A prescription for the error analysis of experimental data in the case of stochastic background is formulated. Several relations are given which allow to establish the significance of mother-daughter relationships obtained from delayed coincidences. Both, the probability that a cascade is produced randomly and the probability that the parameters of an observed event chain are incompatible with known properties of a given species are formulated. The expressions given are applicable also in cases of poor statistics down even to single events.

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Abstract. Excitation functions in the vicinity of theCoulomb barrier have been measured for the formation of evaporation residues in l~176 fusion reactions with 9~ 92,96,98, lOOMo ' 104Ru and l~~ as well as for the system 96Zr + 96Zr. From these data the fusion probability in central collisions was extracted covering a range of 4 orders of magnitude. At the fusion barriers expected from systematics we find that the fusion probability is suppressed by one to three orders of magnitude. It is rising very gradually at higher energies and reaches for the heaviest systems saturation only at energies as high as 30 MeV above the barrier. The observed hindrance of the fusion process increases roughly with the growing Coulomb repulsion between the collision partners, but there is also a distinct influence of their individual nuclear structure. The data are compared to the extra-push model, the surface-friction model and the diabatic fusion model. A parameterisation of the extra-push energy and its fluctuation in terms of a macroscopic quantity like the Coulomb repulsion combined with a microscopic quantity characterizing the nuclear structure is proposed. As a byproduct of this work a new alpha emitter, 191po, could be identi-+6 fled. Its half-life is (15.5_z s) ms, the alpha energy is (7314 _ 20) keV.

In fusion reactions of 4~ 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 spherical N=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

The production of nuclei at the upper end of the periodic table in heavy ion fusion reactions is limited by the high fissility of the compound system. Formation cross-sections decrease rapidly with increasing excitation energy. With targets near the double magic 2~and neutron rich metal ions like 5OTi or 54Cr as projectiles, fusion close to the barrier yields weakly excited compound systems with excitation energies of less than 25 MeV. The asymmetry of the target-projectile combination is still sufficient to expect no dynamic effects on the barrier. Consequently, the irradiation of 2O9Bi with 54Cr as proposed by Oganessian et al /1/ is up to our present knowledge the most appropriate reaction to synthesize element 107 by heavy ion fusion.With the velocity filter SHIP /2/ for separation and surface barrier detectors for identification /3/ of the fusion products we can investigate the predominant decay modes, spontaneous fission and decay over the entire expected halflife range down to us with low background from other reactions.Up to now in experiments to produce element 107 only spontaneous fission was observed. Oganessian et al. /I/ found two spontaneous fission activities in bombardments of 2O9Bi with 54Cr. One with a halflife of (1-2) ms, which was assigned to the 2n channel. The other activity with a halflife of 5 s was explained by an unobserved decay of 261107 leading to 257105 which was regarded to undergo spontaneous fission with the corresponding halflife.The aim of our experiment was to produce element 107 in the same reaction and to identify the isotopes unambiguously by correlated a decay chains to known transitions. We also investigated the reaction 5OTi on 209Bi to find the unknown daughter decays of element 105 and Lr. Experimental SetupThe S4Cr beam from the UNILAC linear accelerator had an average intensity of 6 x 1011 particles/s.The ion source was operated with isotopically enriched material. 209Bi targets with an average thickness of 660 ug/cm 2 had been evaporated on a 30 ug/cm 2 carbon foil and covered with a 30 2 ~g/cm carbon film for radiative cooling. They were mounted on a rotating wheel to stand the high beam intensities /4/. Target thickness was monitored by Rutherford scattering. After in-flight separation by SHIP, the evaporation residues passed two large area detectors for time-of-flight measurement and were implanted into a bench of 7 position sensitive surface barrier detectors. The point of the identification method is that all decays belonging to a decay chain which starts from any incident evaporation residue have to occur at its position of implantation within a window given by the detector resolution. Correlation times are limited by the rates of random events within this position window. The detectors are cooled to 268 K and have an energy resolution of 27 keY FWHM, the position resolution is 0.3 mm FWHM.The evaporation residues are separated with efficiencies of about 20%. The response of the surface barrier detector for ~ decays is 50%, as the evaporation residues are impl...

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