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The PAMELA Silicon Tracker. Lorenzo Bonechi - PAMELA collaboration INFN Sezione di Firenze - Dipartimento di Fisica dell’Universita’ di Firenze. INTRODUCTION MAGNETIC SPECTROMETER PERMANENT MAGNET SILICON TRACKING SYSTEM ( MECHANICS ) PERFORMANCES of the tracking system CONCLUSIONS.

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The PAMELA Silicon Tracker


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    1. ThePAMELASilicon Tracker Lorenzo Bonechi - PAMELA collaboration INFN Sezione di Firenze - Dipartimento di Fisica dell’Universita’ di Firenze • INTRODUCTION • MAGNETIC SPECTROMETER • PERMANENT MAGNET • SILICON TRACKING SYSTEM • (MECHANICS) • PERFORMANCES of the tracking system • CONCLUSIONS

    2. The PAMELA experiment • MAIN TOPICS: • antiproton and positron spectra • search for light antinuclei • SECONDARY TOPICS: • Modulation of GCRs in the Heliosphere • Solar Energetic Particles (SEP) • Earth Magnetosphere  Flight model delivered  Launch from Baikonur (Kazakhstan) @ end 2005 !!! Satellite-borne experiment: Semi-polar orbit  low energy 3-years mission  high statistics PAMELA > 3.104antiprotons 80 MeV/c - 190 GeV/c > 3.105positrons 50 MeV/c - 270 GeV/c RESURS DK1

    3. Satellite and orbit Resurs DK1 • Earth observation • 350 / 610 km • Inclination = 70.4o • Soyuz 2 launcher • Baikonur Cosmodrome • Launch date = end 2005 • 3 year mission Pamela operational During launch / orbital manoeuvres • Housed in an atmospheric pressure vessel • Temperature = 5oC ÷35oC • All subsystems must withstand launch vibrations! • Electronics must withstand up to ~3 krad 350 - 610 km • Total mass ~ 470kg / 345W power budget

    4. Magnetic spectrometer • Magnetic rigidity: R = pc/Ze • Charge sign • Requirements: MDR = 740 GV (4 mm spatial resolution) • Spillover limits: • Antiproton up to 190 GeV • Positron up to 270 GeV The PAMELA subdetectors GF ~20.5 cm2sr

    5. MAGNETIC FIELD MEASUREMENTS • Gaussmeter (F.W. Bell) equipped with 3-axis probe mounted on a motorized positioning device (0.1mm precision) • Measurement of the three components in 67367 points 5mm apart from each other • Field inside the cavity 0.48 T at the center • Average field along the central axis of the magnetic cavity : 0.43 T • Good uniformity • Measurement of external magnetic field – magnetic momentum < 90 Am2 The permanent magnet • 5 magnetic modules • Permanent magnet (Nd-Fe-B alloy) assembled in an aluminum mechanics • Magnetic cavity sizes (132 x 162) mm2 x 445 mm • Geometric Factor:20.5 cm2sr • Black IR absorbing painting • Magnetic shields

    6. DESCRIPTION of the SILICON SENSORS The silicon tracking system • Double Sided (x & y view) • Double Metal on the n side (No Kapton Fanout) • AC Coupled (No external chips) • Produced by Hamamatsu Geometrical Dimensions 70.0 x 53.3 mm2 Thickness 300 mm Leakage Current < 3 mA Decoupling Capacitance > 20 pF/cm Total Defects < 2% p side Implant Pitch 25.5 mm Readout Pitch 51 mm Biasing Resistance (FOXFET) > 50 MW Interstrip Capacitance < 10 pF n side Implant Pitch 67 mm Readout Pitch 50 mm Biasing Resistance (PolySilicon) > 10 MW Interstrip Capacitance < 20 pF

    7. The structure of the tracking system 6 detector planes composed by 3 “ladders” • ladder : - 2 microstrip silicon sensors - 1 “hybrid” with front-end electronics • silicon sensors (Hamamatsu): • 300 mm, Double Sided - x & y view • Double Metal - No Kapton Fanout • AC Coupled - No external chips • FE electronics: VA1 chip • Low noise charge preamplifier • Operating point set for optimal compromise: • total FE dissipation: 37 W on the 36864 channels (6 planes) • Dynamic range up to 10 MIP

    8. Silicon sensors defects Request to Hamamatsu: Defects < 2% Defects: Short Circuit of AC coupling (Most common, not destructive) Short between adjacent strips Open circuit on metal lines It seems to be ‘ perfect ’ BUT… The first batch was OK (Prototype ladders were ‘perfect’, bad strip < 2%) We started the mass production… Huge number of bad strips (>10%)!!!!! After a big ‘fight’ we discovered in many sensors short circuits between adjacent strips at the level of implantation (p side). Hamamatsu replaced all the bad sensors (few months of delay)

