scattering (x/X0  5 10-4) SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  5. x/X0  5 10-4 Low-Mass Materials • Drift chamber gas: • Balancing between • Low multiple scattering • Spatial resolution (dE/dx, vD uniformity) • Stability (gain, plateau) • Aging • Lorentz angle (B < 0.05 T) • Physics-driven choice: • Helium:fill gas, avalanche • Isobutane: primary ionization, quencher • He : i-C4H10 [60:40] • gain = 5…10 105 • vD = 3…4.3 cm/ms • Wires: • Balancing between • Low mass occupancy • Stability (self-sustained currents / Malter) • Tension loss (creeping, load on frames) • Our choice: • Sense wires: 20 / 30 mm Au/W*) • Cathode, Field wires: • 80 / 100 mm annealed Aluminum **) • (I-III: bare Al, IV: gold-plated Al) Other low-mass DCs: CLEO, KLOE, FINUDA, BELLE, CLAS, BaBar *) LUMA **) California fine wire SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  6. Long-Term Stability: Aging C.Garabatos • Accelerated aging • with X-rays (55Fe) • 2 prototypes Io = 6 nA/cm • Expected charge dose in HADES:10 mC/year/cm • Nosignificant gain degradation (<5%) within an equivalent of 2 years running SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  7. 170 cN Tension (cN) DF = 10% 0 1800 Number of days Long-Term Stability: Creep of Al Wires • Bare Aluminum wires: • (annealed) • systematicwire tension loss measurements • HADES MDC: • 80 (100) mm • Low pre-tension 80 (100) cN J.Hehner/H.Daues DL GSI Measured tension loss: 10% in 5 years SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  8. IPN Orsay LHE Dubna FZR GSI MDC Cross Properties • Gross properties: • Active areas: 0.35 – 3.2 m2 • Gas vol.: 15-260 l, flow 10-20 V/day • Cell Size: 5x5 – 14x10 mm2 • 6 Stereo angles: +40, -20, +0,-0,+20, -40 deg., kick angle optimized, cathode wires at 90 deg. • Max. occupancy: 30% (8% average), 0.6 hits per cm • Materials: • Sense wire: 20/30mm Au/W • Cathode, Field wires: 80/100 mm Al • Windows: 12 mm Al-Mylar • Narrow Aluminum frames, 0.5 t load • Operating gas: He-iC4H10 [60-40] Publications: Optimisation of low-mass drift chambers for HADES, Nucl. Instr. Methods A 412 (1998) 38 Development of low-mass drift chambers for the HADES spectrometer, Nucl. Instr. Methods A 477 (2002) 387 SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  9. VD [mm/ns] Y [mm] X [mm] Cell Properties Drift velocity topology (MDC I): • Operating voltages: • Cathode/field: • -1.75 … -2.3 kV • Sense: ground MAGBOLZ / Garfield simulations SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  10. Comparison to Simulations Drift time spectra: x-t correlation: Good agreement!  Improvement of calibration & tracking algorithms Intrinsic spatial resolution: (proton beam, MDC prototype, Silicon strip tracker) SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  11. Present MDC Setup GSI plane I: ORSAY plane IV: Installed (5/2002): 17 out of 24 MDCs • In-beam experiences: • Several commissioning and first • production runs, • C+C, Cr+Al, 1-2 AGeV incident energy, • Moderate intensities: • several 106 projectiles/spill SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  12. Daughter Boards ASD8 LVL1 Bus connector FPC- connector TDC Motherboard FPC Front-End Electronics Analog Daughter Boards: Differential amplifier & discriminator (ASD8 chip) 8 channels, 1 fC intr. noise, 30 mW/channel, adjustable threshold (Straw Tubes, M.Newcomer, IEEE Trans. on Nucl. Sc. 40 (1993)) Signals from Sense wire: td Dt • TDC Features: • semi-customized ASIC • 8 ch., 0.5 ps/ch, common-stop, 1 ms full range • Multi-hit cap. (leading/trailing) • Spike suppression (Dt < 20ns) • Zero suppression • Calibration Mode (…mixed trigger) Dt = “time above threshold” SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  13. Time Above Threshold *) Time above threshold vs. drift time: • Gas system: • He : Isobutane = [60:40] • Re-circulating gas system, • with purification (Bosteels / CERN) • 500 l/h, 10% fresh gas • (17 modules) • Monitoring: • O2, @ input/output • Gas quality monitors (amplitude, drift velocity) Time above threshold: Lower higher O2 contamination high Lower O2 contamination *) efficient offline noise suppression! SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  14. Drift Velocity (mm/ns) Chamber Number In-Beam Performance Hit pattern (central 1.8 AGeV C+C): 6 sectors, 17 chambers Time Resolution (ns) Chamber Number SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  15. Spatial Resolution (microns) Orsay GSI Dubna FZR Chamber Number target veto start foils In-Beam Performance Drift time residuals: Self correlation: “Tracking” with two chambers: Target position along beam axis • Intrinsic spatial resolution 80 - 130 mm • Layer efficiencies > 98% • (systematic studies in progress, cosmic runs) SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

  16. Summary • HADES: High-resolution spectroscopy of “low-momentum” electrons and positrons • Low-mass planar drift chambers: He / Aluminum • Customized read out electronics • Gradual completion: 17 out of 24 chambers in operation • In-Beam performance according to design values GSI Darmstadt LHE Dubna FZ Rossendorf IPN ORSAY SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

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