Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 1 An Overview of Electron Polarimeters and Results of an Intercomparison Joe Grames PST2001 International Workshop on Polarized Sources and Targets Nashville, 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 2 …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 3 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 4 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 (P b ) and the total analyzing power of the polarimeter’s target (A tot ). The target is itself polarized in many polarimeters and A tot 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  = A tot · P b. 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 +P b and then N - events with electron polarization -P b  = A tot · P b = N + + N - N + - N -

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 5 Consider the Jlab 5 MeV Mott polarimeter (electrons scattered from a 1  m gold foil). An Example P b ~ 70% A tot ~ -0.4

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 6 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 =·P b 2 ·P t 2 ·A 2 PbPb PbPb 2

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 7 Polarimeter analyzing power: A tot Precise knowledge of the analyzing power is limited. A tot 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 8 Mott Scattering Spin-orbit coupling of the beam electron 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(  ) P b · 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 9 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, p finite nuclear size at higher energy Ugincius et. al Nucl.Phys. A , p % effect at 5 MeV 20% effect at 14 MeV Uncertainties of Mott A tot (-) A tot

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 10 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 A zz = -7/9 P t ~ 8%  A tot ~6% Good luminosity (~ kHz /  A /  m) COM coincidence for >1000:1 S:B Invasive Limited to <5  A 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 11 Uncertainties in A ij Particle identification Finite energy acceptance (A zz vs. E) Mott background (single vs. double arm) Moliere scattering Levchuk effect (~10%) atomic electron motion of core shells p t ~ 10 keV  2 = 2m e 1 p´ 1 E p t · n meme 1 Jlab Hall B Moller

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 12 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 13 Compton Scattering Asymmetric cross-section between longitudinally polarized electron beam and circularly polarized photon beam. A C =   -     +   Operationally Must work for high luminosity Non-invasive! Easy reversal of target polarization A tot (Energy) Performance suffers at ~1GeV

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 14 JLab Compton Polarimeter Target (laser) polarization P  >99% excellent Fabry-Perot cavity Gain ~ 5  10 3  1-2 kW Chicane delivers backscattered photon 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 15 Calculation of A tot Compton edge energy calibration Low energy threshold resolution Model to describe   and   Background Bremsstrahlung (residual gas) Synchrotron radiation (magnets) Uncertainties of Compton A tot E b =3.2 GeV

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 16 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 17 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 =  ··  bend 2 ( g-2 ) x z

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 18 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. P meas 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 19 Source Strained GaAs photocathode (  = 850 nm, P b >75 %) Accelerator 5.7 GeV, 5 pass recirculation The Experiment

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 20 Spin Dance 2000 Results P meas sin(  Wien +  )

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 21 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 P meas normalized to Mott for reference

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 22 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 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 23 Method 1: precession between injector and end-stations large phase advance (  ~ 10,000 degrees)  to 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 = 2mec22mec2 g e - 2 ·  

Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Dept. Of Energy 24 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 25 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 26 State of electron beam polarimetry Mott recent progress for MeV polarimetery and target thickness model Moller 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 27 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...