1. Swiss Federal Nuclear Safety Inspectorate ENSI, Industriestrasse 19, 5201 Brugg, Switzerland; e-mail:
2. Department of Physics, Science Faculty, University of Zürich, Winterthurerstrasse 190, 8057 Zurich, Switzerland; e-mail:
3. Institute of Radiation Medicine, Helmholtz Zentrum Muenchen, Ingolstaedter Landstr. 1, 85769 Neuherberg, Germany; e-mail: