1. Institut Pluridisciplinaire Hubert Curien-Département de Recherches Subatomiques, UMR7178, IN2P3-CNRS et Université Louis Pasteur (Strasbourg I), 23 rue du Loess, B. P. 28, F-67037 Strasbourg Cedex 2, France
2. Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany
3. INFN-Lab. Nationali di Legnaro and Dipartimento di Fisica, I-35020 Padova, Italy
4. School of Physics and Chemistry, University of Surrey, Guildford, Surrey, GU2 7XH, UK
5. Department of Physics, University of York, York, YO10 5DD, UK
6. Subatech, IN2P3-CNRS et Université-Ecole des Mines, 4 rue A. Kastler, F-44307 Nantes Cedex 3, France