Measurement of electron beam polarization from the energy asymmetry of Compton scattered photons

We describe a new method for precision measurement (/spl sim/1%) of the longitudinal polarization of high-energy electron beams. The method relies on measuring the energy asymmetry of Compton scattered photons which result from collisions between the electron beam and a polarized laser beam. The measurement takes place near the electron-positron interaction point at the SLAC Linear Collider (SLC). The detector described here (PGC) is a threshold gas Cherenkov device which follows a lead radiator. The Cherenkov threshold (14 MeV) is essential for eliminating enormous synchrotron radiation-induced backgrounds at the position of interest in the SLC. We also describe the calibration of the PGC energy response in a test beam and its performance as a working polarimeter for the SLC/SLD physics program. Preliminary results from the 1996 run indicate that PGC measurement with total error at or below the 1% level is indeed achievable, and hence serves as a valuable crosscheck of the polarization scale.