Polarimetry at JLab

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

Marcy Stutzman 15 July 2014 LDRD AESOP

Polarimetry at JLab

• High precision, accurate polarimetry essential at JLabParity 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 DanceJ.M.Grames et al., PRSTAB 7, 042802 (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 3D3 → 5s 3P2 )

Measure polarization of optical fluorescence

Marcy Stutzman 15 July 2014 LDRD AESOP

Determine Pe from Stokes parameters

𝑃𝑒=𝑃3

[𝑎+𝑏𝑃1]

P3 → Electron polarization in the direction of the emission direction

P1 → Analyzing Power P2 → Validity of the kinematic assumptions

a,b exactly computable

for Ar the 5p 3D3 → 5s 3P2

a = 2/3b = 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

Lens

es

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

– Stokes parameter pressure effects– 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 parametersRotating waveplateElectro-optic devicesBeam splitter comparator

Verify optical polarization measurement accuracy to 0.1% or better

Stokes parametersP1 , P2 , P3

Simplified setup without windows, focusing optics, or PMTs: ~$9k equipment + labor

Marcy Stutzman 15 July 2014 LDRD AESOP

Statistical Accuracy: gas targetPolarimeter Pirbhai and Gay JLab proposed

Figure of merit 270 Hz/nA 30 Hz/nA

Pe 0.2000(4) .800(16)

time 13 sec 5 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

EI

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

Lens

es

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 experimentOptical setup ~$9k + labor + overheadElectron spectrometer ~$9k + labor + overhead

Marcy Stutzman 15 July 2014 LDRD AESOP

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