1. Department of Physics, Brock University, St. Catharines, Ontario L2S 3A1, Canada
2. Dept. de Physique, U. de Montréal, Montréal, Quebec H3T 1J4, Canada
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum, Dresden-Rossendorf, PO Box 510119, Dresden 01314, Germany