Low-Mass Drift Chambers for HADES

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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

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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Present HADES Setup _at_ GSI
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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HADES The Heavy Ion Case
Simulation Central 1 AGeV AuAu 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

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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 lt J lt 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 lt 140 mm)
• Use of low-mass materials to minimize multiple
  scattering (x/X0 ? 5 ?10-4)

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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Low-Mass Materials

• Drift chamber gas
• Balancing between
• Low multiple scattering
• Spatial resolution (dE/dx, vD uniformity)
• Stability (gain, plateau)
• Aging
• Lorentz angle (B lt 0.05 T)
• Physics-driven choice
• Helium fill gas, avalanche
• Isobutane primary ionization, quencher
• He i-C4H10 6040
• gain 510 ?105
• vD 34.3 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
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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 (lt5) within an
  equivalent of 2 years running

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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Long-Term Stability Creep of Al Wires

• 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
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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Cell Properties
Drift velocity topology (MDC I)
Operating voltages Cathode/field -1.75 -2.3
kV Sense ground
MAGBOLZ / Garfield simulations
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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)
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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,
• CC, CrAl, 1-2 AGeV incident energy,
• Moderate intensities
• several 106 projectiles/spill

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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 lt 20ns)
• Zero suppression
• Calibration Mode (mixed trigger)

Dt time above threshold
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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Time Above Threshold )
Time above threshold vs. drift time

• Gas system
• He Isobutane 6040
• Re-circulating gas system,
• with purification (Bosteels / CERN)
• 500 l/h, ?10 fresh gas
• (17 modules)
• Monitoring
• O2, _at_ input/output
• Gas quality monitors (amplitude, drift velocity)

Time above threshold Lower ? higher O2
contamination
high
Lower O2 contamination
) efficient offline noise suppression!
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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In-Beam Performance
Hit pattern (central 1.8 AGeV CC) 6 sectors, 17
chambers
Time Resolution (ns)
Chamber Number
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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 gt 98
• (systematic studies in progress, cosmic runs)

C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste
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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
C. Müntz, GSI Darmstadt
SAMBA 2002, Trieste