Mott Measurement Background

1
Mott Measurement Background

• for CEBAF Operations group
• Marcy Stutzman
• 7 February, 2007

Sir Nevill Mott
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Polarized electrons

• Circularly polarized laser light on GaAs
• Polarized electrons generated
• Polarization is longitudinal
• Parallel or anti-parallel to direction of beam
  motion
• Wien filter used to change direction of
  polarization

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Mott scattering

• High energy electron (20 kV- 10 MeV)
• Heavy nucleus atoms (gold)
• Spin-orbit interaction
• Interaction of orbital angular momentum of
  electron (L) and its magnetic moment (µs)
• VSOL µs
• Electron polarization direction transverse to
  beam motion

4
Mott scattering

• Scattering asymmetry
• Sherman function
• Depends on angle, energy, foil thickness
• Ideal target single gold atom
• Reality thin foils, determine thickness
  dependence, extrapolate to zero thickness

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Data Analysis

• Mott data analysis automated
• Looks for asymmetry between up and down counts

6
Real world effects

• Spin must be rotated transverse to get a Mott
  asymmetry (Wien filter 70 or more)
• Foil thickness
• We always use the same target
• Sherman function (analyzing power) known
• Targets cant take much current!
• Use FC1to ensure lt1uA
• FC2 is downstream of the Mott dipole wont help
  during measurement
• Background
• Due to radiation from things like cryomodules
• Auto-Joan disabled so that we can turn off
  injector cryomodules