[1] Univ Trieste, Dipartimento Fis, Trieste, Italy
[2] Sincrotrone Trieste SCpA, Basovizza, Italy
[3] Harvard Univ, Dept Phys, Cambridge, MA USA
[4] Univ Erlangen Nurnberg, Dept Phys, Erlangen, Germany
[5] Ist Nazl Fis Nucl, sez Trieste, Trieste, Italy