Measurement of evaporation residue cross-sections of the reaction Si-30+U-238 at subbarrier energies

The reaction 30Si + 238U ! 268Sg¤ was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of Ec:m: = 144:0MeV for reactions at half thickness of the target we measured three decay chains of 263Sg produced by evaporation of ¯ve neutrons. The cross-section was (67+67 ¡37) pb. At Ec:m: = 133:0MeV we measured three spontaneously ¯ssioning nuclei which we assigned to the isotope 264Sg. The production cross-section was (10+10 ¡ 6 ) pb and a half-life of (120+126 ¡ 44 ) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At Ec:m: = 128:0MeV an upper cross- section limit of 15 pb was measured. The cross-section data reveal a strong in°uence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system 16O + 238U ! 254Fm¤, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasi¯ssion processes.