POLARIMETRY of MeV Photons and Positrons

1
POLARIMETRYof MeV Photons and Positrons

• Overview
• Beam Characterization
• undulator photons
• positrons
• Basics of the Transmission Method
• for photon polarimetry
• for positron polarimetry
• Description of the Layouts and Hardware
• for the photon polarimeter
• for the positron polarimeter
• Expected Polarimeter Performance

2
Undulator Photon Beam I

• Undulator basics (1st harmonic shown only)

E166 undulator parameters
3
Undulator Photon Beam II
photon spectrum, angular distribution and
polarization
4
Positron Beam Simulation
distributions behind the converter target (0.5
r.l. Ti) based on polarized EGS shower
simulations by K. Flöttmann
5
Low-Energy Polarimetry

• Candidate Processes
• Photons Compton Scattering on polarized
  electrons
• forward scattering (e.g. Schopper et al.)
• backward scattering
• transmission method (e.g. Goldhaber et al.)
• Positrons all on ferromagnetic polarized e-
  targets
• Annihilation polarimetry (ee- ? ??)
  (e.g. Corriveau et al.)
• Bhabha scattering (ee- ? ee-)
  (e.g. Ullmann et al.)
• brems/annihilation (e ? ?) plus ?-transmission
  (Compton) polarimetry

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Trade-offs

• Principal difficulties of e polarimetry
• huge multiple-scattering at low energies even in
  thin targets
• cannot employ double-arm coincidence techniques
• or single-event counting due to poor machine
  duty cycle
• low energies below 10 MeV, vulnerable to
  backgrounds
• All of the candidate processes have been explored
  by us
• ? the transmission method is the most
  suitable ?

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Transmission Polarimetry of (monochromatic)
Photons

M. Goldhaber et al. Phys. Rev. 106 (1957) 826.
all unpolarized contributions cancel in the
transmission asymmetry ? (monochromatic case)
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Transmission Polarimetry of Photons

Monochromatic Case
Analyzing Power
But, undulator photons are not monochromatic ?
Must use number or energy weighted integrals ?
9
Transmission Polarimetry of Positrons

• 2-step Process
• re-convert e ? ? via brems/annihilation
  process
• polarization transfer from e to ? proceeds
• in well-known manner
  
• measure polarization of re-converted photons
• with the photon transmission methods
• infer the polarization of the parent positrons
• from the measured photon polarization
• Experimental Challenges
• large angular distribution of the positrons
• at the production target
• e spectrometer collection transport
  efficiency
• background rejection issues
• angular distribution of the re-converted photons
• detected signal includes large fraction of
  Compton scattered photons
• requires simulations to determine the effective
  Analyzing Power
• Formal Procedure

Fronsdahl Überall Olson Maximon Page
McMaster
10
Spin-Dependent Compton Scattering

• Simulation with modified GEANT3
• (implemented by V. Gharibyan)
• standard GEANT is unpolarized
• ad-hoc solution
• - substitute the unpolarized Compton
  subroutines
• with two spin-dependent versions (1
  and -1) and run these in sequence for the same
  beam statistics
• - then determine analyzing power
• from this data
• Gharibyan
  Schüler, Ref. 66

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Polarimeter Layout Overview

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Analyzer Magnets

g 1.919 ? 0.002 for pure iron,
Scott (1962)
Error in e- polarization is dominated by
knowledge in effective magnetization M along the
photon trajectory

active volume Photon Analyzer Magnet 50 mm
dia. x 150 mm long Positron Analyzer Magnet
50 mm dia. x 75 mm long
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Photon Polarimeter Detectors

E-144 Designs
Si-W Calorimeter
Threshold Cerenkov (Aerogel)
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Positron Polarimeter Layout

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Positron Transport System

e transmission () through spectrometer
photon background fraction reaching CsI-detector
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CsI Calorimeter Detector

Crystals from
BaBar Experiment Number of crystals
4 x 4 16 Typical front face of one
crystal 4.7 cm x 4.7 cm Typical backface
of one crystal 6 cm x 6 cm Typical
length 30
cm Density
4.53 g/cm³ Rad. Length
8.39 g/cm² 1.85 cm Mean free path (5
MeV) 27.6 g/cm² 6.1 cm No.
of interaction lengths (5 MeV) 4.92 Long.
Leakage (5 MeV) 0.73
Photodiode Readout (2 per crystal)
Hamamatsu S2744-08 with preamps
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Expected Photon Polarimeter Performance

Si-W Calorimeter
Energy-weighted Mean
Expected measured energy asymmetry and
energy-weighted analyzing power determined
through analytic integration and. with good
agreement, through special polarized GEANT
simulation
Aerogel Cerenkov
See Table 12
all measurements very fast ? only syst. Error
of should matter
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Expected Positron Polarimeter Performance

Simulation based on modified GEANT code, which
correctly describes the spin-dependence of the
Compton process
Number Energy-weighted Analyzing Power vs.
Energy
Photon Spectrum Angular Distr.
10 Million simulated e per point polarity on
the re-conversion target
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Expected Positron Polarimeter Performance I

Analyzing Power vs. Energy Spread
Analyzing Power vs. Target Thickness
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Expected Positron Polarimeter Performance II

Table 13