[1] Politecn Milan, Dipartimento Ingn Nucl, Ce SNEF, I-20133 Milan, Italy
[2] Ist Nazl Fis Nucl, Sez Milano, I-20133 Milan, Italy
[3] Politecn Milan, Dipartimento Elettron & Informaz, I-20133 Milan, Italy
[4] Brookhaven Natl Lab, Instrumentat Div, Upton, NY 11973 USA
[5] PNSensor GmbH, D-80803 Munich, Germany
[6] Max Planck Inst Halbleiterlabor, D-81739 Munich, Germany
[7] Max Planck Inst Extraterr Phys, D-85741 Garching, Germany
[8] Univ Siegen, FB Phys, D-57068 Siegen, Germany