The GDH-Mّller-Polarimeter at ELSA

The design, setup and performance of a new two-arm coincidence, large acceptance and variable energy Møller-Polarimeter used within the GDH-Experiment at the Electron Stretcher Accelerator Facility (ELSA) in Bonn is described. The polarimeter employs a dipole magnetic spectrometer and lead glass detectors to detect both Møller scattered electrons in coincidence. A target system consisting of three different pairs of coils provides the magnetic field to magnetize Vacoflux foils in three different space orientations. This allows to measure all beam polarization components. The variable geometry of the detector system enables adjustments to the kinematic conditions between 0.8 and 3.5 GeV electron beam energy. A large center of mass acceptance of θ̄=[65°;115°] provides the measurement of the longitudinal electron beam polarization, for example at 1.9 GeV with a statistical precision of 1% within 10 min (Ie≈500 pA). Unprecedented systematic studies of Møller polarimetry as well as direct measurements of the transversal beam polarization components have been performed with this device.