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Silicon Pad Detectors for tracking and particle identification in heavy ion collisions

Silicon Pad Detectors for tracking and particle identification in heavy ion collisions. Heinz Pernegger for the Phobos collaboration Massachusetts Institute of Technology March 31, 2000. Outline. Brief introduction of the PHOBOS experiment at RHIC Layout of silicon pad detectors

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Silicon Pad Detectors for tracking and particle identification in heavy ion collisions

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  1. Silicon Pad Detectors for tracking andparticle identification in heavy ion collisions Heinz Pernegger for the Phobos collaboration Massachusetts Institute of Technology March 31, 2000

  2. Outline • Brief introduction of the PHOBOS experiment at RHIC • Layout of silicon pad detectors • Measurement signal pad detectors • Wafer parameters • signal response in beam tests and lab measurements • Measurement of energy loss in silicon pad detectors • Measurement of energy loss for low and high momentum • Comparison to simulation • Test of particle identification capabilities Heinz Pernegger

  3. The PHOBOS Collaboration A “small” experiment with 72 collaborators from: Argonne National Laboratory Brookhaven National Laboratory Institute of Nuclear Physics Krakow Jagiellonian University Krakow Massachusetts Institute of Technology National Central University, Taiwan University of Rochester University of Illinois Chicago University of Maryland Heinz Pernegger

  4. 1.) Phobos at RHIC • Aim of Phobos: Search for Quark-Gluon Plasma in Au-Au collisions • Phobos searches for events with very high charged multiplicity and will study them with the spectrometer • particle multiplicity over full solid angle • reconstruct tracks in mid-rapidity range with low Pt threshold and identify them • Allows to measure particle spectra, particle correlation Au 100 GeV/n Au 100 GeV/n Heinz Pernegger

  5. Layout of the Experiment 1m • 1 layer Silicon multiplicity & Vertex Detector (~20,000 readout channels) • 14 layer Silicon Spectrometer Arms (~60,000 readout channels/arm) • Time of Flight Wall Magnet (top half not shown) Heinz Pernegger

  6. Full track reconstruction and particle identification in mid rapidity reconstruction efficiency 85% Pt Threshold 50MeV for p and 200 MeV p Measure charged multiplicity in 4p study event with high multliplicity 2 Silicon detectors with different aimsMultiplicity Detector - Spectrometer dE/dx 650 MeV/c p /K 1200 MeV/c K/p TOF 1200 MeV/c p /K 2000 MeV/c K/p Single event multiplicity Si TOF Heinz Pernegger

  7. What do our silicon detectors do • The octagon and ring multiplicity detector: • 1 large layer of silicon around the beam pipe • Measuring total multiplicity for charged particles • expect very high occupancy (90-100% ?) • cover pseudorapidity of -5.5 to + 5.5 and full phi coverage • The vertex detector: • Determining the interaction point with an accuracy of 50 um in a range of +/- 10 cm around the nominal interaction point • uses 2 layers of silicon on each side of the interaction region • The Spectrometer • does 3d-tracking of charged particles (for about 1% of full solid angle) • operates inside a 2 T magnetic field • uses 14 layers of highly segmented silicon detector for tracking • uses the silicon signal for particle identification for pions, kaons and protons Heinz Pernegger

  8. The Spectrometer Detector 66 sensors x 256 ch 21 sensors x 256 ch. 28 sensors x 512 ch. 18 sensors x 500 ch. 1x1mm to 0.7x19mm 57896 channels/arm 8 sensors x 1536 ch. Heinz Pernegger

  9. A central event in the spectrometer Heinz Pernegger

  10. bias bus signal lines 1.2um ONO vias 0.2um ONO p+ Implant Polysilicon Drain Resistor 300um 5000kOhm nSi n+ Silicon Pad Sensor Double Metal, Single sided, AC coupled, polysilicon biased produced by ERSO, Taiwan PN junction Thin ONO Thick ONO Polysilicon Heinz Pernegger

  11. Double metal layer for readout of pad detectors • Use a metal 1 layer as electrode and the metal 2 layer to route signals to the detector edge: AC coupled Pad (p-implant + metal 1pad) polisilicon bias resistor metal 2 readout line contact hole metal 1- metal 2 Advantages of this readout scheme: + readout on detector edge: minimizes multiple scattering (no extra material in active area) + simplifies the readout: different pad geometries can be routed to a single type bond pad array + can use conventional Si-strip detector readout chips on pad detectors Disadvantages: - double metal structure adds to the total detector capacitance - increases capacitance between pads Heinz Pernegger

  12. Module Assembly SMD/conn to hybrid Chip to hybrid Hybrid bonding Test of - VA chips - flex cables - hybrids Find electronic problems backpl. contact Module gluing MS rework Module rework (fast) Module function test Module bonding Hybrid calibration Sensor tests SURVEY measurement Module calibration Module SOURCE test Mounting on frames Determine number of defect channels with channel/channel calibration good if < 5% Scan the full module surface with Sr90 source on automated test station good if peak S/N > 10:1 Heinz Pernegger

