1. University of Gent, Department of Subatomic and Radiation Physics, B-9000 Gent, Belgium
2. European Communities, Joint Research Centre, Institute for Reference Materials and Measurements Retieseweg 111, B-2440 Geel, Belgium
3. CEA-Cadarache, DEN/DER/SPRC/LEPh, F-13108 Saint Paul lez Durance, France