1. Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany
2. Helmholtz Institut Mainz, 55099 Mainz, Germany
3. Department of Physics, University of California, Berkeley, California 94720-7300, USA
4. Department of Physics and Astronomy, Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico 87106, USA
5. Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA