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5 MeV Mott Measurement

5 MeV Mott Measurement. Overview: polarized electrons Mott scattering injector polarimeter training & qualification. for CEBAF Operations group Joe Grames, Marcy Stutzman February 14 th , 2007. Sir Nevill F. Mott at the ceremony with his Nobel Prize for Physics, 1977. ( N + - N - ).

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5 MeV Mott Measurement

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  1. 5 MeV Mott Measurement • Overview: • polarized electrons • Mott scattering • injector polarimeter • training & qualification for CEBAF Operations group Joe Grames, Marcy Stutzman February 14th, 2007 Sir Nevill F. Mott at the ceremony with his Nobel Prize for Physics, 1977

  2. (N+ - N-) Polarization = (N+ + N-) What is a polarized beam? A beam is polarized in a specific direction if the average value of all the spins along that direction is not zero. (9 - 1) = 80% (9 + 1)

  3. Polarized electron beam • Circularly polarized laser light on GaAs • Polarized electrons generated • Polarization is longitudinal • Parallel or anti-parallel to direction of beam motion (this is called the helicity) • Wien filter used to change direction of polarization • Polarization direction changes, but beam orbit unchanged (use injector steer script)

  4. (R1 – R2) = Aexp Analyzer • Pbeam= (R1 + R2) What is a polarimeter? A polarimeter is a tool which analyzes the polarized beam in a way that an observer (you) may physically detect and measure. R1 Beam R2

  5. Mott scattering • “Low” energy electrons (20 kV- 10 MeV) • Heavy nucleus atoms (e.g., gold Z=79) • “Spin-orbit” interaction • Interaction of orbital angular momentum of electron (L) and its magnetic moment (µs) • VSO~ L· µs • Sensitive to electron polarization direction transverse to beam motion

  6. Sherman function • Scattering asymmetry • Sherman function • Depends on electron energy, target material, scattering angle

  7. Polarimeter targets • Targets • Ideal target: single heavy nucleus, e.g., Au, Ag or Cu • Real target: thick foils (nm or mm), diluted Sherman function, extrapolation to zero thickness

  8. Target Corrector Detector Hut Dump Dipole Target/ Viewer Ladder Vacuum Valve Target Chamber Setup Viewer Start with beam to FC2. A script reliably steps you through Mott setup. Perform measurement of beam polarization. The script restores conditions when finished.

  9. (R1+ - R1-) AR1exp = (R1+ + R1-) Why flip the helicity? We reverse the sign (+ or -) of the beam polarization at ~30 Hz to cancel differences between the two detectors (R1 or R2). For each detector we measure an asymmetry: (R2+ - R2-) AR2exp = (R2+ + R2-) We combine asymmetries for the two detectors (AR1exp AR2exp) to arrive at a “super asymmetry” Aexp is proportional to the polarization. Aexp = S • Pbeam Analyzer (known) measured unknown

  10. Detector Spectra • Detector package • E (energy) • DE (discriminates photons) • Mott data analysis automated • Looks for asymmetry between up and down counts

  11. Detector Asymmetry electronic threshold inelastic elastic

  12. Mott operation • Spin must be rotated transverse to get a Mott asymmetry (Wien filter ~70° or more) • Target • We routinely use our 1 mm gold foil • Extrapolated Sherman function well known • Beam current 0.5 to 1 mA sufficient for 5 min run • Use FC1 to ensure <1uA • FC2 is downstream of the Mott dipole – won’t help during measurement • Backgrounds • Field emission from 0L03/0L04 cryomodules • “Auto-Joan” disabled so that we can turn off injector cryomodules

  13. Training • Qualification – means sitting w/ expert to watch & learn, then you do w/ expert present & finally “signed off”: • Preparing for Mott • Delivering beam to polarimeter • Changing the Wien angle • Beam setup troubleshooting • Adjusting detector HV • Making a measurement • Logging the data • Backing out of Mott

  14. Okay, Sir Nevill Mott says, “ let’s measure the beam polarization!”

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