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.