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

2. 2 Alice Setup

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

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

5. 5 ALICE TPC Layout 500 cm

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

7. 7 Test-Setup at SPS Beamline

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!

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

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

11. 11 TPC working principle - 3D-imaging

12. 12 TPC cccupancy pad-time space occupancy (pad-time) @ inner chambers up to 50% (!)  cluster finding & tracking very involved

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   <200 ns temporal signal is diffusion limited  oversampling Conclusion –choose the time/pad area which yields still reasonable signal (S/N > 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

14. 14 Example: Pad Plane pad size: 4  7.5 mm 2 ≈ 570 000 pads (36 sectors)

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

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

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 BIT <10 MHz - Ped. Subtr. - Tail Cancell. - Zero Suppr. Multi-Event Buffer 1 MIP = 4.8 fC S/N = 30 : 1 Dynamic = 30 MIP L2: < 100  s 200 Hz DDL (4096 CH / DDL) Power consumption: < 100 mW / channel Power consumption: < 100 mW / channel 570132 channels Digital Circuit RAM TPC FEE: Basic Components

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

19. 19 FEE: the Altro-16 Chip 8 ADCs Data & Pedestal Memories 8 ADCs Digital Filter & Control Logic

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

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

22. 22 Occupancy & Tracking occupancy: clear side: 51% absorber side: 42% dN/dy= 8300

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%) *)

24. 24 Projected Performance- dE/dx dE/dx resolution ≈ 9 % @ 8300 primary tracks

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