Electron Beam Polarimetry

1
Electron Beam Polarimetry
Sirish Nanda Jefferson Laboratory PREX
Workshop Aug 19, 2008

• Motivation
• Polarimetry Principle
• Present Polarimeters
• Future Prospects
• Conclusion

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Motivation

• Accurate knowledge of electron beam polarization
  is necessary for meaningful interpretation of
  physics results in spin asymmetry measurements.
• How accurate?
• Single Spin Asymmetry Experiments
• eElectron polarization uncertainty limits
  physics observables
• Require electron beam polarimetry dp/p lt 1
• Double Spin Asymmetry Experiments
• eHadron polarization uncertainty limits physics
  observables

g
e- N ? e- N Nucleon Structure, Parton
Distributions(PDF), Standard Model(SM) e- A ? e-
A Classical Nuclear Physics, PREX e- e N ? X
SM
g
g
g
g
e- N ? e- N Nucleon Structure, Polarized
target e- N ? e- N Nucleon Structure, FPP e- N
? e- X Deep Inelastic Scattering, PDF
g
-
g
g
g
3
Parity Violating Experiments
Electron Beam Polarization accuracy is often the
dominant uncertainty Planned experiments at JLab
1 Polarimetry accuracy is needed, 0.5 would be
better
4
Electron Polarimetry Principle

• Electron Polarization (Pe) is deduced from
  Scattering Asymmetry (Aexp) from a target
  sensitive to spin dependent interaction with
  known Analyzing power (A)
• Aexp Pe.A
• Spin-Orbit interaction
• Mott Scattering
• - Sensitive to only transverse polarization of
  the electron
• - Applicable to low energy, typically lt 10 MeV
• - Intrusive measurement
• - Accuracy limited by Analyzing power (Sherman
  function)
• Spin-Spin interaction
• Moller Scattering
• - Sensitive to both transverse and longitudinal
  components
• - Figure of merit insensitive to electron energy
• - Intrusive measurement
• - Accuracy limited by target polarization
  knowledge
• Compton Scattering
• - Sensitive to both transverse and longitudinal
  components
• - Figure of merit increases with electron and
  photon energies
• - Non-Intrusive measurement

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Mott Polarimetry

• Coulomb scattering from High Z nucleus
• Measure Left-Right Asymmetry
• Analyzing power given by Sherman Function S(q),
  back
• angle peaked.
• Target thickness, spin rotation, background, and
• radiative corrections
• Unsuitable for high energies

?(?) ??(?) (1 S(??.Pb)
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JLab 5 MeV Mott Polarimeter
Elastic
Gold Scattering Spectrum and Asymmetry
In operation in Jlab Injector Intrusive to beam
delivery to Halls Typical Accuracy dp/p 1
3 Provides good cross check to Hall Polarimetry
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Møller Polarimetry

• Scattering of polarized electrons from polarized
  electrons
• Polarized electron target generally is a
  magnetized Iron foil with 8 polarization
• For free electron Møller scattering
• of longitudinally polarized electrons

• Raw Asymmetry is large at -7/9
• Detect e- pair at 90o CM angle in coincidence
• Asymmetry independent of electron beam energy
• Accuracy limited by target Polarization
  uncertainties
• Target heating limits maximum beam current to
  few mA
• Intrusive measurement, Beam conditions different
  from the experiment

Levchuk Electrons in Fe are bound. One must
correct for this quasi-elastic scattering Levchuk
corrections are generally lt 1, Large
Acceptance in the spectrometer reduces the
correction
8
Hall A Møller Polarimeter

• QQQD Spectrometer
• Diople Mid-plane shield for direct beam pass
• Large acceptance
• - Reduced Levchuk corrections
• Pb-Glass Calorimeter
• Target Supermendeur foil, polarized in-plane
• Low field applied (240 G)
• Tilted 20o relative to beam direction
• Target polarization known to 2
• Improvement in works

Energy range 0.8 - 6 GeV Beam current lt 2
mA dp/P Accuracy 3
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Hall B Møller Polarimeter

• Hall B Møller uses similar target design as Hall
  A ? Fe alloy in weak magnetic field
• Two-quadrupole system rather than QQQD
• Detector acceptance not as large Levchuk effect
  corrections important
• Dominant systematics NIM A 503 (2003) 513
• Target polarization 1.4
• Levchuk effect 0.8

