AESOP: Accurate Electron Spin Optical Polarimeter Marcy L. Stutzman, Matt Poelker; Jefferson Lab Timothy J. Gay; University of Nebraska

Marcy Stutzman 15 July 2014 LDRD AESOP 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 Proposal: Calibrate the Mott to an absolute uncertainty of 0.5% Spin Dance J.M.Grames et al., PRSTAB 7, (2004)

Marcy Stutzman 15 July 2014 LDRD AESOP 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

Marcy Stutzman 15 July 2014 LDRD AESOP AESOP: Accurate Electron Spin Optical Polarimetry Dayhoff (<1956) Excite gas target with polarized electron beam using exchange excitation of atomic fluorescence (Russell-Saunders triplet state Argon: 5p 3 D 3 → 5s 3 P 2 ) Measure polarization of optical fluorescence

Marcy Stutzman 15 July 2014 LDRD AESOP Determine P e from Stokes parameters P 3 → Electron polarization in the direction of the emission direction P 1 → Analyzing Power P 2 → Validity of the kinematic assumptions a,b exactly computable for Ar the 5p 3 D 3 → 5s 3 P 2 a = 2/3 b = 4/27

Electron source - Old Horizontal Gun 2? Voltage dependent decelerator V. Wien 127° cylindrical deflector Target Chamber ~10” H. Wien Target Chamber Large chamber Turbo pump Gas inlet system Gas target pump ~10 -4 Torr Beam direction Optical polarimetry Dump Electrical feedthroughs pumping 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 Lenses Proposed AESOP Layout

Marcy Stutzman 15 July 2014 LDRD AESOP Requirements to achieve goals Optical polarization measurement accuracy Electron beam energy spread Characterization of pressure dependent effects –S–Stokes parameter pressure effects –C–Cascade pressure effects Target pressure isolation from electron source Magnetic field isolation (Hanle depolarization)

Marcy Stutzman 15 July 2014 LDRD AESOP 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

Marcy Stutzman 15 July 2014 LDRD AESOP Optical polarimetry verification setup Light source  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 Stokes parameters P 1, P 2, P 3 Simplified setup without windows, focusing optics, or PMTs: ~$9k equipment + labor

Marcy Stutzman 15 July 2014 LDRD AESOP Statistical Accuracy: gas target PolarimeterPirbhai and GayJLab proposed Figure of merit270 Hz/nA30 Hz/nA Pe0.2000(4).800(16) time13 sec5 minutes 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 Nebraska data 1996 in less than 100s data collection 0.2% statistical + 0.2% systematic polarization 0.4% absolute electron polarization and 0.3% Mott precision 0.5% Mott polarimeter for CEBAF

Marcy Stutzman 15 July 2014 LDRD AESOP 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 superlattice photocathodes, 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 E I E dE

Marcy Stutzman 15 July 2014 LDRD AESOP Voltage dependent decelerator Electron source - Old Horizontal Gun 2? V. Wien 127° cylindrical deflector Target Chamber ~10” H. Wien Optical polarimetry Dump Electrical feedthroughs pumping Lenses Measure Energy Spread – Electron Source – Deceleration – Electron spectrometer – Scanning slit, electrometer – Measure energy profile – Equipment ~$9k + labor

Marcy Stutzman 15 July 2014 LDRD AESOP 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

Marcy Stutzman 15 July 2014 LDRD AESOP ¿e?