Linearly polarised photon beams at the BGO-OD experiment at ELSA

Abstract

The BGO-OD experiment, located at the electron stretcher facility ELSA in Bonn, is constructed for the study of meson photo-production reactions off the nucleon. The two main detector parts of the experimental setup are a central electromagnetic BGO calorimeter covering most of the solid angle and a magnetic spectrometer enclosing the most forward angles. This combination allows the detection of complicated mixed, charged and neutral, final states over almost the full solid angle and is optimised for the investigation of low momentum transfer processes at extreme forward angles. Physics proposals for the BGO-OD experiment include the measurement of the photon beam asymmetry Σ in different decay channels. The highly energetic photon beam is produced from the electron beam of ELSA via the process of bremsstrahlung. Due to the continuous energy spectrum, the energy of the produced bremsstrahlung photon is a priori unknown. The energy E_γ corresponds to the difference between the energy E₀ of the incoming electron beam and the energy E_e^- of the post-bremsstrahlung electron. With a detector setup known as a photon tagger, consisting of a dipole magnet and a scintillator hodoscope, it is possible to measure the momentum of the post-bremsstrahlung electron and therefore to determine the energy of the produced bremsstrahlung photon. The hodoscope of the BGO-OD photon tagger is designed, assembled and commissioned within this thesis. An energy range from 10%E₀ to 90%E₀ is covered with 120 plastic scintillators. The energy width of two adjacent scintillators ranges between 0.55%E₀ and 2.28%E₀. Since the post-bremsstrahlung electrons are detected with a time resolution of better than 210 ps, the tagger system also provides the time reference for the BGO-OD experiment. Using a linearly polarised photon beam allows the measurement of the photon beam asymmetry. A linearly polarised photon beam can be achieved through coherent bremsstrahlung off a diamond crystal properly aligned with respect to the incoming electron beam. For the alignment of the crystal, the Stonehenge technique performed with a goniometer system is made operational at the BGO-OD experiment within this thesis and has been successfully used since November 2014. Additionally, the C++ based program called COBRIS used to determine the degree of polarisation of the produced bremsstrahlung photon beam is further improved. A more precise calculation of the degree of polarisation and the determination of its error based on the uncertainty of the input parameters is now possible. A cross check on the determination of the degree of linear polarisation is performed through the measurement of the photon beam asymmetry in π⁰ photo-production off the proton. The results are compared to calculations of the Bonn-Gatchina partial wave analysis, which is a representation of existing accurate measurements. The energy range from 1100 MeV to 1500 MeV was studied, in which the expected degree of linear polarisation amounts to at least ~10%. From the comparison of the extracted photon beam asymmetries with the BnGn calculations it can be concluded, that the determination of the degree of linear polarisation is accurate at least within the errors of the extracted beam asymmetry. The relative error on the degree of polarisation derived from the error of the photon beam asymmetry is in the order of 10%. A more likely relative error of the degree of polarisation of 1% - 2% is determined from COBRIS.