Electron polarimetry working group update
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Wolfgang Lorenzon (Michigan) EIC Collaboration Meeting Stony Brook Dec 7-8, 2007. Electron Polarimetry Working Group Update. EIC Electron Polarimetry Workshop August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) http://eic.physics.lsa.umich.edu/ (A. Deshpande, W. Lorenzon).

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Electron Polarimetry Working Group Update

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Wolfgang Lorenzon


EIC Collaboration MeetingStony Brook

Dec 7-8, 2007

Electron Polarimetry Working GroupUpdate

W. Lorenzon SBU Dec-2007

EIC Electron Polarimetry Workshop

August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) http://eic.physics.lsa.umich.edu/(A. Deshpande, W. Lorenzon)

W. Lorenzon SBU Dec-2007

Workshop Participants

BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1

Accelerator/Source: 3 / Polarimetry: 12 / students/postdocs (*): 5

W. Lorenzon SBU Dec-2007

Goals of Workshop

Which design/physics processes are appropriate for EIC?

What difficulties will different design parameters present?

What is required to achieve sub-1% precision?

What resources are needed over next 5 years to achieve CD0 by the next Long Range Plan Meeting (2012)

→ Exchange of ideas among experts in electron polarimetry and source & accelerator design to examine existing and novel electron beam polarization measurement schemes.

W. Lorenzon SBU Dec-2007

How to measure polarization of e-/e+ beams?

Three different targets used currently:

1. e- - nucleus: Mott scattering 30 – 300 keV (5 MeV: JLab)spin-orbit coupling of electron spin with (large Z) target nucleus

2. e - electrons: Møller (Bhabha) scat. MeV – GeVatomic electron in Fe (or Fe-alloy) polarized by external magnetic field

3. e - photons: Compton scattering > GeVlaser photons scatter off lepton beam

W. Lorenzon SBU Dec-2007

Polarimeter Roundup

W. Lorenzon SBU Dec-2007

Phys. Rev. ST Accel. Beams 7, 042802 (2004)

Results shown include statistical errors only

→ some amplification to account for non-sinusoidal behavior

Statistically significant disagreement

The “Spin Dance” Experiment (2000)

Systematics shown:


Møller C 1%


Møller B 1.6%

Møller A 3%

Even including systematic errors, discrepancy still significant

W. Lorenzon SBU Dec-2007

Lessons Learned

Include polarization diagnostics and monitoring in beam lattice design

minimize bremsstrahlung and synchrotron radiation

Measure beam polarization continuously

protects against drifts or systematic current-dependence to polarization

Providing/proving precision at 1% level very challenging

Multiple devices/techniques to measure polarization

cross-comparisons of individual polarimeters are crucial for testing systematics of each device

at least one polarimeter needs to measure absolute polarization, others might do relative measurements

Compton Scattering

advantages: laser polarization can be measured accurately – pure QED – non-invasive, continuous monitor – backgrounds easy to measure – ideal at high energy / high beam currents

disadvantages: at low beam currents: time consuming – at low energies: small asymmetries – systematics: energy dependent

Møller Scattering

advantages: rapid, precise measurements – large analyzing power – high B field Fe target: ~0.5% systematic errors

disadvantages: destructive – low currents only – target polarization low (Fe foil: 8%) – Levchuk effect

New ideas?

W. Lorenzon SBU Dec-2007

Compton vs Møller Polarimetry





  • Detect g at 0°, e-< Ee

  • Strong  need <<1

  • at Ee< 20 GeV

  • Plaser~100%

  • non-invasive measurement

  • syst. Error: 3 → 50 GeV (~1 → 0.5%) hard at < 1 GeV: (Jlab project: ~0.8%)

  • rad. corr. to Born < 0.1%

  • Detect e-at qCM~90°

  •  good systematics

  • beam energy independent

  • ferromagnetic target PT~8%

  • beam heating (Ie < 2-4 mA), Levchuck eff.

  • invasive measurement

  • syst. error 2-3% typically0.5% (1%?) at high magn. field

  • rad. corr. to Born < 0.3%

W. Lorenzon SBU Dec-2007

New Ideas

Polarized Hydrogen in a cold magnetic trap (E. Chudakov et al., IEEE Trans. Nucl. Sci. 51, 1533 (2004))

use ultra-cold traps (at 300 mK: Pe~ 1-10-5, density ~ 3∙1015 cm-3 , stat. 1% in 10 min at 100 mA)

expected depolarization for 100 mA CEBAF < 10-4

limitations: beam heating → “continuous” beam & complexity of target

advantages: expected accuracy < 0.5% & non-invasive, continuous, the same beam

Problem: very unlikely to work for high beam currents for EIC (due to gas and cell heating)

Jet Target: avoids these problems

VEPP-3 100 mA, transverse

stat 20% in 8 minutes (5 ∙ 1011 e- /cm2 , 100% polarization)

What is electron polarization in a jet?

