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Ion charge measurement with the AMS-02 silicon tracker

Ion charge measurement with the AMS-02 silicon tracker. 1rst Int. Workshop on High Energy cosmic-Radiation Detection October 17-18, 2012 IHEP CAS, Beijing. Martin Pohl, Pierre Saouter Center for Astroparticle Physics University of Geneva Alberto Oliva CIEMAT Madrid.

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Ion charge measurement with the AMS-02 silicon tracker

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  1. Ion charge measurementwith the AMS-02 silicon tracker 1rst Int. Workshop on High Energy cosmic-Radiation Detection October 17-18, 2012 IHEP CAS, Beijing Martin Pohl, Pierre Saouter Center for Astroparticle Physics University of Geneva Alberto Oliva CIEMAT Madrid

  2. Si TrackerChargeMeasurement Strip crosstalk Gain (at VA level, usingH, He and C) Charge loss (position/angle dependence) MIP scale conversion (saturation, non-linearities) From ADC to energydeposition Detector relatedcorrections Pathlength correction Beta/Rigidity correction (layer dependent) PDFZ(Edep) Likelihood From floating point charge estimator to integer charge (Z) Fromenergydepositionto floatingpointchargeestimators (Q)

  3. Si TrackerChargeMeasurement • Physics: • From physics to ADC: • Si material properties • Nuclear charge: z2 • β and βγ: eV/μm • Path length in Si: dx • Ionisation yield: eVfC • Charge collection efficiency on strips • ASIC response function • Channel cross talk: ADC

  4. The AMS SiliconTracker • 9 planes: 18 to 26 ladders • Ladder : 7 to 15 double-sidedsiliconsensors. • Implantationpitch p(n) side 27.5 (104) μm • Readout pitch p(n) side 110 (208) μm (1/4 and 1/2 stripsread out) • Signal usually collected by several adjacent strips (cluster) • Doublethreshold to eliminate insignificant strips Ionization Energy Loss Cluster Amplitude

  5. Front-endelectronics p-side n-side VA64hdr 10 VAson the p-side (Y direction) 6 VAson the n-side (X direction) Each VA reads64 channels • Each VA produces a signal with different characteristics • In particular differences in the gain are observed • FEE response curve is deliberately non-linear, different for p and n

  6. Example of Gain Differences for He for p-side VAs of Ladder +307 Helium Sample Raw ADC Typical ~10%, max ~35% x 10

  7. Amplitude distribution (protons, single VA) Single VA Distribution for Proton. • Landau function convoluted with a Gaussian • MPV to characterize the gain of a given VA

  8. Cluster pulse integral (single ladder) as function of ion charge Si • Two sides behave differently: • Maximum dynamic range • Good resolution at low charge • Two ~ linear response regimes • Same behavior expected for all VA n side B p side Alpat B. & al., 2004 (2003 Cern and GSI Test Beam)

  9. Charge Calibration Sample Selection • Uncalibrated charge response with rather good resolution • Define charge samples using truncated mean of hits on nside, corrected for impact angle • 1σ selection ranges around MPV He H • Avoid any bias in selection: • separate ranges for each layer • truncated mean excluding layer under study • (see later) C O Li B N Be

  10. Charge Calibration Sample Selection • Proton • Helium • Carbon X-side Clusters VA Number

  11. Reference MPV values for each charge • Proton • Helium • Carbon Readout Region Individual VA gains equalized on reference value

  12. Good linearity of VA64 response Gain Corr. Fact • Gain factor inde-pendent of particle impact location • Small offset due to thresholds on seed and adjacent strips

  13. Gain Correction Factors and Offsets At most 10% correction needed. Offset must be taken into account in gain correction!

