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Elena Vannuccini (INFN Florence) on behalf of the PAMELA collaboration

In-flight performances of the PAMELA magnetic spectrometer. Elena Vannuccini (INFN Florence) on behalf of the PAMELA collaboration. The PAMELA experiment. PAMELA flight model. MAIN TOPICS: CR antiproton and positron spectra: ~10 4 antiprotons  80 MeV/c - 190 GeV/c

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Elena Vannuccini (INFN Florence) on behalf of the PAMELA collaboration

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  1. In-flight performances of the PAMELA magnetic spectrometer Elena Vannuccini (INFN Florence) on behalf of the PAMELA collaboration

  2. The PAMELA experiment PAMELA flight model • MAIN TOPICS: • CR antiproton and positronspectra: • ~104antiprotons 80 MeV/c - 190 GeV/c • ~105positrons 50 MeV/c - 270 GeV/c • search for light antinuclei • SECONDARY TOPICS: • Modulation of GCRs in the Heliosphere • Solar Energetic Particles (SEP) • Earth Magnetosphere • PAMELA on board of Russian satellite Resurs DK1 • Orbital parameters: • inclination ~70o ( low energy) • altitude ~ 360-600 km (elliptical) • active life >3 years ( high statistics) More about PAMELA: E.Mocchiutti – H01 – 14 July 11:00 M.Pearce – E19 – 18 July 09:55 •  Launched on 15th June 2006 • First switch-on on 21st June 2006 • Detectors in nominal conditions (no problems due to the launch) • Tested different trigger and hardware configurations • Commissioning phase successfully ended on September 15th 2006 •  PAMELA in continuous data-taking mode since then! Launch from Baykonur Elena Vannuccini

  3. PAMELA detectors Main requirements  high-sensitivity antiparticle identification and precise momentum measure + - • Time-Of-Flight • plastic scintillators + PMT: • Trigger • Albedo rejection; • Mass identification up to 1 GeV; • - Charge identification from dE/dX. • Electromagnetic calorimeter • W/Si sampling (16.3 X0, 0.6 λI) • Discrimination e+ / p, anti-p / e- • (shower topology) • Direct E measurement for e- • Neutron detector • plastic scintillators + PMT: • High-energy e/h discrimination GF: 21.5 cm2 sr Mass: 470 kg Size: 130x70x70 cm3 Power Budget: 360W • Spectrometer • microstrip silicon tracking system+ permanent magnet • It provides: • - Magnetic rigidity R = pc/Ze • Charge sign • Charge value from dE/dx Elena Vannuccini

  4. Maximum energy determined by spectrometer performances (wrong determination of charge sign) Antiprotons Unexplored Region • Secondary component • CR propagation • Primary source ((?)) • Dark matter • Extragalactic primordial p-bar • Spectrometer required performances: • 4 mmresolution on the bending view (x)  MDR = 740 GV  spillover limit 190 GeV • ( MDR = Maximum Detectable Rigidity  DR/R=1 @ R=MDR where R=pc/Ze ) Elena Vannuccini

  5. Magnetic module Magnet elements Aluminum frame The 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: 21.5 cm2sr • Black IR absorbing painting • Magnetic shields Magnetic tower Base plate prototype Elena Vannuccini

  6. The magnetic field • 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@ center • Average field along the axis:0.43 T • Good uniformity • External magnetic field: magnetic momentum < 90 Am2 Elena Vannuccini

  7. The tracking system • 6 detector planes, each composed by 3 ladders • Mechanical assembly • aluminum frames • carbon fibers stiffeners glued laterally to the ladders • no material above/below the plane • 1 plane =0.3% X0 reduced multiple scattering • elastic + rigid gluing Carbon fibers LADDER First assembled plane Test of plane lodging inside the magnet Elena Vannuccini

  8. Silicon detector ladders • 2 microstrip silicon sensors • 1 “hybrid” with front-end electronics • Silicon sensors (Hamamatsu): • 300 mm, double sided- x & y view • AC coupled (no external chips) • double metal (no kapton fanout) • • 1024 read-out channels per view • - strip/electrode coupling ~ 20 pF/cm; • channel capacitance to ground: • - junction: < 10 pF • - ohmic: < 20 pF • Bias: • VY -VX = + 80 V fed through guard ring surrounding the strips • Bias resistor: • - junction: punch-through, > 50 MΩ; • - ohmic: polysilicon, > 10 MΩ. • Leakage current < 1 μA/sensor. Elena Vannuccini

  9. cluster S/N = 7/6 (x/y) -2 -1 0 1 2 S/N = 4 Signal-to-noise In-flight basic performaces X view Y view N ~ 4 ADC counts N ~ 9 ADC counts • Tracking system calibrated @ every orbit (95 min) • Data acquisition in compressed mode (~5%) • 12 x 250 B ~ 3 kB/ev • (5 kB/ev all detectors) • system is stable • good signal-to-noise performaces Y view larger noise  worse performances X view lower noise  better performances Elena Vannuccini

