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Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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An Overview of Electron Polarimeters and Results of an Intercomparison
Joe Grames
PST2001
International Workshop on Polarized Sources and TargetsNashville, Indiana, USA
September 30 - October 4, 2001
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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…and an Intercomparison• Spin Dance 2000 experiment• Comparison of 5 JLab electron polarimeter• High precision spin-based energy measurements
Electron polarimeters…• Purpose and desirable parameters• Meaning of analyzing power• And how it can be difficult to measure
• Unique Jefferson Lab capability
Overview
• Low & high energy Mott polarimeters• Compton polarimeter• Three Moller polarimeters
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Jefferson Lab - E. Chudakov, H. Fenker, A. Freyberger, J. Grames, J. Hansknecht, J. Mitchell, M. Poelker, C. Sinclair, M. Steigerwald, M. Tiefenback
CEA Saclay, DSM/DAPNIA/SPHN - C. Cavata, S. Escoffier, F. Marie, T. Pussieux, P. Vernin
Florida International University - R. Nasseripour, B. Raue
Karkov Institute - V. Gorbenko
Massachusetts Institute of Technology - D. Higinbotham, R. Suleiman
North Carolina Ag. and Tech. State University - S. Danagoulian
Old Dominion University - V. Dharmawardane
University of Virginia - R. Fatemi, K. Joo, M. Zeier
Universitaet Bonn - T. Reichelt
Vrije Universiteit - B. Zihlmann
Jefferson Lab Polarimeter Collaboration
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Electron Beam Polarimetry
Electron polarimetry is the technique of separating scattered particles for detection using some physical interaction between the polarization of the beam under test (Pb) and the total analyzing power of the polarimeter’s target (Atot).
The target is itself polarized in many polarimeters and Atot is then proportional to the product of the target polarization and the analyzing power of the interaction (cross-section). The experimental asymmetry is defined as = Atot · Pb.
This asymmetry can be measured by either reversing the polarity of the beam or target
and while detecting the scattered particles from each state, e.g., measuring N+ events
with electron polarization +Pb and then N - events with electron polarization -Pb
= Atot · Pb =N+ + N-
N+ - N-
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Consider the Jlab 5 MeV Mott polarimeter (electrons scattered from a 1m gold foil).
An Example
Pb ~ 70%Atot ~ -0.4
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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What purpose does electron beam polarimetry serve?
• keV - MeV region for sources and nuclear parity experiments• GeV and greater for parity and nucleon spin structure experiments• Future experiments are pushing toward ~1% absolute polarimetry• Eager users!
What are desirable (necessary) features?
• Large analyzing power• Designs with reduced sensitivity to inherent systematics• Non-invasive method does not disrupt experiment• High luminosity to rapidly achieve small statistical uncertainty
N = ·Pb2 ·Pt
2 ·A2Pb
Pb
2 -1
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Polarimeter analyzing power: Atot
Precise knowledge of the analyzing power is limited.
Atot is not a directly measured quantity:• measurement requires difficult double-scattering experiments
• the analyzing power is determined by theory and simulation
Factors that effect knowledge of the total analyzing power• inferred target polarization• detector acceptance• multiple scattering
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Mott Scattering
Spin-orbit coupling of the beamelectron and the target nucleus.
Operational Points• Unpolarized, high-Z, solid targets (Au, Pb)• Useful in the keV to MeV energy range
14 MeV on Pb (MAMI, 1994) 5 MeV on Au (JLAB, 1995)
• Sherman function is large (~ 30-50%)• Invasive• Multiple/plural scattering in thick targets
= 1+ S() Pb·k k´
| k k´|
Sherman function
Jlab 5 MeV Mott polarimeter
Sherman Phys.Rev. 103(6) 1956, p1601-7
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Target thickness effects• Dilution by multiple/plural scattering• Sherman function sets scale• Target thickness extrapolation necessary• ~MeV double scattering important
Uncertainty of Sherman function• coulomb screening at lower energy Ross et.al Phys.Rev A 38(12) 1988, p6055-8
• finite nuclear size at higher energy
Ugincius et. al Nucl.Phys. A158 1970, p418-32
1.5% effect at 5 MeV20% effect at 14 MeV
Uncertainties of Mott Atot
(-)
Ato
t
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Moller Scattering
QED spin-spin interaction of a polarized beam electron and a polarized target electron.
Operational Points• Nucleon probe energies (GeV range)• Large asymmetry Azz = -7/9 • Pt ~ 8% Atot ~6%• Good luminosity (~10-100 kHz /A /m)• COM coincidence for >1000:1 S:B• Invasive• Limited to <5A by target heating
Jlab Hall C Moller
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Uncertainties in Aij
Particle identification• Finite energy acceptance (Azz vs. E)• Mott background (single vs. double arm)• Moliere scattering
Levchuk effect (~10%)• atomic electron motion of core shells• pt ~ 10 keV
2 = 2me
1p´
1
E
pt · nme
1
Jlab Hall B Moller
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Target Polarization Effects
Conventional Moller• iron-alloy (Fe, Co, V)• in-plane magnetization (tilted, B ~ 100 G)• absolute calibration in beam environment• thickness inhomogeneity lead to uncertainty between magnetization and flux
Novel Moller Design (I. Sick, et. al)• spin-polarization versus magnetization known for pure iron ~ 0.25%• out-of-plane (normal targets) • insensitive to target thickness• high field saturation (~4 Tesla)• field direction and uniformity• target heating
split SC coils (4 Tesla)
pure Fe
Jlab Hall B Moller
Jlab Hall C Moller
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Compton ScatteringAsymmetric cross-section between longitudinally polarized electron beam and circularly polarized photon beam.
AC = -
+
Operationally• Must work for high luminosity• Non-invasive!• Easy reversal of target polarization• Atot(Energy)• Performance suffers at ~1GeV
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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JLab Compton PolarimeterTarget (laser) polarization P >99% excellent
Fabry-Perot cavity Gain ~ 5103 1-2 kW
Chicane delivers backscatteredphoton and electron
c ~ 20 mrad
Increased complexity
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Calculation of Atot
• Compton edge energy calibration• Low energy threshold resolution• Model to describe and
Background• Bremsstrahlung (residual gas)• Synchrotron radiation (magnets)
Uncertainties of Compton Atot
Eb=3.2 GeV
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Polarized Source & Beam
The polarized electron beam is produced by photoemission from a strained GaAs crystal held at a potential of -100 kV. The degree of polarization depends upon the properties of the crystal and the wavelength and degree of polarization of the incident laser light.
The magnitude of the electron polarization is determined by the average number of electrons with a spin along a defined direction. That defined direction is determined by the circular laser polarization (momentum).
P = = 80%
GaAs
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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The Role of Spin Precession in Testing
Spin precession is dominantly contributed to by the dipole magnetic fields in proportion to the beam’s energy and bend angle.
A Wien filter is a suitable spin rotator for a 100 keV electron beam; an electric field rotates the spin, while a crossed magnetic field balances the Lorentz force. The net rotation is called the Wien angle (Wien).
spin = · ·bend2
( g-2 )
x
z
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Where does “spin dance” come from?
The measured experimental asymmetry is proportional to the component of the total beam polarization along some analyzing component of a polarimeter.
Pmeas sin(Wien + )
By varying the Wien angle the measurable component of the beam polarization will vary sinusoidally.
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Source Strained GaAs photocathode (= 850 nm, Pb >75 %)
Accelerator 5.7 GeV, 5 pass recirculation
The Experiment
Polarimeter I ave Px Py Pz
Injector Mott 2 A x xHall A Compton 70 A xHall A Moller 1 A x xHall B Moller 10 nA x xHall C Moller 1 A x
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Spin Dance 2000 Results
Pmeas sin(Wien + )
Polarimeter (deg)
Hall A Compton 10984.2 0.8
Hall A Moller 10983.9 0.7
Hall B Moller 10500.4 0.6
Hall C Moller 10023.0 0.7
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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• Uncertainties are based on statistics and do not include any systematics.• Polarimeters of 3 types (Mott, Moller, Compton) indicate agreement.• Uncertainty in Wien angle induces < 0.2% relative effect.
Relative Analyzing Powers Compared
Pmeas normalized to Mott for reference
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Spin Based Energy Measurements
Why measure the beam energy by spin precession?
Precise (~10-4) alternative methods exist and are quick (1-2 hours)• magnetic spectrometer• elastic electron-proton scattering
Polarimetry + cursory measurements for a spin dance take 2-3 days
The answer is:• carefully done yields relative energy at ~10-5, certainly 10-4
• independent calibration• and the information is free
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Method 1: precession between injector and end-stations • large phase advance ( ~ 10,000 degrees) 10-4 to 10-5 relative uncertainty• requires knowledge of injector energy, linac gradients, and all bend angles
Method 2: precession between end-stations at final beam energy• small phase advance ( ~ 500 degrees) ~10-3 relative uncertainty• requires knowledge of end-station bend angles
Two methods with different precision
E =2mec2
ge - 2·
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Energy Results Summarized
+35 MeV
-35 MeV
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Hall B 35 MeV shift?
= -2.9°
or
= -0.22°
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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State of electron beam polarimetryMott
• recent progress for MeV polarimetery and target thickness modelMoller
• inclusion of corrective effects, e.g., Levchuk effect• saturated foil targets
Compton• high gain cavities providing compatibility at lower current
Disagreement amongst polarimeters greater than quoted systematics.
Agreement of independent polarimeters (Mott, Moller, Compton).
Often, the polarimeter is simply viewed as the tool. To reach the 1% absolute mark the polarimeter must be the experiment, not the tool.
Conclusions
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy
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Part of a letter by L.H. Thomas to Goudsmit (25 March 1926). Reproduced from a transparency shown by Goudsmit during his 1971 lecture. The original is presumably in the Goudsmit archive kept by the AIP Center for History of Physics.
One last remark...