  13. The Spectrometer Modules Heinz Pernegger

  14. Sensor Testing and Module Assembly Hughes 2470-V bonder Inspection stations Probe Stations Clean Rooms Gluing Station Heinz Pernegger

  15. Test results of detector properties • Statistics on Silicon parameters • Tests on finished modules • signal uniformity for different sensor geometries • cross-talk between pads • noise for different geometries • Overall module performance • Readout electronics • VA-HDR1 chips in 64 and 128 channel versions • “Viking” type electronics produced by IDE AS, Norway • consists of preamp , RC-CR shaper, track-hold stage + multiplexed analog output • peaking time 1.0ms • high dynamic range > 100 MIP input signals Heinz Pernegger

  16. Measurement of detector capacitances • Metal 1 to Metal 2 capacitance (1.2mm oxide/nitride insulator layer) • Backplane capacitance of a Type 5 pad Neighbour columns grounded 1 MHz source frequency line width = 10mm line length = 6cm Metal 1-Metal 2 capacitance: from test structure: 4.5pF/cm from detector: 4.7pF/cm Neighbour pads grounded 1 MHz source frequency pad width = 0.667mm pad length = 19 mm Backplane capacitance: from p-n diode: 5.3pF/pad from detector: 5.4pF/pad Vfd=105V Heinz Pernegger

  17. Readout line quality • Detect broken readout lines by measuring C back-plane • Other typical detector parameter • leakage currents active area @ Vfd = 3-5mA • polysilicon resistors = 5MW • depletion voltage = 100-110V 70 mm Broken readout line M1 p+ “Cb” 22 mm n+ Currently typical 5% broken readout lines - work on improvement to <2% Heinz Pernegger

  18. Sensor parameters Full Depletion Voltage [V] Leakage Current [uA] PolySilicon Resistance [Mohm] Operational Range [V] Heinz Pernegger

  19. Signal uniformity across sensors • Relative signal across the pads (row-wise) +/- 2 % full scale row i-1 row i row i+1 Relative signal Row number Very uniform signal response within a sensor better than +/- 1% Heinz Pernegger

  20. Measurement of cross talk on small pads • Use reference system to predict hit position on Type 1 detector • plot signal distribution for predicted pad and all neighbouring pads bot top top left hit right C= <Spad>/Scenter cross talk %: top: <0.1 bot: 0.6* left: <0.1 right: <0.1 bot +20 -20 0 +20 -20 0 left right center * crosstalk in analog readout chain 200 +20 +20 -20 0 -20 0 0 Pad signal (ADC) Pad signal (ADC) Pad signal (ADC) (Detector related) cross talk less than 0.6% on Type 1 detector Heinz Pernegger

  21. Measurement of cross talk on large pads • Expect largest cross talk of all Phobos detector types due to readout line to pad capacitance (9.4 pF) Row 0 Row 1 C= <Spad>/Scenter cross talk %: row 0: 0.9 row 1: 0.6 row 3: 0.5 -5 0 +5 -5 0 +5 Row 3 Hit in Row 2 0 100 -5 0 +5 Pad signal (ADC) Pad signal (ADC) Cross talk less than 1% on Type 5 detector Heinz Pernegger

  22. Calculated and measured noise values Noise largely dominated by constant part preamp-only noise main detector noise source : bias resistor and pad to pad capacitance Heinz Pernegger

  23. Assembled Modules Peak Signal/Noise Number of defect channels (%) • Mean module S/N = 16:1 • Mean number of detefect channels = 1% Heinz Pernegger

  24. Measured energy loss for low momentumpions and kaons • Phobos “lives” on the Silicon analog signals • The multiplicity is directly calculated from the analog signal • The spectrometer needs it for particle identification and track reconstruction • The aim of this measurement was • to measure and understand theresponse of our detector for the low momentum pions and kaons • measure the dE/dx loss and straggling for kaon and pions versus momentum • This allows us to: • compare and tune our Geant simulation • test the particle identification Heinz Pernegger

  25. Test setup at AGS TOF start (Degrader) Phobos 4 planes of TOF stop Cerenkow Paddle (Trg) type 1 modules • The Silicon detector: • use first 4 planes of the spectrometer (12k channels) • small pads -> good and full tracking • high S/N -> good energy loss measurement • 8 sensors & 96 chips -> minimize systematic error, give redundancy and allow cross checks • The TOF and Cerenkow: • provides pi/K separation and particle identification in the low p range • suppress e- back ground of secondary beams Heinz Pernegger

  26. How do we process the signal? • The basic step in the signal calibration: • calibrate the gain and linearity of on each channel • convert the measured charge to energy deposited using a constant of 3.62eV for the creation of 1 electron/hole pair • correct for the measured detector thickness • The intrinsic detector signal: • Landau part described by restricted Bethe-Bloch • Intrinsic gaussian contribution to the energy loss due to variation of Ionization potential for e- in different Si- shell (Shulek et al. ) • electronic noise (5keV in our case) • The measurements: • make a convolute Landau+Gauss fit to distribution • determine the most probable signal of the Landau part to measure dE/dx loss • use sigma of gaussian part and FWHM to characterize the energy straggling Heinz Pernegger

