Polarized Positron Sources

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Polarized Positron Sources
May 19 - 20, 2004 ICAR Workshop

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• Armen Apyan,
• Mayda Velasco
• Hans Braun

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1. The Concept

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• ? decay of
• Naturally existing radioactive isotopes
• Short life isotopes produced by an accelerator
• are longitudinally polarized
• Electron Positron pair production of circularly
  polarized photons.
• Positron beam is longitudinally polarized at the
  upper limit of the positrons energy

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2. Polarized Positrons by ee- Pair Production

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• Circularly polarized photons from high energy
  electron beam passing through an undulator and
  pair production in converter target.
• Circularly polarized photons from Compton
  backscattering of laser beam on electron beam and
  pair production in converter target.
• Circularly polarized photons by bremsstrahlung of
  longitudinally polarized electron beam in thin
  amorphous target and pair production in the same
  target.

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Coherent Bremsstrahlung in Oriented Single
Crystals

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• An energy spectrum with discrete peaks due to the
  varying contributions of different reciprocal
  lattice vectors.
• A polar angle distribution which is correlated
  with the energy of the photon.
• Tunable peak energies by crystal choice and
  orientation.

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4. Proposed Technique

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• Separate target for production of circularly
  polarized photons
• Our choice is aligned Diamond crystal
• tight lattice, low Z, high Debay
• temperature and low mosaic spread.

Step-1 Produce Circularly polarized ?

• Separate target for positron production
• High Z amorphous material or
• aligned crystal

Step 2 Convert ?-s to positrons
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Photon and Positron Polarization

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Ee- 10 GeV
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Amorphous Tungsten as a Circularly Polarized
Photon Source

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• Rad. Length ? 3.5 mm
• Thickness 0.3 mm (8.6 XL )
• Chosen energy region
• 6.5GeVltE?lt7.5 GeV
• 90 photons first gen.
• Photon number-- 0.007?/e-

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Aligned Diamond Crystal as a Circularly Polarized
Photon Source

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• Orientation
• ?3.9 mrad wrt axis lt001gt
• ?0, on the (110) plane
• Effective r.l. ? 7cm
• Thickness 1 cm (14 XL )
• Chosen energy region
• 6.5GeVltE?lt7.5 GeV
• 90 photons first gen.
• Photon number-- 0.03?/e-

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Photon Spectra and Circular Polarization Degree

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Positron Number and Polarization

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• Target
• Amorph. Tungsten 0.3 mm
• Positrons 5GeVltEelt6.5GeV
• W 0.005e/e-
• C -- 0.015e/e-

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Positron Polarization

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5GeVlt Ee lt6.5GeV Polarization ? 45
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Summary and Conclusion

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• Aligned Crystals are better choice to produce
  intense beams of circularly polarized photons.

• The photon emission angle for amorphous target is
  me-/Ee- , Tungsten ?? ? 50?rad
• Diamond ?? ? 10 ?rad in coherent peak

• The coherent effects in crystals become more
  pronounced at higher energies , so the intensity
  of produced photons become larger as the initial
  electron energy increases.