    9. Implantation procedure problems! Transverse ‘cuts’ on the junction side reduce the interstrip resistance

    10. The mechanical assembly • Requirements: • 1 plane made by 3 ladders • no material above/below the plane (1 plane = 0.3% X0!!!) • survive to the launch phase (7.4 grms, 50 g shocks!!!) • good alignment precision • thermal stresses (5-35 0C) • Solution:Carbon fibers stiffeners glued laterally to the sensors • very high Young module carbon fiber (300 Gpa) • pultrusion technology • Elastic + Rigid gluing A very thin (2.5 mm) Mylar foil is glued on the plane to increase the safety of the whole spectrometer during integration and flight phases No coating on the bonding

    11. The first silicon plane

    12. Mylar film protecting the plane

    13. Test plane lodgingon the magnet

    14. The flight model of the magnetic spectrometer

    15. Strip noise X view Y view <SIG>GOOD = 9.2 <SIG>GOOD = 4.4 MIP signal 50 GeV/c proton (CERN-SPS 2003) Detector performances (1)

    16. Spatial resolution ETA4 sx = (2.77 ± 0.04) mm ETA3 ETA2 sy = (13.1 ± 0.2) mm ETA2 Simulation of silicon detector: best p.f.a. angle-dependent non-linear ETA algorythm(n=number of used strips) 40-100 GeV pions (CERN-SPS 2000) beam-test of a small tracking-system prototype Detector performances (2)

    17. 40-150 GeV/c protons • Track selection cuts: • Nx  5 Ny  4 • Hit views 1x and 6x • 95%-efficiency cut on c2 Multiple scattering Nx & sx MDR ~ 1 TV Momentum resolution 2003  Last beam-test of PAMELA flight model @ CERN-SPS

    18. Very preliminary! Very preliminary! • Very preliminary results: • no efficiency correction • first-order alignment • no ETA p.f.a. On-ground muon results 2005  acquisition of atmospheric particles during PAMELA test before delivering  Check of spectrometer systematics with positive and negative muons

    19. Conclusions PAMELA apparatus integrated and delivered to the Russian space agency launch foreseen for the end of 2005 Detectors tested with particle beams and atmospheric muons during integration phase Spectrometer: sx~3 mm at 0o, sx< 4 mm up to 10o MDR up to 1 TV  The spectrometer meets the requirements for the PAMELA mission

    20. ----------------------------------------------------------------------------------------------------------------------------

    21. Antiproton flux Positron charge ratio The PAMELA experiment MAIN TOPICS: • fluxes measurement • Search for light Antinuclei • Modulation of GCR’s in the Heliosphere • Solar Energetic Particles (SEP) • Earth Magnetosphere • … spectra 80MeV/c … 190GeV/c e+ spectra 50MeV/c … 270GeV/c SECONDARY TOPICS:

    22. Expected Fluxes in 3 Years • ‘Semi-Polar’ orbit (700)  Low energy particles • Wide energy range + 3 years mission  Reliable measurements

    23. Pamela Subdetectors Pamela Subdetectors 1.2 m Mass ~450 kg Acceptance ~20.5 cm2sr • TRD • Threshold device. Signal from e±, no signal from p,p • 9 planes of Xe/Co2 filled straws (4mm diameter). Interspersed with carbon fibre radiators  crude tracking. • Aim: factor 20rejection e/p (above 1GeV/c) (2. 105 with calorimeter) • Anticoincidence system • Defines acceptance for tracker • Plastic scintillator + PMT • Time-of-flight • Trigger / detects albedos / particle identification (up to 1 GeV/c) / dE/dx • Plastic scintillator + PMT • Timing resolution = 120ps • Si Tracker + magnet • Measures rigidity • 5 Nd-B-Fe magnet segments (0.4T) • 6 planes of 300mm thick Si detectors • ~3mm resolution in bending view demonstrated, ie: MDR = 740GV/c • +/-10 MIP dynamic range • Si-W Calorimeter • Measures energies of e±. • DE/E = 15% / E1/2 + 5% • Si-X / W / Si-Y structure. • 22 Si / 21 W  16X0 / 0.9l0 • Imaging: EM - vs- hadronic discrimination,longitudinal and transverse shower profile

    24. e+ The PAMELA Magnetic Spectrometer • Magnetic System • It produces an intense magnetic field region where charged particles follow curved trajectories • Tracking System • It allows to determine six points in the high field region to reconstruct the particle trajectory and so its momentum and charge sign • Momentump=mgv • Charge sign (e+/e-)(p/p) • If B uniform and perpendicular to p, then B

    25. e+ B A glossary of magnetic spectrometersfor cosmic rays studies • Momentum p = qBr (r=radius of curvature) • Rigidity R = p/q = Br • Deflection h = 1/R = q/p • DR/R = Dh/h = R Dh (Dh = constant  point’s measurement error) • Maximum Detectable Rigidity (MDR) : spatial resolution

    26. Permanent magnet elements The “Magnetic Tower” Geometry of a magnetic block MAGNETIC SYSTEM Base Plate prototype Aluminum frame The PAMELA Magnetic Spectrometer • 5 magnetic modules • permanent magnet assembled in an aluminum mechanics • Nd-Fe-B alloy • magnetic cavity sizes: • (132 x 162)mm2 x 445mm • field inside the cavity: • 0.48 T at the center • places for detector planes and electronics boards lodging • Geometric Factor: 20.5 cm2sr • Black IR absorbing painting (not shown in the picture!)

    27. Main field component along the cavity axis Main field component for z=0 (II) Main field component for z=0 (I) The PAMELA Magnetic System Magnetic field measurement • Gaussmeter F.W. Bell equipped with 3-axis probe mounted on a motorized positioning device (0.1mm precision) • Measurement of the three components in 67367 points 5mm apart from each other • Average field along the central axis of the magnetic cavity: 0.43 T • Good uniformity !

    28. The“ladder” Thedetector planes Thesilicon sensor The PAMELA Tracking System The TRACKER • 6 detector planes • each plane: composed by 3 “ladders” • the “ladder”: 2 microstrip silicon sensors + 1 hybrid circuit with front-end electronics (VA1 chip) • silicon sensors: double sided; double metalization; integrated decoupling capacitance • resolutions: • MDR > 740 (GV/c)

    29. Few words on the electronics…. • Requirements: • Very small power consumption • (60 W all included for 36864 readout channels) • Very low noise • (3 mm resolution required!!!!) • Redundancy and safety • (satellite experiment) • Protection against highly ionizing cosmic rays • (Mainly Single Event Effect tests) • Very big data reduction • (4 GB/day of telemetry, 5 Hz trigger rate, 30 GB/day of data, >90% reduction is mandatory) • Solutions: • CMOS low power analog and digital electronics • VA1 chips: ENC = 185 e- + 7.5 e-C(pF) • Small input Capacitance (<20pF) • Decoupling between front-end and read-out • Big modularity, hot/cold critical parts • Selection of components (dedicated tests) • Limiting circuits on the power lines • Architectural `tricks’ (error correction codes, majority logic etc.) • 12 dedicated DSP (ADSP2187) with highly efficient compression alghoritm

    30. Tracker front-end: thermal test • MAGNETIC FIELD MEASUREMENTS • Test were done in the following conditions: • Mechanical aluminum frames with iron replacing the magnet; • Different paints; • System closed in a vacuum chamber to avoid air convection; • Tracker planes replaced by mechanical planes (mechanical silicon + raw alumina) with the front-end electronics replaced by resistors; • Several PT1000 temperature sensors glued on the plane and on the iron blocks.

    31. Silicon gluing points Siliconic glue

    32. Vibrations tests in Galileo (Florence) First resonance frequency: 340 Hz!!!! Test plane survived to +6db spectrum (10.4 g rms) and repeated 50 g/5 ms + 40g/1 ms shocks

    33. ZOP compression algorithm No Zero Suppression (Losses of particles in case of bad strips or change in the pedestals!!!) We use a reversible alghoritm (Zero Order Predictor, ZOP) Deventstrip = ADC eventstrip - PEDstrip - CNevent Deventstripis distributed around 0 First word is transmitted Following word is transmitted if above/below ns . . A word is transmitted with the corresponding address if the preceding one was not transmitted If a cluster is identified (Deventstrip> N s)  +/- 2 strips are transmitted On 2000 Beam Tests: 94.6% compression factor no loss of resolution no loss of efficiency 3.3 kB/event (tracker)

    34. Some results on the compression… Compression time<1ms Compression factor>96% • Decompressed data • Non compressed data First Plane Signal/Noise • Decompressed data • Non compressed data Last Plane Resolution Dx (mm)

    35. 2002: production of flight model detector planes Performances obtained with cosmic rays in Firenze : s/n for MIP

    36. Spatial resolution (July 2000 beam test with 5 ladder prototype MS) Dp/p versus p • DISTRIBUTION • = 1 / R = q / p July 2000: CERN SPS • FINAL LADDERS • FINAL ELECTRONICS • SMALLER MAGNETIC SYSTEM

    37. Signal/Noise s/n  26 s/n  52 Signal non bending view bending view 300 GeV/c Electron event non bending viewbending view July 2002: CERN SPS During the last test (June 2002) the spectrometer flight model has been tested to determine the performances