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BaBar Silicon Vertex Tracker Status and Prospects

BaBar Silicon Vertex Tracker Status and Prospects. Adam Cunha UC Santa Barbara for the BaBar SVT Group 7 November 2005 Vertex2005, Nikko, Japan. adamcun@slac.stanford.edu. Outline. Overview of BaBar Overview of Silicon Vertex Tracker (SVT) Recent SVT issues and solutions

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BaBar Silicon Vertex Tracker Status and Prospects

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  1. BaBar Silicon Vertex TrackerStatus and Prospects Adam Cunha UC Santa Barbara for the BaBar SVT Group 7 November 2005 Vertex2005, Nikko, Japan adamcun@slac.stanford.edu

  2. Outline • Overview of BaBar • Overview of Silicon Vertex Tracker (SVT) • Recent SVT issues and solutions • Pedestal shift • Leakage current increase in outer layers • Impact of possible chip loss • SVT performance projections • Conclusion Adam Cunha

  3. The BaBar Experiment • Scientific Objective • Study CP violation in B meson decays. • Over-constrain CKM quark mixing matrix. SVT Performance Requirements • Δz resolution < 130 µm (average Δz for B0 decays = 280µm). • Single vertex resolution < 80 µm. • Stand-alone tracking for pT < 100 MeV/c with 80-90% efficiency. Reconstruct CP state µ+ µ- π+ J/ψ π- B0 KS e+ e- B0 Boosted Υ(4S) Δz=(βγc)Δt K- Determine time between decays from vertices l- slow pion Adam Cunha

  4. The BaBar Detector SLAC: PEPII Collider Electromagnetic Calorimeter 6580 CsI crystals e+ ID, p0 and g reco Instrumented Flux Return 19 layers of RPCs (LSTs) m+ and KL ID Cherenkov Detector (DIRC) 144 quartz bars K,p separation e+ [3.1 GeV] Drift Chamber 40 layers Tracking + dE/dx e- [9 GeV] Silicon Vertex Tracker 5 layers of double sided silicon strips Adam Cunha

  5. The BaBar SVT Be Beam Pipe Magnet • 5 Layers of double-sided, AC-coupled silicon • 0.94 m2 of Si • Φ and z strips • Inner 3: Precision Vertexing • Outer 2: Pattern recognition, Low Pt tracking • Custom rad-hard readout IC (the AToM chip). • Low-mass design (Kevlar/carbon fiber mechanical support) Adam Cunha

  6. SVT Modules z-side upilex fanout Si wafers φ-side Adam Cunha

  7. HDI Readout Board 8.3 mm 5.7 mm HDI Board AToM Chip • ATime over Threshold Machine • Time Over Threshold Readout • 128 Channels per chip • Simultaneous • Acquisition • Digitization • Read-out • Internal charge injection for calibration Berg Connector Mounting Buttons AToM Chips Upilex Fanout AToM Chip Adam Cunha

  8. Inside the AToM Chip Digitization pipeline of single channel Sparsification Readout Buffer Chan # CAC TOT Counter Time Stamp PRE AMP 15 MHz Shaper Comp Buffer Si Revolving Buffer 193 Bins Event Time Event Number Thresh DAC Buffer CAL DAC CINJ Serial Data Out Adam Cunha

  9. Radiation Monitoring (SVTRAD) • Purpose • Monitor accumulated dose • Protect against too high dose rates: • Acute damage threshold: 1 krad/ms • Chronic (10min) threshold: 50 mrad/s • Specifications • Reverse-biased (50V) Si PIN diodes. • Active area 1cm x 1cm x 300µm • 2 rings (FWD/BWD) of 6 diodes • Near Layer-1 SVT electronics • Details • DC coupled readout monitors total (leakage + radiation) current • Large leakage current subtraction with temperature corrections (thermistors) • Trigger beam dump when acute/chronic dose rate exceeds maximum • + 2 CVD Diamonds (new) Cross-section view of SVT Adam Cunha

  10. SVT Performance and Bugeted Radiation Damage Adam Cunha

  11. SVT Performance Phi side • Average hit efficiency 97% • Slow pion efficiency 70% for PT>50 MeV • Average zed hit resolution 10 - 40 μm • (Track-angle dependent) • No radiation-induced change in performance observed so far. Efficiency z side Z Resolution (μm) Efficiency Adam Cunha Adam Cunha

  12. Expected Damage to Electronics Radiation tests were performed on the AToM chip in 2001 using 60Co sources at SLAC and using 60Co sources at LBL. (Also tests at Elettra, mentioned later) G.C., A.Perazzo decrease ~ 4%/Mrad Gain In the real system, the gain decreases by ~5%/Mrad, noise increases by 15-20%/Mrad Noise increase ~ 16%/Mrad 1 Mrad 4 Mrad Exactly the same numbers we found with the 60Co Dose (Mrad) No digital failure observed up to 5 MRads Adam Cunha

  13. Signal to Noise Projections SVT layer-1 Signal/Noise 25 5000 Total S/N 20 4000 Signal/Noise Total Noise Noise (ENC) 3000 15 Limit: S/N=10 2000 10 ATOM noise 1000 5 Shot noise 0 0 0 2.5 10 5 7.5 Dose (Mrad) S/N Limit of 10  Radiation budget: 5 Mrad Adam Cunha