  27. Pions at low momentum: the measured signal Peak at 80keV 500 MeV/c 1GeV/c Peak at 150keV 130 MeV/c 285 MeV/c Heinz Pernegger

  28. Most probable energy loss for high momentum pions preliminary Landau most probable energy loss [keV] Data Geant • We measure a 4 % logarithmic rise of dE/dx (0.5 - 8GeV/c) for pions • Geant agrees very well with our measurement Heinz Pernegger

  29. Go to even lower momentum for pions: 130 +- 10 MeV/c Heinz Pernegger

  30. Kaon on Pion at the same momentum • Use the peak (Landau mp) to determine the dE/dx • use the width to measure the straggling Heinz Pernegger

  31. The measured dE/dx versus bg compare to scaled Bethe-Bloch • Scaling accounts for most probable to mean (as in BB) difference (determined at 1GeV) Heinz Pernegger

  32. Putting it to work: Particle Identification with 4 planes only? • Test our particle ID capabilities with 4 of 14 on mixed kaon + pion data sample • The particle momenta are nicely at the limit of our claimed pi/K separation (650MeV/c) • Use the TOF measurement to determine efficiency and purity • define: • efficiency e(pi)= N(pi->pi)/N(pi) • contamination c(pi)= N(K->pi)/N(K) • and vise-versa for Kaons Heinz Pernegger

  33. First approach: Truncated mean with 3 of 4 measurements p 500MeV/c 620MeV/c K • Works up to 620 MeV/c but worsens at 750MeV/c • requires very careful tuning of the cut • cut strongly depends on relative fraction of p/K 750 MeV/c Heinz Pernegger

  34. Second approach: Using a Maximum-Likelyhood estimation for pi/K • based on calculated signal probabilities for p and K hypothesis: Slog(f(Si)) = max • f…probability density function for pion or kaon at fixed momentum • requires knowledge of signal distribution at different p Heinz Pernegger

  35. The particle ID efficiency with 4 planes likelyhood likelyhood Truncated mean Truncated mean • Good efficiency already with 4 planes in both cases • eff (pi) > 85 to 90 % at 750MeV/c • eff (K) = 85% at 750MeV/c Heinz Pernegger

  36. The assembled spectrometer • Installed the fully assembled spectrometer in December in Phobos and made system tests during January Heinz Pernegger

  37. Conclusion • Phobos uses silicon pad detectors for • reconstruction of low momentum proton and pions in a 14 layer spectrometer • particle identification with dE/dx measurements • The detectors • the measured cross talk less than 1% • signal uniformity better than +/- 1 % • measured Signal / Noise 14:1 to 18:1 • The spectrometer • Made detailed studies on the dE/dx for our particle ID • Was installed in December • Full system tests after installation showed >98% functional channels • Detector noise in-situ is close to detector/electronics limit • Stability tests in area showed excellent stability • Looking forward to the first RHIC physics run in June! Heinz Pernegger

  38. Defects associated with double metal structure Non Func. Channels [%] Broken Signal Lines [%] Shorted Channels [%] Shorted Coupling Cap [%] Heinz Pernegger

  39. Spectrometer Acceptance Heinz Pernegger

  40. Momentum Resolution Heinz Pernegger

  41. Signal amplitude for different pad sizes Smp=21500 e- (=78keV) Small pads Large pads • Acquired with 90Sr b source • Average source signal: 21081 e- • Signal MP agrees with capacitive loss calculation (charge sharing between detector and VA input capacities) S/N mp =16.4 Heinz Pernegger

  42. ... In 9 different configurations MOD1 *6 (9) MOD2 *3 (5) MOD6 *5 (9) M4T4 *4 (7) M4T5 *7 (13) M7T2 *3 (5) M7T3 *5 (9) Heinz Pernegger M5T4 *4 (7) M5T5 *16 (30)

  43. Extracting the gaussian component of energy loss (“Shulek correction”) s/Smp=0.078 Heinz Pernegger

  44. GEANT simulation versus DATACompare it for pions at 285MeV/c (“Phobos typical”) without gaussian addition Heinz Pernegger

  45. The selection contamination with 4 planes • Very little contamination already with 4 planes in both cases • c (pi) <15% % at 750MeV/c and reaches levels of 5% beyond 600MeV • using Maximum Likelyhood produces slightly better purity Heinz Pernegger

  46. Summary: Measured Signals versus Geant and Bethe-Bloch • GEANT: • Geant reproduces the most probable energy loss extremely well!!! • Geant has trouble with the straggling (distribution is too sharp) • Adding gaussian componenet to account for the Shulek correction significantly improves the modelling of energy straggling • Bethe-Bloch • need to apply an restricted energy loss calculation due to escaping d electrons • can reproduce the momentum behaviour quite well once is it normalized at one point. Heinz Pernegger

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