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POLARIMETRY of MeV Photons and Positrons

POLARIMETRY of MeV Photons and Positrons. Overview Beam Characterization undulator photons positrons Basics of the Transmission Method for photon polarimetry for positron polarimetry Description of the Layouts and Hardware for the photon polarimeter

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POLARIMETRY of MeV Photons and Positrons

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  1. POLARIMETRYof MeV Photons and Positrons Overview Beam Characterization undulator photons positrons Basics of the Transmission Method for photon polarimetry for positron polarimetry Description of the Layouts and Hardware for the photon polarimeter for the positron polarimeter Expected Polarimeter Performance SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  2. Undulator Photon Beam undulator basics(1st harmonic shown only) E166 undulator parameters SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  3. Undulator Photon Beam E166 undulator: photon spectrum, angular distr. and polarization SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  4. Positron Beam Simulation distributions behind the converter target (0.5 r.l. Ti) based on polarized EGS shower simulations by K. Flöttmann SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  5. Low-Energy Polarimetry Candidate Processes • Photons: Compton Scattering on polarized electrons • forward scattering (e.g. Schopper et al.) • backward scattering • transmission method (e.g. Goldhaber et al.) • Positrons: all on ferromagnetic = polarized e- targets • Annihilation polarimetry (e+e-  ) (e.g. Corriveau et al.) • Bhabha scattering (e+e-  e+e-) (e.g. Ullmann et al.) • brems/annihilation (e+  ) plus -transmission (Compton) polarimetry Principle difficulties of e+ polarimetry: • huge multiple-scattering at low energies even in thin targets • cannot employ double-arm coincidence techniques or single-event counting due to poor machine duty cycle • low energies below 10 MeV, very vulnerable to backgrounds All of the candidate processes have been explored by us:  the transmission method is the most suitable  SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  6. Transmission Polarimetry of (monochromatic) Photons M. Goldhaber et al. Phys. Rev. 106 (1957) 826. all unpolarized contributions cancel in the transmission asymmetry  (monochromatic case) SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  7. monochromatic case Transmission Polarimetry of Photons Analyzing Power: But, undulator photons are not at all monochromatic:  Must instead use integrated numbers or energies  SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  8. Transmission Polarimetry of Positrons 2-step process: • re-convert e+   via brems/annihilation process • polarization transfer from e+ to  proceeds in well-known manner • measure polarization of re-converted photons with the photon transmission method discussed earlier • infer the polarization of the parent positrons from the measured photon polarization experimental challenges: • huge angular distribution of the positrons at the production target: • e+ spectrometer collection & transport efficiency • background rejection issues • huge angular distribution of the re-converted photons • detected signal includes large fraction of Compton scattered photons • requires extensive simulations to determine the effective Analyzing Power formal procedure: Fronsdahl & Überall; Olson & Maximon; Page; McMaster SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  9. Polarimeter Layout Overview SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  10. Analyzer Magnets g‘ = 1.919  0.002 for pure iron Scott (1962) Error in e- polarization is dominated by knowledge in effective magnetization M along the photon trajectory: active volume Photon Analyzer Magnet: 50 mm dia. x 150 mm long Positron Analyzer Magnet: 50 mm dia. x 75 mm long SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  11. Photon Polarimeter Detectors Si-W Calorimeter Aerogel threshold Cerenkov SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  12. Positron Polarimeter Layout E166 Proposal Presentation K.P. Schüler SLAC EPAC 12 June 2003

  13. Positron Transport System e+ transmission (%) through spectrometer photon background fraction reaching CsI-detector SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  14. CsI Calorimeter Detector Crystals: from BaBar Experiment Number of crystals: 4 x 4 = 16 Typical front face of one crystal: 4.7 cm x 4.7 cm Typical backface of one crystal: 6 cm x 6 cm Typical length: 30 cm Density: 4.53 g/cm³ Rad. Length 8.39 g/cm² = 1.85 cm Mean free path (5 MeV): 27.6 g/cm² = 6.1 cm No. of interaction lengths (5 MeV): 4.92 Long. Leakage (5 MeV): 0.73 % Photodiode Readout (2 per crystal): Hamamatsu S2744-08 with preamps SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  15. Expected Photon Polarimeter Performance Si-W Calorimeter energy-weighted mean: Expected measured energy asymmetry and energy-weighted analyzing power determined through analytic integration and. with good agreement, through special polarized GEANT simulation Aerogel Cerenkov See Table 12 all measurements very fast  only syst. Error of should matter SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  16. Simulation based on modified GEANT code which correctly describes the spin-dependence of the Compton process Expected Positron Polarimeter Performance Photon Spectrum & Angular Distr. number & energy-weighted Analyzing Power vs. Energy 10 Million simulated e+ per point & polarity on the re-conversion target SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  17. Expected Positron Polarimeter Performance Table 13 SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  18. Expected Positron Polarimeter Performance Analyzing Power vs. Target Thickness Analyzing Power vs. Energy Spread SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

  19. Spin-Dependent Compton Scattering SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

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