5 MeV Mott Measurement 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 Overview: • polarized electrons • Mott scattering • injector polarimeter • training & qualification
14
Embed
5 MeV Mott Measurement for CEBAF Operations group Joe Grames, Marcy Stutzman February 14 th, 2007 Sir Nevill F. Mott at the ceremony with his Nobel Prize.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.
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