The Compton backscattering polarimeter of the A4 experiment

The A4 collaboration at the Institut für Kernphysik, University of Mainz, is conducting experiments on single-spin asymmetries in the elastic electron-nucleon-scattering which require polarized beams. In order to measure the absolute beam polarization, we have installed a Compton backscattering polarimeter in front of the target, using for the first time the internal cavity concept. A maximum intra-cavity intensity of 90 W has been measured, and in August 2003, first backscattered photons have been detected. Recently, first Compton asymmetries have been measured.