[1] Department of Physics, University of Potsdam, Am Neuen Palais, PF 601553, D-14415 Potsdam, Germany
[2] Department of Medical Physics, University of Patras, 26 500 Rion Patras, Greece
[3] Char., Johannes-Muller Inst. P., Humboldt University, Tucholsky Strasse 2, D-10117 Berlin, Germany