Evaporation residues produced in heavy-ion fusion-reactions of
50Ti and
207Pb,
208pb have been investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into position-sensitive surface-barrier detectors, where their
decay or their decay by spontaneous fission was measured. The production of weakly excited compound nuclei in reactions of projectiles and targets close to the doubly magic
48Ca and
208Pb was confirmed by the observation of the 1n deexcitation channel in the reaction
50Ti +
208Pb.
The spontaneous fission activities of
255104 and
256104 could be confirmed; improved halflives are (1.4
–0.2
+0.2
)s and (7.4
–0.7
+0.9
)ms. Furthermore
-decay of
255104 (
b
a
=0.48 ±0.07) and an indication for a small
-branch (
b
a
= 0.022
–0.018
+0.073
) of
256104 was found.
A total kinetic energy of (207±13)MeV released in spontaneous fission of256104 was measured.
A statistical model analysis of the evaporation-residue cross sections is shown to give evidence for a dynamic hindrance to complete fusion in the system50Ti +208Pb, as well as for non-equilibrium components in the deexcitation mechanism.
This work is part of the Ph. D. Thesis of F.P. Heßberger
The authors thank N. Angert and the UNILAC staff for the excellent performance of the accelerator. We are indebted to M. Müller and W. Jacoby for the development of the highly efficient sources for isotopically enriched material and H. Folger, W. Hartmann and W. Thalheimer for the skilful preparation of the large area Pb-targets. Further we are indebted to the GSI experimental electronics group, namely M Richter and K. Poppensieker, for their helpful assistance. We also thank H.G. Burkhard who is in charge of the mechanical set-ups for the SHIP experiments.