A fourfold Bhabha/Møller polarimeter for positrons/electrons

A polarimeter with high statistica power is introduced for relative measurements of the longitudinal polarization of positrons or electrons at kinetic energies above 1 MeV. A fourfold construction improved the statistical power and the instrumental symmetry. In each of the four polarimeters five detectors viewed a cylindrical scatterer of FeCoV. The polarimeters reached an average sensitivity S 0.058, approaching the theoretical optimum of 0.079. A fast data-acquisition system was dedicated to high event rates at small dead time. In case of ß+-polarization measurements the systematic errors were negligible to 10-3 accuracy