A high-rate coincidence Mّller polarimeter

A coincidence Møller polarimeter has been built and operated to measure the electron beam polarization for the recent E143 experiment at SLAC. This system can measure the polarization of the electron beam to high statistical precision (∼1.5%) in about 15 min, despite the 10−4 duty factor of the SLAC beam. The use of two arrays consisting of seven detectors each – with a total acceptance of 40° in the center-of-mass frame – reduced the sensitivity to many potential systematic errors. The large number of measurements, taken under a wide variety of experimental conditions, enabled a detailed search for the presence of systematic errors in the data.