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GLAST calorimeter performance Energy, position & direction Background rejection

GLAST calorimeter performance Energy, position & direction Background rejection. Calibration procedure In-flight calibration using cosmic rays Energy measurement Low energy: correction for losses in the tracker High energy: correction for shower leakage Position measurement

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GLAST calorimeter performance Energy, position & direction Background rejection

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  1. GLAST calorimeter performanceEnergy, position & directionBackground rejection Régis Terrier PCC Collège de France

  2. Calibration procedure In-flight calibration using cosmic rays Energy measurement Low energy: correction for losses in the tracker High energy: correction for shower leakage Position measurement Longitudinal position Transverse position Background rejection Outline Régis Terrier PCC Collège de France

  3. Energy measurement – Astrophysical drivers • Energy range: from 20 MeV to 300 GeV (up to 1 TeV) • Spectral measurements • Straight and broken power laws • Cut-off energies (pulsars…) • High energy lines (WIMPS?) • Thin calorimeter (8.4 X0) with 1.1 X0 tracker • Wide field of view • A large fraction of events are well contained Régis Terrier PCC Collège de France

  4. Calibration procedure • Calibration in two steps: • Beam tests of a group of 4 modules on the ground to validate simulations • In flight calibration using cosmic rays nuclei H, He, C, N, O, Ne, Mg, Si, and Fe spanning nearly full dynamic range of CAL readout • Calorimeter measures deposited energy • Elements of calibration process: • Extract multiMIP events. • Identify likely Galactic Cosmic Rays • Fit tracks • Accept events with clean track through log • Identify charges. • Identify charge-changing interactions. • Identify mass-changing interactions. • Fit dE/dx. • Accumulate energy losses and light asymmetries. Régis Terrier PCC Collège de France

  5. Calibration - Nuclei identification • CAL module tested on GSI beam • 700 MeV/A Ni beam • polystyrene target upstream • Nuclear species are determined • even in presence of a spectrum Régis Terrier PCC Collège de France

  6. GLAST Tracker is 1.1Xo thick large fraction of energy never reaches CAL Use the tracker as a sampling calorimeter Find hits in a cone around fitted track coneopening angle 5 MS Around 90% of the hits in this cone are due to the track (as opposed to electronic noise) Energy Measurement below 500 MeV Hits in upper layers Hits in lower layers Energy in Calorimeter Régis Terrier PCC Collège de France

  7. Energy Measurement below 500 MeV 68% Containment slope = - 0.44 20% Fitted Gaussian slope = - 0.59 Ecal 10% Ecorr 5% 20% 68% Containment slope = - 0.33 10% Fitted Gaussian slope = - 0.51 5% 50 100 200 400 Energy (MeV) Régis Terrier PCC Collège de France

  8. Shower containment in the calorimeter limited by: -Losses in the tracker - Leakage from the back, cracks and sides of CAL Strong non-linearity of response at low and high energies Necessity to correct for: - Longitudinal leakage - Shower Profile fitting Last layer correction Lateral leakage GLAST – Energy losses Less than 30% containment at very high energies (>300 GeV) Tracker energy loss (diffusion dE/dX) For high energies (several tens of GeV) at least 30% leakage Régis Terrier PCC Collège de France

  9. Minimizing global width on MC data on a layer by layer basis Contribution from last layer only Energy deposited in the last layer is proprtionnal to the number of escaping particles Energy estimate given by: Ecorr = f(Esum,) Elast +Esum f depends on deposited energy and angle Restore linearity and provides good energy resolution Works as long as shower maximum is contained Shower leakage - Last layer correction Last layer 68% containment gaussian 0°30°45°60° losses in thetracker Régis Terrier PCC Collège de France

  10. Minimize: Longitudinal energy density profile model: Parameters: E0 incident energy free z0 shower starting point parameters  fixed to their  mean value at E0 Mean profile fitting Restores linearity over the whole energy range Gives energy measurement even when shower maximum is not contained Good energy resolution up to very high energies (~20% at 1 TeV normal incidence) Shower leakage - Mean shower profile fitting Fit Last layer Sum Régis Terrier PCC Collège de France

  11. 1999-2000 SLAC beam tests Prototype GLAST tower CAL, TKR at end station A CAL module made of 80 crystals 2.1 cm thick For 20 GeV normal incidence electrons: 7% (raw energy sum) 5% (profile fitting) 4% (last layer) Cf Do Couto e Silva et al. 2001 Energy reconstruction - Performances Régis Terrier PCC Collège de France

  12. Energy reconstruction performances Depending on energy and angle, different corrections have to be applied Fitted energy resolution for very high energies At angles larger than 50°, less than 6% resolution at 1 TeV Régis Terrier PCC Collège de France

  13. Position et Direction - Motivations • Motivations : • - Hadronic background rejection • - Pointing improvement for high energies • - CAL only direction determination • Hodoscopic array of crystals provides 1 position per layer: • Longitudinal: for 1 crystal • With x barycenter of energy deposition along crystal axis • The crystal with highest energy deposition yields the best estimate. • Transverse: energy weighted mean of crystal positions • Pb: Both methods are biased. Régis Terrier PCC Collège de France

  14. Longitudinal position • SLAC 97 Beam test • 3x3x19 cm crystals • High performance readout • In practice, limited by • electronic noise • systematics Régis Terrier PCC Collège de France

  15. For large incidence angles, the barycentre position in a crystal is different from the shower axis effect due to the mixing of longitudinal and transverse shower profiles Longitudinal bias Position –Longitudinal bias Increasing profile Shower Maximum Decreasing profile Régis Terrier PCC Collège de France

  16. Bias due to layer segmentation (size of a crystal larger than lateral extension of the shower) baryrec barytrue Correction using the usual S shape function depends on depth in the CAL incident energy For non-zero incidence angles, previous shift has to be added Position – Transversal bias 3rd layer 5th layer 7th layer Régis Terrier PCC Collège de France

  17. After correction, longitudinal dispersion is twice as good as transverse dispersion Barycenter dispersion as function of energy: Position measurement Transverse Longitudinal Errors after correction Note that this doesn ’t include systematics due to non linearity of preamps, charge injection and asymetry calibration precision Régis Terrier PCC Collège de France

  18. Background rejection 1:106 required constraint comes from extragalactic gamma ray background ACD efficiency is 0.9997 CAL & TKR need to reject remaining background Background rejection CR protons EGRB CR electrons e Upward moving particles: shower orientation High energy hadrons (over a few GeV) CAL is 0.5 int 40 to 80% incident protons interact and produce a shower - Lateral and longitudinal spread - barycenter position - difference between CAL and TKR directions 0.99 efficiency easily achievable p Régis Terrier PCC Collège de France

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