1. CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
2. OPTIS, ZE de la Farlède, F-83078 Toulon Cedex 9, France
3. University of Basel, Department of Physics, CH-4056 Basel, Switzerland
4. THALES-SESO, 305 Rue Louis Armand, CS 30504, F-13593 Aix en Provence Cedex 3, France