Compton Scattering Basics
The counting rates in each Cherenkov channel are measured for parallel
and anti-parallel combinations of the photon and electron beam helicities.
The Compton spectrum is characterized by a kinematic edge at 17.4 GeV
(180 degree backscatter in the center-of-mass frame), and the
zero-asymmetry point at 25.2 GeV (90 degree backscatter in the
center-of-mass frame). The Compton asymmetry function is modified from
the theoretical asymmetry function by detector resolution effects.
This effect is about 1% for the Cherenkov channel at the Compton edge.
Detector position scans are used to precisely locate the Compton edge.
The position of the zero-asymmetry point is then used to fit for the
spectrometer dipole bend strength. Once the detector energy scale is
callibrated, each Cherenkov channel provides an independent measurement
of electron polarization. The Compton edge is in channel 7, and we use
this channel to precisely determine electron polarization.