1. Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Bonn, Germany
2. European Organization for Nuclear Research (CERN), CH-1211 Geneva, Switzerland
3. Institute for Materials Science and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 45141 Essen, Germany