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

2. 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.
Polarization =
(N+ - N-)
(N+ + N-)
(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. What is a polarimeter? (R1 – R2) = Aexp Analyzer • Pbeam = (R1 + R2)
A polarimeter is a tool which analyzes the polarized beam in a way that an observer (you) may physically detect and measure.
= Aexp
(R1 – R2)
(R1 + R2)
Analyzer • Pbeam = 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. A script reliably steps you through Mott setup.
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. Why flip the helicity? AR1exp = (R1+ - R1-) (R1+ + R1-) (R2+ - R2-)
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:
AR1exp =
(R1+ - R1-)
(R1+ + R1-)
(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 Mott data analysis automated
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!”