5 MeV Mott Measurement

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

Overview:• polarized electrons• Mott scattering• injector polarimeter• training & qualification

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)

(9 + 1)= 80%

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)

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.

= Aexp

(R1 – R2)

(R1 + R2)Analyzer • Pbeam =

Beam

R1

R2

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

Sherman function

• Scattering asymmetry

• Sherman function– Depends on electron

energy, target material, scattering angle

• Targets– Ideal target: single heavy

nucleus, e.g., Au, Ag or Cu– Real target: thick foils (nm or

m), diluted Sherman function, extrapolation to zero thickness

Polarimeter targets

Vacuum Valve

SetupViewer

Target Corrector

Dump Dipole

Detector Hut

TargetChamber

Target/ViewerLadder

Start with beam to FC2.A script reliably steps you through Mott setup.Perform measurement of beam polarization.The script restores conditions when finished.

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:

AR1exp =

(R1+ - R1-)

(R1+ + R1-)

We combine asymmetries for the two detectors (AR1exp AR2

exp) to arrive at a “super asymmetry” Aexp is proportional to the polarization.

Aexp = S • Pbeam

measured

unknown

Analyzer (known)

AR2exp =

(R2+ - R2-)

(R2+ + R2-)

Detector Spectra

• Detector package– E (energy)– E (discriminates

photons)

• Mott data analysis automated

• Looks for asymmetry between up and down counts

Detector Asymmetryelectronicthreshold inelastic elastic

Mott operation• Spin must be rotated transverse to get a

Mott asymmetry (Wien filter ~70° or more)• Target

– We routinely use our 1 m gold foil– Extrapolated Sherman function well known– Beam current 0.5 to 1 A 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

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

Training

Okay, Sir Nevill Mott says,

“ let’s measure the beam polarization!”