1. University of Washington, Physics Department and Electrical and Computer Engineering Department, Seattle, Washington 98105, USA
2. Helmholtz-Institut, GSI Helmholtzzentrum für Schwerionenforschung, 55128 Mainz, Germany
3. Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany
4. Department of Physics, University of California, Berkeley, California 94720-7300, USA