1. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
2. Centre for Energy Research, Hungarian Academy of Sciences, H-l12l Budapest, Konkoly Thege Miklos ut 29-33, Hungary
3. Joint Institute for Nuclear Research, Flerov Laboratory of Nuclear Reactions, Dubna, Moscow Region 141980, Russia