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 Review

Joe Grames

JLab Parity Workshop

October 26, 2001

Priorities for electron polarimeters Issues facing 1% polarimetry

Spin Dance 2000 Results

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

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What priorities must Jlab electron beam polarimetry serve?

• keV - MeV region for sources and/or nuclear parity experiments• GeV for parity and nucleon spin structure experiments• Future experiments are heading toward 1% absolute polarimetry• Eager users (and the broader community!)

What are desirable (necessary) features?

• Large, precisely known analyzing power• Designs with reduced sensitivity to known systematics• Techniques that do not disturb the experiment (non-invasive)• 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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The JLab Electron Polarimeters

Polarimeter Target Ptarget I ave Px Py Pz

Injector Mott ZA 5 A x x

Hall A Compton > 99% 100 A x

Hall A Moller e ~ 8 % 2 A x x

Hall B Moller e ~ 8 % 5 nA x x

Hall C Moller e ~ 8 % 2 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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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 affect knowledge of the total analyzing power:• Mott (Coulomb screening, finite nuclear size, multiple scattering)• Compton (energy resolution, radiative background)• Moller (detector acceptance, Levchuk effect, target polarization)

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 Spin Dance 2000 Collaboration

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 & Spin Precession

The polarized electron beam is produced by photoemission from a strained GaAs crystal held at a potential of -100 kV using circularly polarized light of the appropriate wavelength.

Spin precession accumulates in the transport arcs. At 6 GeV the totalprecession exceeds 10,000 deg.

spin = · ·bend2

( g-2 )

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy

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Why call it a “spin dance” ?

.

Pmeas sin( Wien + )

Measuring the beam polarization at various Wien angles determines both the spin polarization and direction.

To offset the spin precession in theaccelerator we use a Wien filter near the electron gun to set (Wien)the beam polarization direction.

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

Spin Dance 2000 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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Relative Analying Power 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.

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 consider measuring the beam energy by spin precession when precise (~10-4) alternative methods exist and are quick (1-2 hours)?

Answer 1: large cummulative spin precession (10,000 degrees) between the injector and end-stations yields 10-4 to 10-5 relative uncertainty and issensitive to accelerator setup (injector energy, linac gradients, bend angles).

Answer 2: small precession (500 degrees) between end-stations at final beam energy yields ~10-3 relative uncertainty but requires only knowledge of end-station bend angles.

This information comes essentially free at the cost necessary to intercompare theelectron polarimeters. It provides two independent methods which can be usedfor cross-calibration and are sensitive to the accelerator where the others are not.

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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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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Polarimeters often disagree at a level larger than quoted systematics, particularly beyond the level hoped for by users.

A spin dance experiment aims to intercompare polarimeter analyzing powers by using a common test beam to improve polarimeter systematics.

Spin Dance 2000 indicates agreement of independent polarimeter techniques (Mott, Moller, Compton), but also demonstrates systematic differences.

Spin based energy measurements at Jlab are high precisision, are sensitive to accelerator parameters, and are a by-product of the polarimeter comparison.

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