POLARIMETRY of MeV Photons and Positrons

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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

SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
2
Undulator Photon Beam

• undulator basics (1st harmonic shown only)

E166 undulator parameters
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
3
Undulator Photon Beam
E166 undulator photon spectrum, angular distr.
and polarization
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
4
Positron Beam Simulation
distributions behind the converter target (0.5
r.l. Ti) based on polarized EGS shower
simulations by K. Flöttmann
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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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
• Principle 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, very vulnerable to
  backgrounds
• All of the candidate processes have been explored
  by us
• ? the transmission method is the most suitable
  ?

SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
6
Transmission Polarimetry of (monochromatic)
Photons

M. Goldhaber et al. Phys. Rev. 106 (1957) 826.
all unpolarized contributions cancel in the
transmission asymmetry ? (monochromatic case)
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Transmission Polarimetry of Photons
monochromatic case

Analyzing Power
But, undulator photons are not at all
monochromatic ? Must instead use integrated
numbers or energies ?
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
8
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 method
  discussed earlier
• infer the polarization of the parent positrons
• from the measured photon polarization
• experimental challenges
• huge angular distribution of the positrons
• at the production target
• e spectrometer collection transport
  efficiency
• background rejection issues
• huge angular distribution of the re-converted
  photons
• detected signal includes large fraction of
  Compton scattered photons
• requires extensive simulations to determine the
  effective Analyzing Power
• formal procedure

Fronsdahl Überall Olson Maximon Page
McMaster
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Polarimeter Layout Overview

SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
10
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
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Photon Polarimeter Detectors

Si-W Calorimeter
Aerogel threshold Cerenkov
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Positron Polarimeter Layout

SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
13
Positron Transport System

e transmission () through spectrometer
photon background fraction reaching CsI-detector
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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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
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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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
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
16
Expected Positron Polarimeter Performance
Simulation based on modified GEANT code which
correctly describes the spin-dependence of the
Compton process

Photon Spectrum Angular Distr.
number energy-weighted Analyzing Power vs.
Energy
10 Million simulated e per point polarity on
the re-conversion target
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Expected Positron Polarimeter Performance

Table 13
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Expected Positron Polarimeter Performance

Analyzing Power vs. Target Thickness
Analyzing Power vs. Energy Spread
SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler
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Spin-Dependent Compton Scattering

SLAC EPAC 12 June 2003
E166 Proposal Presentation K.P. Schüler