Slide 1

Como, October 15-19, 2001H.R. Schmidt, GSI Darmstadt 1 The Time Projection Chamber for the CERN- LHC Heavy-Ion Experiment ALICE ALICE Detector overview TPC overview TPC components Field Cage Readout Chambers Front End Electronics Simulated Performance Slide 2 2 Alice Setup Slide 3 3 TPC Comparison NA49 (fixed target, 1995) STAR (collider, 2000) ALICE (collider, 2005) #channels182 k140 k570 k gasNeCO 2 (90-10) (vertex), ArCh 4 CO 2 (90-5-5) (main) ArCh 4 (90-10)NeCO 2 (90-10) gain 2 10 4 (vertex) 5 10 3 (main) 3 10 3 2 10 4 FC sizeW=390 cm, L=390 cm, H=180 cm (main), V=27 m 3 L=420 cm, R=210 cm V=56 m 3 L=500 cm, R=250 cm V=88 m 3 minimal pad size 3.5 16 mm 2 = 56.0 mm 2 2.85 11.5 mm 2 = 32.8 mm 2 4 7.5 mm 2 = 30.0 mm 2 Luminosity [cm -2 s -1 ] 10 25 2 10 25 1 10 27 Slide 4 4 TPC challenges ALICE Pb+Pb @ s=5.5 TeV/A 6000-16000 charged tracks in TPC acceptance (+ 30% BGND) problem #1: high track density problem #2: high track density problem #3: high track density space charge problem stability of operation (sparks, breakdown) accelerated ageing high occupancy deteriorated tracking efficiency deteriorated dE/dx and momentum resolution large number of channels huge data volume per event cost of front-end electronics ~ #of channels Slide 5 5 ALICE TPC Layout 500 cm Slide 6 6 TPC Field Cage- Structural Layout TPC: vessels + central electrode + end plates + chambers + supplies ~7600kg Four separate vessels: ~1800kg (flanges incl.) Outer containment vessel Inner containment vessel Outer field cage vessel Inner field cage vessel Two end plates for readout ~1500kg Slide 7 7 Test-Setup at SPS Beamline Slide 8 8 Exposure to secondaries from Pb beam CERES Pb-beam:6000/cm 2 s (secondaries) ALICE:360 /cm 2 s Stable to 60 kV! Global irradiation of the FC led to charge accumulation on the insulating surface Injection of water vapor reduced the tendency of discharges (increase of surface conductance); stable up to 100 kV. The test was thus repeated at lower primary Pb beam rates (no water). Field cage stable up to 100 kV! Slide 9 9 Beam Tests with GSI Heavy-Ion Beams Investigate chamber operation at high gain and/or particle load Result: Stable operation with NA35 chamber, ALICE full size chamber will tested this fall Slide 10 10 Short Term Test of Long Term Behavior Total accumulated charge in 2000 hrs: 60 mC/cm ALICE Pb+Pb: 1.1 mC/cm year Slide 11 11 TPC working principle - 3D-imaging Slide 12 12 TPC cccupancy pad-time space occupancy (pad-time) @ inner chambers up to 50% (!) cluster finding & tracking very involved Slide 13 13 How to measure in a high track/cluster density environment? R- (pad direction):smaller pads limitations: # of channels (cost!) HV-GND gets critial PRF is diffusion limited oversampling Z (time direction): fewer time bins limitations: signal/noise gets critical for 20) for a give pad area optimize aspect ratio minimize diffusion: cold gas, use high drift field 400 V/m, central electrode at 100 kV (!) transverse diffusion Slide 14 14 Example: Pad Plane pad size: 4 7.5 mm 2 570 000 pads (36 sectors) Slide 15 15 Space Charge Problem Two source of positive charge in drift volume: a)direct ionization by charged particle ion 4 cm 2 /Vs >> electron => quasi stationary positive charge in drift b)ion feed back from amplification region depends on gating ineffeciency: gain 10 4 requires at least 10 -4 gating inefficiency! varies with luminosity => rate dependent corrections Get field line plot from blumet al or tdr Slide 16 16 Gating Inefficiency for Positive Ions Gating ineff. < 0.5 10 -4 (2 ) measurement sensitivity limit gate open/close primary ionization : 20 ions/cm gas gain 10 4 2 10 5 ions/cm track from amplification gate < 0.5 10 -4 < 10 ions/cm from amplification Slide 17 17 TPC DETECTOR 88 s L1: 5 s 200 Hz Digital Circuit RAM 8 CHIPS x 16 CH / CHIP FEC (Front End Card) - 128 CHANNELS (CLOSE TO THE READOUT PLANE) FEC (Front End Card) - 128 CHANNELS (CLOSE TO THE READOUT PLANE) 8 CHIPS x 16 CH / CHIP ADC Custom IC (CMOS 0.35 Custom IC (CMOS 0.25 ) Commercial PASA 128 CHIPS x 1 CH / CHIP CSA Semi-Gauss. Shaper 10 BITSlide 18 18 TPC FEE: Mounting 36 trapezoidal sectors Inner chamber Outer chamber FEC C1 : 18 FECs C6 : 20 FECs C4 : 20 FECs C3 : 18 FECs C2 : 25 FECs FRONT VIEW C5 : 20 FECs SIDE VIEW 128 channels Front End Card (FEC) Capton Cable 140mm 190mm Slide 19 19 FEE: the Altro-16 Chip 8 ADCs Data & Pedestal Memories 8 ADCs Digital Filter & Control Logic Slide 20 20 Zero suppression with adaptive threshold Cluster finding masking the cluster neighborhood updating the threshold with the average over a number of samples FEE: Adaptive Zero Suppression fixed threshold Slide 21 21 FEE: Digital Tail Cancellation Filter Test of the digital filter with signals from ALICE TPC prototype Two different analogue readout: NA35 PASA, NA49-FTPC PASA Digitization with 10-MHz 10-bit ADC Experimental results 123456789 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 NA49-FTPC Analogue Readout microseconds Normalized Amplitude Input data 4 EXP 3 EXP us 051015 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 NA35 TPC Analogue Readout microseconds Normalized Amplitude Input data 4 Exp 3 Exp Slide 22 22 Occupancy & Tracking occupancy: clear side: 51% absorber side: 42% dN/dy= 8300 Slide 23 23 Projected Performance- Tracking good 88 % fake 2 % *) with vertex cut 2.4% (1.8%) *) 5 GeV/c: p t /p t 14% (7%) *) Slide 24 24 Projected Performance- dE/dx dE/dx resolution 9 % @ 8300 primary tracks Slide 25 25 Summary and Conclusion ALICE TPC: conventional TPC (design similar to NA49, STAR) optimized for extreme particle densities tests show: chamber operational and stable under high particle load simulations show: acceptable performance even at highest anticipated multiplicities