1. Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany
2. University of Cape Town, Private Bag, Rondebosch 7701, South Africa
3. Université Catholique de Louvain, Chemin du Cyclotron, 2, B-1348 Louvain-la-Neuve, Belgium
4. iThemba Laboratory for Accelerator Based Sciences, PO-Box 722, Somerset West 7129, South Africa