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The interplay of shell structure and rotational motion in the nuclear level density is discussed. The result of a schematic calculation reveals a washing out of spherical shell effects at much lower excitation energies than expected if only the intrinsic level density in the independent particle model is considered.

The energy dependence of the fusion probabilities for central collisions of the systems124Sn +92,94,96Zr and86Kr +123Sb was determined from neutron-evaporation-residue cross sections. Near the barrier as calculated from one-dimensional barrier-penetration models, the fusion probability was found to be reduced by several orders of magnitude.

Evaporation residues from the fusion of the nearly symmetric systems86Kr+123Sb and124Sn+94Zr have been detected. The compound nucleus218Th could be produced with an excitation energy as low as 21 MeV. The barriers for the formation of a compound nucleus were deduced from the evaporation residue cross sections. There is no evidence for an additional amount of energy needed to fuse these systems, if compared with the extrapolation of fusion barrier systematics or with results of fusion model calculations not including friction phenomena. One of the systems reaches a value ofZ 1·Z 2=2,000, about the highest value where evaporation residues have ever been observed.

In fusion reactions of40Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

Theα-decays of199Rn (E α = (6.989±0.010) MeV,T 1/2>300ms) and of199m Rn (E α = (7.060±0.015) MeVT 1/2=(61 −2 +32 ) ms) were identified by anα-α-correlation method. They proceed via separated decay paths to195Po and195m Po, respectively. The spins of the involved states are discussed by aid of the shell model, and a tentative spin assignment is given.

The major problems in the investigation of the heaviest elements at the upper end of the periodic table are the small formation cross sections which are far below the nanobarn region and their short halflives of the order of milliseconds. The heaviest elements known up to now were I07 and I09. Two a-emitting odd-odd isotopes of these elements were identified by a-chains to known daughter isotopes /1,2/. Element I08 could not be observed so far /3,4/. Our aim was to produce element 108 by complete fusion of S'Fe with 2~ followed by evapora- tion of one neutron which leads to the odd mass isotope '6sI08. To allow an unambiguous identi- fication of the new element in case of its a-decay, we had also to investigate the unknown daughter isotope 2~I06.