New fiber laser technology (Jeff Martin for Hall C)

Gain switched fiber laser

huge luminosity boost when locked to Jlab beam structure (30 ps pulsesat499 MHz)

lower instantaneous rates than high power pulsed lasers

external to beam line vacuum → easy access

in-house experience (Jlab source group)

excellent stability, low maintenance

Compton e- analysis (Kent Paschke for PV-DIS experiments)

dominant challenge: determination of analyzing power Az

zero-crossing e- analysis: two points of well-defined energy (Compton edge, zero crossing)

linear fit of zero crossing: integrate between two points

absolute calibration (only input is QED)

weak dependence of energy resolution & no need to calibrate calorimeter

W. Deconinck, A. Airapetian

Hybrid Electron Compton Polarimeterwith online self-calibration


separates polarimetry from accelerator

scattered electronmomentum analyzed in dipole magnet measured with Si or diamond strip detector

pair spectrometer (counting mode)

e+e– pair production in variable converter

dipole magnet separates/analyzes e+ e–

sampling calorimeter (integrating mode)count rate independent

Insensitive to calorimeter response


A2 Workshop Summary

Electron beam polarimetry between 3 – 20 GeV seems possible at 1% level: no apparent show stoppers (but not easy)

Imperative to include polarimetry in beam lattice design

Use multiple devices/techniques to control systematics


crossing frequency 3–35 ns: very different from RHIC and HERA

beam-beam effects (depolarization) at high currents

crab-crossing of bunches: effect on polarization, how to measure it?

measure longitudinal polarization only, or transverse needed as well?

polarimetry before, at, or after IP

dedicated IP, separated from experiments?

Workshop attendees agreed to be part of e-pol working group

coordination of initial activities and directions: W. Lorenzon

members: A. Airapetian, D. Gaskell (long. polar.), W. Franklin (trans. polar.), E. Chudakov (Møller targets)

Design efforts and simulations just starting

W. Lorenzon SBU Dec-2007


Longitudinal Polarimetry

Pair Spectrometer

Geant simulations with pencil beams

(10 GeV leptons on 2.32 eV photons)

Coincidence Mode:

- acceptance (from <1.51 GeV (“zero crossing”) to >2.63 GeV (Compton edge)

- resolution (2%-3.5%)

Single Arm Mode:

- analyzing magnet relates momentum and

position of pair produced e - e+

- provide well defined e - or e+ beams to calibrate

the Compton photon calorimeter


- include beam smearing (a, b functions)

- fix configuration (dipole strength, length, position, hodoscope position and sizes, … - estimate efficiencies, count rates

e+e– coincidence mode

all 18 hodo channels

e+e– single arm mode

single hodo channels


Longitudinal Polarimetry (II)

Compton electron detection

- using chicane design, max deflection from e- beam: 22.4 cm (10 GeV), 6.7 cm (3 GeV) deflection at “zero-crossing”: 11.1 cm (10 GeV), 3.3 cm (3 GeV)

→e- detection should be easy


- include realistic beam properties →study bkgd rates due to halo and beam divergence

- adopt Geant MC from Hall C Compton design

- learn from Jlab Hall C new Compton polarimeter

7.5 GeV beam2.32 eV laser

  • Compton photon detection

  • Sampling calorimeter (W, pSi) modeled in Geant

  • based on HERA calorimeter

  • study effect of additional energy smearing

No additional smearing

additional smearing: 5%

additional smearing: 10%

additional smearing: 15%


Transverse Polarimetry

Energy Dependence

- analyzing power as function of scattered photon energy

- large variation in energy of peak analyzing power

20 GeV studies

- using pencil beams - peak asymmetry in gamma spectrum at ~6 GeV for 20 GeV electron beam of

- resolution of ~1 m needed in vertical centroid for 1% polar. measurement for 50 m flight path

3 GeV studies

- peak asymmetry in gamma spectrum at ~200 MeV for 3 GeV electron beam

- position sensitive detector of 10*10 cm2 will subtend relevant region for asymmetry at lowest energy for 50 m flight path


Transverse Polarimetry (II)

  • Plans:

  • Asymmetries appear adequate for transverse polarimetry, even at low energies.

  • Inclusion of transverse electron polarimetry within IP polarimeter appears feasible with compact position-sensitive detector in photon arm. Flight path greater than 50 m desirable.

  • Next steps:

    • Include beta functions and emittance at IP

    • Projection of asymmetry vs. position for asymmetry for EIC energies

    • Begin simulation to determine effective analyzing power of calorimeter

    • Use of electron vertical information?


Møller Polarimetry

  • Hydrogen Atomic Jet

  • Just started investigations

  • Several problems to address:

    • Breit-Rabi measurement analyzes only part of jet

    • → uniformity of jet has to be understood

    • large background from ions in the beam: most of them associated with jet (hard to measure)

    • origin of background observed in Novosibirsk still unclear (in contact with them)

    • clarification of depolarization by beam RF needed

    • → might be considerable



  • Electron Polarimetry working group has been formed

    • kick-off at A2 Workshop in Aug 2007

    • design efforts and simulations have started

    • dialog with accelerator groups at BNL / JLab

  • There are issues that need attention (crossing frequency 3-35 ns; beam-beam effects at high currents; crab crossing effect on polarization)

  • JLAB at 12 GeV will be a natural testbed for future EIC Polarimeter tests

    • evaluate new ideas/technologies for the EIC

  • No serious obstacles are foreseen to achieve 1% precision for electron beam polarimetry at the EIC (3-20 GeV)

W. Lorenzon SBU Dec-2007


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