  14. Unexpected Phenomenon Adam Cunha

  15. Unexpected Phenomenon:Pedestal Shift • Noise pedestal (Threshold offset) started to increase • Behavior associated with AToM chip location, not with strip location • Why problem? One pedestal setting per AToM chip Chip 4 HDI Card in horizontal plane Pedestal Threshold offset (counts) Channel Adam Cunha 20 threshold DACs = 1fC

  16. Pedestal Shift (cont.) • Sets in at an integrated radiation dose of 1 Mrad • Correlated with most irradiated channel (note highly non-uniform: peaked sharply in the horizontal plane). • Effect reproduced at Elettra (test beam at Trieste, Italy) AToM Chip narrow e-beam 2 Mrad 1 Mrad Groups of 8 channels Threshold offset (counts) Delta Threshold (counts) Effect reproduced @ Elettra Integrated Radiation Channel Adam Cunha Pedestal recovers

  17. Pedestal Shift (cont.) • Cause: Uneven radiation above 1 Mrad leads to asymmetry in AToM chip electronics • Solution: Adjust threshold by chip is successful Make threshold adjustment… Layer 2 Module 4 Layer 2 Module 4 …recover efficiency. 10% inefficiency level Adam Cunha

  18. Unexpected Phenomenon 2 Adam Cunha

  19. Unexpected Phenomenon:Leakage Current Increase Apr May Jun 300uA ILeak (A) 10uA Days in 2004 • Since May 2004 an anomalous increase in the bias current for some modules has been observed • Only Layer-4 modules: not a simple radiation damage effect • No geometrical correlation • Consequences: increasing occupancies Adam Cunha

  20. Leakage Current Increase No beams Decrease in leakage current Beams play a role in leakage current increase Adam Cunha

  21. Leakage Current Increase If we vary the reference potential of the metal strips (see next slide)it leads to a decrease in the leakage current Leakage Current “Phenomenon is reference-voltage dependent” Time Jan 24 Jan 25 More humidity helps to stop the effect Leakage Current “Humidity plays a role” 0 1 2 Adam Cunha Time (hrs)

  22. Leakage Current Increase Hypothesis: Accumulation of static charge on the silicon surface. The charge is beam-induced drifts because of the field between the facing sides of different layers. -20V +20V Pside E -20V Nside +20V Pside Nside DVL5-L4=+40V ILeak (A) By varying the potential drop across the air between the layers we can control the effect 1800 0000 0600 1200 Time (hrs) Adam Cunha

  23. Leakage Current Increase Static charge on passivated surface • Charge accumulation causes an increase in the electric field at junction edge, inducing a soft junction breakdown. • Charge accumulation due to trickle injection  SVT bias always on. Adam Cunha

  24. Leakage Current Increase Using humid air and a new reference voltage setting, the situation now is under control Increased Humidity 180 μA Leakage Current 100 μA 1 June 05 - 1 July 05 Adam Cunha

  25. SVT Status & Future Prospects Adam Cunha

  26. Current SVT Status • 95% of detector is fully functional: • 6 out of 208 readout sections not working • 300 p-stop shorts/pinholes (mainly from before 2001) • 2% unbonded or otherwise dead channels • Redundancy proven to be sufficient Backward Forward short 2 chips masked short Both noisy Faulty AToM Chips Adam Cunha

  27. Radiation Dose History & Future Midplane Modules Bottom Modules Radiation Budget Pedestal Shift • Dose Projections: • Midplane modules will reach 5 Mrad budget in 2008 • Top & Bottom modules will reach 1 Mrad in 2006 – pedestal shift `00 `02 `04 `06 `08 Date `00 `02 `04 `06 `08 Adam Cunha

  28. Impact of Losing Chips on Physics Set E = 2 midplane chips OFF in L1& 2 (1/9 of L1/2 readout) B ®J/Y Ks 35.3% 34.5% Scenarios with up to all mid-plane L1-2 modules OFF: (UNREALISTIC) Efficiency (%) Soft p 56% 51% However, no loss of Δz precision! Adam Cunha

  29. Conclusions • SVT has been successfully operated in BaBar since 1999 • Effects of radiation damage • Pedestal shift effect solved using bimonthly threshold optimization • Electronics noise/gain degradation limits detector lifetime • Rapid leakage current increase was alarming, but has been mostly reversed • Adjustable reference voltages implemented • Increased humidity levels also help charge dissipation • Physics performance only slightly reduced in any realistic damage scenario. Adam Cunha

  30. The End Adam Cunha

  31. Extra Slides Adam Cunha

  32. Schematic of Signal Readout Power Supplies Back Cables MUX Power Front Cables HDI Link Inside detector Matching Card Si Wafers HDI Kapton Tail DAQ Link Fiber Optic to DAQ • HDI:High Density Interconnect. • Mounting fixture and cooling for readout ICs. • Kapton Tail:Flexible multi-layer circuit. • Power, clock, commands, and data. • Matching Card: Connects dissimilar cables. Impedance matching (passive). • HDI Link: Reference signals to HDI digital common. • DAQ Link: Multiplex control, demultiplex data. • Electrical -- optical conversion. Adam Cunha

  33. Pedestal Shift Onset Adam Cunha

  34. Trickle Injection Fill & Coast; Ramp down SVT during each injection Lumi Trickle Injection!(began March 2004)SVT biased all the time LER HER Adam Cunha

  35. Occupancy Projections Predicted from background studies at various beam currents & luminosities. Note: Electronics noise (e.g. AToM pedestal level) NOT included Adam Cunha

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