  14. Deviation of VA MPV values from Linear Fit Systematic error ~ 3%

  15. Gain Correction Effect on H, He and C Samples • No Correction • Gain Correction • Including Offsets RMS improves by factor of 3.5

  16. Gain Systematics • Layer 1 • Layer 2 • Layer 3 • Layer 4 • Layer 5 • Layer 6 • Layer 7 • Layer 8 • Layer 9 • Each point is mean of VA response per layer, with RMS as error • RMS is larger for layer 1 • Systematics less than 0.5% << statistical error on gain factor

  17. Track Truncated Mean n Side He Before Correction After Gain Corrections H Number Nuclei C O B Li N Be Ne Mg Si Na F

  18. Zoom on High Charges n Side C Before Correction After Gain Corrections O B N Be Ne Mg log (Number Nuclei) Si Na F

  19. Resolution of Charge Estimator After Gain Correction • n side before correction • n side after gain correction A. Oliva

  20. Track Truncated Mean p Side He Before Correction After Gain Corrections Number Nuclei H C O Li B Be Ne

  21. Charge Collection Efficiency Charge loss ~30%for Helium Particle very near a readout strip. Particle passes in between two readout strips. 0 Capacitive coupling between strips allowsto estimate impact positionof the traversing particle (COG). Loss of collection efficiency in thenon-readout region

  22. Charge Collection:Impact Point and Angle Z Y X Z XZ Projected Track θXZ X

  23. Implant structure and n/p side differences • n - side: 1 out of 2 strips read out + saturation • p - side: 1 out 4 strips readout + non linearity at low charges (B,C,O) different charge collection behavior Charge Loss For Carbon Sample ADC ADC N-Side / Z=6 / ~28% P-Side / Z=6 / ~35%

  24. Track Truncated Mean n Side C O B N Be Ne F • No Corr • Gain Corr • Gain + Charge Loss

  25. Resolution of Charge Estimator After Correction

  26. Track Truncated Mean p Side C O Li B Be Mg Si Fe

  27. Path Length Correction Normalization to 300 μm of Silicontraversed.

  28. Beta Correction: Layer-by-Layer (II) Z = 1 Z = 2 Layer 1 Layer 1 Effect of TRD + upper TOF Effect of TRD + upper TOF Z = 1 Z = 2 Layer 4 Layer 4

  29. Beta Correction: Layer-by-Layer (III) Layer 8 Z = 1 Z = 2 Layer 8 Z = 1 Layer 9 Z = 2 Layer 9 Effect of RICH + lower TOF Effect of RICH + lower TOF

  30. Beta Correction β > βTOF Protons βTOF TOF measures βinsideAMS Helium β< βTOF

  31. Tracker Charge Measurement n Track Truncated Mean n–Side (c.u.) Z>10 should use p-side Track Truncated Mean p–Side (c.u.)

  32.  MIP Correction  • Transforms corrected response into charge units. • Accounts for saturation and non-linearity • Directly provided as an outcome of the charge loss correction n side p side • Gives almost linear charge estimator • Some residual deviation left in the non-linearity regions

  33.  Joint Track Charge Estimator  • Combine the n and p measurement with a weighted sum. • Weights depend on the number of hits used • Weights assumed to be independent of Z (approximately correct) H x 10-3 C He x 10-2 O Be Si Fe

  34. Going to PDF Layer 2 charge distributions 2 1 6 8 5 3 7 4 10 12 14 9 26 • This shapes should be understood in detail • Tails from wrong hit associated to tracks, interactions… • Specific ladder behavior • Dependencies on external parameters: t, T …

  35. Carbon: Rigidity=215 GV, P=1288 GeV, Ekin/A=106 GeV/n ZTRK_L1=6.1 ZTRD=5.9 ZTOF_UP=5.9 ZTRK_IN=5.8 ZTOF_LOW=5.8 ZRICH=6.1

  36. Boron: Rigidity=187 GV, P=935 GeV, Ekin/A=93 GeV/n ZTRK_L1=4.9 ZTRD=4.5 ZTOF_UP=5.0 ZTRK_IN=4.9 ZTOF_LOW=5.1 ZRICH=5.2

  37. Tracker and ToF H He C Li N O B Be Ne Mg Si F Na Al S P Cl Ar Fe Ca K Sc Ti Ni Cr V

  38. Conclusions • AMS Si tracker shows excellent nuclear charge identification: • Excellent charge separation • Simple unfolding of species • Complete calibration chain in place: • Floating point charge estimator • Probabilistic approach based on PDF • Redundancy of subdetectors is key to systematic accuracy: • Tracker • ToF • RICH • Chemical composition of cosmic rays GeV to TeV

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