JPS Conf. Proc. 10, 032003 (2016) [5 pages]
Proceedings of the 10th International Workshop on the Physics of Excited Nucleons (NSTAR2015)
Polarised Photon Beams for the BGO-OD Experiment at ELSA
1Physikalisches Institut, Nussallee 12, 53115 Bonn, Germany
2Helmholtz-Institut für Strahlen- und Kernphysik, Nussallee 14-16, 53115 Bonn, Germany
3INFN sezione di Pavia, Via Agostino Bassi, 6-27100 Pavia, Italy
4Justus-Liebig-Universität Gießen, II. Physikalisches Institut, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
5National Science Center Kharkov Institute of Physics and Technology, Akademicheskaya St. 1, Kharkov, 61108, Ukraine
6INFN sezione Catania, 95129 Catania, Italy
7Università degli Studi di Messina, Via Consolato del Mare 41, 98121 Messina, Italy
8INFN Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
9University of Rome “Tor Vergata”, Physics department, Via della Ricerca Scientifica 1, 00133 Roma, Italy
10INFN sezione di Torino, Via P.Giuria 1, 10125 Torino, Italy
11Dipartimento di Fisica, Università di Torino, via P. Giuria 1, 10125 Torino, Italy
12INFN sezione di Roma, c/o Dipartimento di Fisica - Università degli Studi di Roma “La Sapienza” P.le Aldo Moro, 2, 00185 Roma, Italy
13Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Roma, Italy
14The University of Edinburgh, James Clerk Maxwell Building, Mayfield Road, Edinburgh EH9 3JZ, U.K.
15Petersburg Nuclear Physics Institute, Gatchina, Leningrad District, 188300, Russia
16Russian Academy of Sciences Institute for Nuclear Research, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia
17Institut für Physik, Klingelbergstrasse 82, 4056 Basel, Switzerland
18INFN - LNF, Via E. Fermi 40, 00044 Frascati, Italy
Received October 1, 2015

The new BGO-OD experiment at the electron accelerator ELSA, of the University of Bonn, is designed to study the reaction dynamics of nucleon excitations in meson photoproduction. It consists of a central BGO calorimeter with a magnetic spectrometer in forward direction. The physics programme includes the measurement of polarisation observables using linearly and circularly polarised photon beams. Linear polarisation is obtained by coherent bremsstrahlung off a diamond crystal, and circular polarisation is obtained via bremsstrahlung from longitudinally polarised electrons. The degree of linear polarisation is determined from the bremsstrahlung spectrum itself. To determine the polarisation of the circularly polarised photon beam, the polarisation of the electron beam is measured by a Møller polarimeter. As a preliminary consistency check, the (linear) polarisation observable, Σ, was compared to world data for π0 and η photoproduction. To determine the degree of circular polarisation, a Møller polarimeter was setup and first measurements of the electron beam polarisation performed.

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