[en] Spontaneous fission and α-decay of ^253-257104 and ²⁵⁸106 were investigated in irradiations of ^204,206,208Pb with ⁵⁰Ti and ²⁰⁹Bi with ⁵¹V, respectively. New spontaneous fission activities were identified and assigned to ²⁵³104, ²⁵⁴104, and ²⁵⁸106. The half-lives were measured as T_1/2=(48⁺¹⁷_-10) μs for ²⁵³104, T_1/2=(23±3) μs for ²⁵⁴104, and T_1/2=(2.9^+1.3_-0.7) ms for ²⁵⁸106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios b_α limits corresponding to b_α≤0.1 for ²⁵³104, b_α≤0.015 for ²⁵⁴104, and b_α≤0.2 for ²⁵⁸106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of ²⁵⁷104 and ²⁵⁵104 have been improved. An isomeric state decaying by α-emission was identified in ²⁵⁷104 and attributed to a low lying 11/2^-[725] state. A small α-decay branch for the even-even nucleus ²⁵⁶104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even-even nuclei up to ²⁶⁴Hs (Z=108), the heaviest even-even nucleus identified so far. (orig.)

[en] Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 were produced by the reaction ²⁰⁹Bi(⁵⁰Ti,xn) ^259-xDb (x = 1, 2, 3) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction ²⁰⁸Pb(⁵⁰Ti,1n)²⁵⁷Rf. The measured α-decay data of ²⁵⁷Db and its daughter products resulted in the identification of α-decaying isomeric states in ²⁵⁷Db and ²⁵³Lr. Two new isotopes, ²⁵⁶Db and ²⁵²Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by delayed α-α coincidences. The measured half-lives are (1.6^+0.5_-0.3) s for ²⁵⁶Db and (0.36^+0.11_-0.07) s for ²⁵²Lr. Besides α-decay, a spontaneous fission activity of T_1/2 = (2.3^+1.1_-0.6) s was observed and attributed to an electron-capture branch of ²⁵⁶Db, which feeds the fissioning nucleus ²⁵⁶Rf. A branching ratio of 0.36±0.12 was obtained. The isotope ²⁵⁵Rf was produced by the reaction ²⁰⁷Pb(⁵⁰Ti,2n)²⁵⁵Rf. Improved decay data have been obtained by means of α- and α-γ spectroscopy. (orig.)

No abstract available

[en] The progress in investigations of the heaviest known elements in the recent years was only possible with an elaborate target technology. Large area thin and homogeneous targets of Pb and Bi, mounted on a rotating wheel to allow high projectile currents, were used in the experiments performed at the velocity filter SHIP at GSI Darmstadt. Various techniques for the on-line monitoring of the targets will be described and their importance for the experiments will be discussed. The experimental results from the investigations of the heaviest elements will be summarized briefly. (orig.)

[en] Measured α-decay spectra of the isotopes ²¹⁵Th and ²¹³Ra were used to investigate the influence of energy summing of α-particles and conversion electrons, and Auger electrons, respectively, emitted after α-decays into excited daughter levels, on the shape of α-spectra. The isotopes were produced in heavy ion fusion reactions and were implanted into silicon surface barrier detectors after in-flight separation from the projectile beam. It was shown that the summing effects will cause (a) the occurrence of additional lines in the α-energy spectra and (b) a change in the relative intensities of α-lines belonging to the α-decay pattern of a certain isotope. The results for ²¹⁵Th and ²¹³Ra are used for a discussion of the α-decay properties of the isotope ²⁵⁵104 and its α-decay products ²⁵¹No and ²⁴⁷Fm. Especially the large discrepancies of the α-energies of ²⁴⁷Fm measured in our experiments and literature values are interpreted as due to a summing between α and electron energies. (orig.)

[en] Evaporation residues from the fusion reactions ⁴⁸Ca+^206,208Pb and ⁵⁰Ti+^207,208Pb, ²⁰⁹Bi were investigated using the velocity filter SHIP. Four new isotopes ^258,257105 and ^254,253Lr were identified and α decay of ²⁵⁵104 and ²⁵⁶104 was observed for the first time. A hindrance to complete fusion by a factor of about 30 at bombarding energies close to the classical barrier was deduced from a measured excitation function for ⁵⁰Ti+²⁰⁸Pb. The mass extents for ²⁵⁶104 and ²⁶⁰106, obtained from their α-decay energies, were used to determine the ground-state shell effects and a semi-empirical description of the half-life T_SF for even-even nuclei. The abrupt decrease in T_SF for elements from nobelium to element 104 was found to be correlated with a decrease in the fission barrier width, which we regard as confirmation of the expected drop in the outer fission barrier to below the ground state. The total kinetic energies released in SF of ²⁵⁶104 and ²⁵⁸104 were measured and found to be 207±13 MeV and 220±15 MeV respectively. These values agree with those predicted from empirical systematics. (orig.)

[en] The author reviews the experiments for the production of the elements 104-109, performed at the GSI Darmstadt using the UNILAC and the SHIP spectrometer. Furthermore shell effects in collision dynamics are discussed. (HSI)