1. The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
2. Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan
3. Institut de Physique Nucléaire, 91406 Orsay Cedex, France
4. Institut für Physik, Universität Rostock, D-18051 Rostock, Germany