Transcript Mott Measurement Background
5 MeV Mott Measurement
for CEBAF Operations group Joe Grames, Marcy Stutzman February 14 th , 2007 Overview: • polarized electrons • Mott scattering • injector polarimeter • training & qualification
Sir Nevill F. Mott at the ceremony with his Nobel Prize for Physics, 1977
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 ( 9 1 ) + 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.
Analyzer • P beam = ( R1 ( R1 – R2 ) + R2 ) = A exp 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 (µ
s
) – V SO ~ L· µ
s
• Sensitive to electron polarization direction transverse to beam motion
Sherman function
• Scattering asymmetry • Sherman function – Depends on electron energy, target material, scattering angle
Polarimeter targets
• Targets – Ideal target: single heavy nucleus, e.g., Au, Ag or Cu – Real target: thick foils (nm or m m), diluted Sherman function, extrapolation to zero thickness
Target Corrector Vacuum Valve Setup Viewer Detector Hut Target Chamber Target/ Viewer Ladder Dump Dipole 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: A R1 exp =( R1 R1 + + R1 ) + R1 ) A R2 exp =( R2 R2 + + R2 ) + R2 ) We combine asymmetries for the two detectors (A arrive at a “super asymmetry” A exp R1 exp A R2 exp ) to is proportional to the polarization.
measured
A exp = S • P beam Analyzer (known)
unknown
Detector Spectra
• Detector package – E (energy) – D E (discriminates photons) • Mott data analysis automated • Looks for asymmetry between up and down counts
Detector Asymmetry
electronic threshold inelastic elastic
Mott operation
• Spin must be rotated transverse to get a ° or more) • Target – We routinely use our 1 – Beam current 0.5 to 1 m m m gold foil – Extrapolated Sherman function well known 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
Training
Qualification & learn, then you do w/ expert present & finally “signed off”: – means sitting w/ expert to watch • 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