  10. Saturated clusters X view Y view Charge identification capabilities Beam-test data (@GSI 2006) flight data 12C projectiles on Al and polyethylene targets (track average) 4He B,C 3He d Be p Li • Good charge discrimination of H and He • Single-channel saturation at ~10MIP affects B/C discrimination Elena Vannuccini

  11. Spatial resolution Sensor instrinsic resolution Spatial resolution studied by means of beam-test of silicon detectors and simulation Simulation Simulation COG ETA4 ETA2 ETA3 ETA2 Non-linear algorythm with 2,3,4 strips Center-Of-Gravity • junction side (X): 3 mm @0o, < 4 mm up to 10o ( determines momentum resolution) • ohmic side (Y): 8÷13 mm Sensor alignment(relative to mechanical positions) Track-based alignment: minimization of spatial residuals as a function of the roto-traslational parameters of each sensor @ground proton beam and atmospheric muons (cross-check) ~100±1 mm @flightobserved displacements relative to ground alignment~10 mm Necessary to align in flight !! Elena Vannuccini

  12. (ymeas-yfit) (xmeas-xfit) In-flight alignment  Done with relativistic protons (high statistics) Flight data Simulation Spatial residuals (1st plane) protons 7-100 GV X side Y side • After alignment: • residuals are centered • width consistent with nominal resolution Elena Vannuccini

  13. 100 x multiple scattering spatial resolution (x) R (GV) Momentum resolution Beam test - protons Iterative c2 minimization as a function of track state-vector components a MDR ~ 1TV η = 1/R magnetic deflection sR/R = sh/h Maximum Detectable Rigidity (MDR) def: @ R=MDR sR/R=1 MDR = 1/sh Trajectory evaluated by stepwise integration of motion equations by means of Runge-Kutta method (not-homogeneous B field) • Measured at beam test with protons of known momentum (CERN SPS, 2003) • In-flight: (possible) global distortions after alignment procedure  deflection offset • cross-check with electrons and positrons • energy measured by the calorimeter DE/E < 10% above 5GeV Elena Vannuccini

  14. Spectrometer systematics • z < 1 due to Bremstahlung effect in the material above the spectrometer • the pdf of z depends only on the amount of traversed material deflection offset Calorimeter calibration uncertanty electrons Positrons 5÷20 GeV P0~10-5 P0~0.403 Kolmogorov probability between ze- and ze+ (P0 = 0 ÷ 1) with free parameter Dh Dh ~ -10-3 GV -1 Elena Vannuccini

  15. S1 CARD CAT S2 . TOF TRK CAS S3 CAL S4 ND High-energy antiproton analysis Event selected from 590 days of data Basic requirements: • Clean pattern inside the apparatus • single track inside TRK • no multiple hits in S1+S2 • no activity in CARD+CAT • Minimal track requirements • energy-dependent cut on track c2 (~95% efficiency) • consistency among TRK, TOF and CAL spatial information • Galactic particle • measured rigidity above geomagnetic cutoff • down-ward going particle (no albedo) Elena Vannuccini

  16. electron (17GV) Antiproton (19GV) 1 GV 5 GV Antiproton identification • dE/dx vs R (S1,S2,TRK) and b vs R proton-concistency cuts • electron-rejection cuts based on calorimeter-pattern topology -1  Z  +1 p (+ e+) p e-(+ p-bar) “spillover” p p-bar Elena Vannuccini

  17. 10 GV 50 GV Proton spillover background p p-bar “spillover” p MDR = 1/sh evaluated event-by-event by the track fitting routine • MDR account for: • number and distribution of fitted points along the trajectory • spatial resolution of the single position measurements • magnetic field intensity along the trajectory Elena Vannuccini

  18. Proton spillover background Minimal track requirements MDR > 850 GV • Strong track requirements: • strict constraints on c2 (~75% efficiency) • rejected tracks with low-resolution clusters along the trajectory • - faulty strips (high noise) • - d-rays (high signal and multiplicity) Elena Vannuccini

  19. 10 GV 50 GV High-energy antiproton selection p p-bar Elena Vannuccini

  20. 10 GV 50 GV High-energy antiproton selection p p-bar Elena Vannuccini

  21. 10 GV 50 GV High-energy antiproton selection p p-bar R < MDR/10 Elena Vannuccini

  22. Antiproton/proton ratio Preliminary! ~300 p-bar Elena Vannuccini

  23. Antiproton/proton ratio Preliminary! Secondary production CR+ISM p-bar + … Elena Vannuccini

  24. Conclusions PAMELA is in space, continuously taking data since July 2006 Detectors have been calibrated and in-flight performances has been studied  PAMELA now ready for science!! Magnetic spectrometer: - basic performances (noise, cluster signal, spatial resolution...) are nominal - tracking system alignment completed (incoherent+coherent) • Spectrometer performances (momentum resolution) fulfill the requirements of the experiment • Preliminary results about high-energy antiproton abundance could be obtained!! Work in progress to extend antiproton measurement further in energy  thanks! 

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