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

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

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 rare r,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

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

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) x/X0  5 10-4 Other low-mass DCs: CLEO, KLOE, FINUDA, BELLE, CLAS, BaBar *) LUMA **) California fine wire SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

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 No significant gain degradation (<5%) within an equivalent of 2 years running SAMBA 2002, Trieste C. Müntz, GSI Darmstadt

Long-Term Stability: Creep of Al Wires 1800 170 cN Tension (cN) Number of days DF = 10% Bare Aluminum wires: (annealed) systematic wire 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

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 IPN Orsay FZR LHE Dubna GSI 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

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

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

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

Front-End Electronics Motherboard TDC FPC Daughter Boards ASD8 LVL1 Bus connector FPC- connector 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

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

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

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

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