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
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

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

7. 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

8. MDC Cross Properties Gross properties: Active areas: 0.35 – 3.2 m2
Gas vol.: l, flow V/day
Cell Size: 5x5 – 14x10 mm2
6 Stereo angles: +40, -20, +0,-0,+20, 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

9. 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

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. 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

13. 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:
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. 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

15. 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 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