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

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. = A exp (R1 – R2) (R1 + R2) Analyzer P beam = 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 (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

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 Setup Viewer Target Corrector Dump Dipole Detector Hut Target Chamber Target/ Viewer Ladder 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 - ) We combine asymmetries for the two detectors (A R1 exp A R2 exp ) to arrive at a “super asymmetry” A exp is proportional to the polarization. A exp = S P beam measured unknown Analyzer (known) A R2 exp = (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 Asymmetry electronic threshold inelasticelastic

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!”