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Hall C Møller Polarimeter

• 2 quadrupole optics maintains constant tune at
  detector

• Target pure Fe foil, brute-force polarized out
  of plane with 3-4 T superconducting magnet

• Total systematic uncertainty 0.47 NIM A 462
  (2001) 382

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Møller Polarimetry Future Plans

• Hall A High Field Transverse target

• Hall A and C High current 50?A
• Half-foil 1 ?m target
• Kicker located upstream of target
• Beam excusion 12 mm at target

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Møller Polarimetry with Atomic Hydrogen Target
E. Chudakov and V. Luppov, IEEE Trans, Nucl. Sci
51, 1533 (2004)
Atomic Hydrogen in an Ultra-Cold Trap

• Target polarization known precisely, in
  principle!
• Non-Intrusive measurement
• Samples the same beam as the experiment, similar
  to Compton Polarimetry
• Expected Accuracy lt 0.5 over broad Energy range
• Exiting new prospect, but unproven on the bench.
  Needs substantial development!

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Compton Polarimetry
s
E,E
k,k
s -
ltAthgt2
kmax340 MeV

• Non-Intrusive measurement
• Accuracy improves with higher beam and photon
  energies
• - Figure-of-Merit s x A2 k2 x E2

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Hall A Compton Polarimeter

• Electron Detector
• Silicon microstrip, 600 mm pitch
• 4 planes, 48 strips/plane

• Electron Beam
• 1.5 6 GeV, Up to 100 mA
• Clean beam lt10-11 halo _at_ 5mm

• Photon Detector
• 5x5 array of 20x20x200 mm Lead Tungstate

• Photon Target
• 1064 nm Fabry-Perot cavity
• 1 kW of cavity power
• Crossing angle 20 mrad

• Accuracy at the 1 level for recent HAPPEx
  experiments _at_ 3 GeV PRL 98 (2007) 032301

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Hall A Compton Upgrade
Goal
1 beam polarimetry from 1 to 11 GeV. - PREX
is the driver
Scope

• New Electron Detector
• High resolution silicon microstrips
• 240mm pitch, 4 planes 192 strips/plane
• - Movable in dispersive plane for wide energy
  coverage

• New Photon Detector
• High Res. single crystal GSO calorimeter
• 60ns resp., Preserves counting abilities
• Integrating FADC

High Power Green Fabry-Perot Cavity - 1.5kW
Power _at_532 nm Twice the Analyzing power of
present IR cavity
Participating Institutions Saclay, Syracuse,
Clermont-Ferrand, Uva, Duke,
Carnegie-Mellon, William Mary, Jefferson Lab
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Expected Performance
Simulation by David Lhuillier lt 1 error _at_0.85
GeV obtained in about 4 hrs with 50 uA beam
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Hall C Compton Polarimeter

• The Hall C Compton polarimeter will be very
  similar to the Hall A Compton with some
  differences in the details
• Chicane ? vertical drop 57 cm. LOA 11.1 m
• Electron detector ? diamond strip detector with
  200 mm pitch
• Photon detector ?CsI (?) ? Recycle MIT-Bates
  crystal
• Single Pass RF pulsed 20 W green laser matched to
  e-beam micro pulse
• - Equivalent to 400 W CW
  power
• Crossing Angle ? 20 mrad
• Expected Systematic
  Uncertainty _at_ 1GeV 1

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Higher Energy HERA TPOL
Alex Traper
Trans. Pol. Spatial asymmetry measurement Laser
Ar Ion 514 nm 10W CW Det 240 mm pitch Si mstrips
and Calorimeter
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HERA LPOL
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SLAC SLC Compton Polarimeter
M. Woods, Polarimetry Workshop, Jlab 2003
Beam 45.6 GeV, 3.5x1010e- _at_ 120Hz 0.7
mA Laser 532 nm pulsed, 50mJ _at_ 7ns, 17Hz
rep Crossing Angle 10 mrad e- detector 9
channel threshold gas Cherenkov g detector
Quartz Fiber Calorimeter
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Conclusion

• Absolute beam polarization measurement needs
  redundant polarimety
• Mott polarimetry is limited to low enegry
  injector, important cross check
• Moller polarimetry provides 1 polartimetry,
  limited beam currents
• Atomic hydrogen Moller polarimetry is interesting
  candidate for further development
• Compton polarimetry routinely does 1 level
  polarimetry at high energies
• Plans underway to achieve lt1 at 1 GeV

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Green FP Cavity in development Compton Lab
Photograh Alan Gavalya