Linearly polarised photon beams at ELSA and measurement of the beam asymmetry in \pi^{0}_{} photoproduction off the proton

At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on the basis of the experimental conditions is obtained. Polarisation degrees of \(\ensuremath P_{\gamma}=40\) % are typically achieved at half of the primary electron energy. The determination of \(\ensuremath P_{\gamma}\) is confirmed by measuring the beam asymmetry, \( \Sigma\) , in \( \pi^{0}_{}\) photoproduction and by a comparison of the results to independent measurements using laser backscattering.