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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 – for the positron polarimeter Expected Polarimeter Performance SLAC EPAC 12 June 2003 E166 Proposal Presentation K.P. Schüler

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

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

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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

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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

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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

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

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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

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

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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

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

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

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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

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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

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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

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16 Expected Positron Polarimeter Performance Simulation based on modified GEANT code which correctly describes the spin-dependence of the Compton process 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

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

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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

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

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