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Polarimetry at JLab

AESOP: Accurate Electron Spin Optical Polarimeter Marcy L. Stutzman , Matt Poelker ; Jefferson Lab Timothy J. Gay; University of Nebraska. Polarimetry at JLab. High precision, accurate p olarimetry essential at JLab Parity violation experiments: MOLLER, SOLID

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Polarimetry at JLab

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  1. AESOP: Accurate Electron Spin Optical PolarimeterMarcy L. Stutzman, Matt Poelker; Jefferson LabTimothy J. Gay; University of Nebraska

  2. Polarimetry at JLab • High precision, accurate polarimetry essential at JLab Parity violation experiments: MOLLER, SOLID • Stringent polarimetry requirements for EIC • Improvements in precision of Compton, Moeller and Mott polarimeters • Discrepancies persist Spin Dance J.M.Grameset al., PRSTAB 7, 042802 (2004) Proposal: Calibrate the Mott to an absolute uncertainty of 0.5%

  3. Experimental Mott Scattering • Polarization = Asym. x Sherman fn. • Sherman function must be calculated • Updating with GEANT4 simulations • Improving theoretical understanding • Hardware upgrades to reduce background • 0.3% precision anticipated • Accuracy depends on Sherman function Proposal: Use AESOP to measure beam polarization absolutely Use p to measure Sherman function

  4. AESOP: Accurate Electron Spin Optical Polarimetry Excite gas target with polarized electron beam using exchange excitation of atomic fluorescence (Russell-Saunders triplet state Argon: 5p 3D3 → 5s 3P2) Measure polarization of optical fluorescence Dayhoff (<1956)

  5. Determine Pe from Stokes parameters a,b exactly computable for Arthe 5p 3D3 → 5s 3P2 a = 2/3 b = 4/27 P3 → Electron polarization in the direction of the emission direction P2 → Validity of the kinematic assumptions P1 → Analyzing Power

  6. Voltage dependent decelerator 127° cylindrical deflector Proposed AESOP Layout Electron source - Old Horizontal Gun 2? H. Wien V. Wien Electrical feedthroughs pumping Lenses Target Chamber Gas inlet system Target Chamber ~10” Optical polarimetry Dump Beam direction • Custom fabricated chamber • 304L Stainless steel, heat treated • Exact dimensions determined through modeling • Likely 6” diameter transport, 10” diameter for bend • Either circular or rectangular cross sections • mounts for electron optics • ports for pumping and electronics Gas target pump ~10-4Torr Large chamber Turbo pump

  7. Requirements to achieve goals • Optical polarization measurement accuracy • Electron beam energy spread • Characterization of pressure dependent effects • Stokes parameter pressure effects • Cascade pressure effects • Target pressure isolation from electron source • Magnetic field isolation (Hanle depolarization)

  8. Optical polarimeter accuracy Trantham and Gay (1996) Demonstrated 0.8% accuracy in Stokes parameter measurement Astronomy: 0.001% acccuracy demonstrated Acquisition and characterization of high quality optics essential

  9. Optical polarimetry verification setup Light source Stokes parameters P1 , P2 ,P3 • Use existing laser • Generate light with known linear and circular polarization components Test optical polarimeter setup Measure Stokes parameters Rotating waveplate Electro-optic devices Beam splitter comparator Verify optical polarization measurement accuracy to 0.1% or better Simplified setup without windows, focusing optics, or PMTs: ~$9k equipment + labor

  10. Statistical Accuracy: gas target JLab : Design with smaller optical aperture for higher accuracy, lower background 0.2% statistical accuracy using 1μA polarized electron beam near 80% polarization: ~5 minutes 0.2% statistical + 0.2% systematic polarization 0.4% absolute electron polarization and 0.3% Mott precision Nebraska data 1996 in less than 100s data collection 0.5% Mott polarimeter for CEBAF

  11. Energy spread in electron beam • Cascade threshold: 830 meV • dE must be below ~100 meV to avoid cascade effects Expect low dE from thin strained superlatticephotocathodes, µA • Orlov(2004) achieved dE ~10 meV at mA currents Must use electron spectrometer to measure • Measure dE • Measure any polarization variation across dE (25 meV slices of beam) • DC and CW illumination I I dE E E

  12. Measure Energy Spread 127° cylindrical deflector Electron source - Old Horizontal Gun 2? • Electron Source • Deceleration • Electron spectrometer • Scanning slit, electrometer • Measure energy profile • Equipment ~$9k + labor H. Wien V. Wien Electrical feedthroughs pumping Lenses Voltage dependent decelerator Target Chamber ~10” Optical polarimetry Dump

  13. Significance of AESOP to Lab • Accurate , precise polarimetry essential at CEBAF • Parity violation experiments, Electron Ion Collider • Improved Mott precision with upgrade, but reliant on Sherman function calculations (0.3% precision, ~1% accuracy) • AESOP can be used to send beam of known polarization to Mott, perform absolute calibration of device (0.4% absolute) • Calibrated 0.5% Mott polarimeter • Nuclear Physics hall polarimetry • EIC polarimetry • Many challenging tasks, but all should be possible • Demonstrations of crucial components possible prior to undertaking full experiment Optical setup ~$9k + labor + overhead Electron spectrometer ~$9k + labor + overhead

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