1. Universität Bonn , Regina – Pacis – Weg 3, D-53115 Bonn , Germany
2. Scottish Universities Physics Alliance, University of Saint Andrews , North Haugh, Saint Andrews, Fife KY16 9SS , UK
3. Helmholtz – Institut für Strahlen – und Kernphysik, Universität Bonn , Nussallee 14-16, D-53115 Bonn , Germany
4. Astronomical Institute, Faculty of Mathematics and Physics, Charles University , V Holešovičkách 2, CZ-180 00 Praha 8 , Czech Republic