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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 evaporation of one neutron which leads to the odd mass isotope '6sI08. To allow an unambiguous identification of the new element in case of its a-decay, we had also to investigate the unknown daughter isotope 2~I06. Experimental SetupIn an 240 h experiment we used an "Fe beam with an average intensity of lO ~' particles/s provided by the UNILAC heavy ion accelerator. The specific beam energy of 5.02 MeV/u corresponds to an excitation energy (18+2) MeV in the compound system. The ion souFce was operated with isotopically enriched material, diluted in gold to 16 atomic percent. The integral particle dose was 6xlO ~7. The experimental setup is described in Ref. /2/. Isoto~ically enriched 2~ targets of 400 Ng/cm evaporated onto a 35 N~/cm 2 carbon backing and covered with 25 pg/cm" carbon foil were mounted on a rotating target wheel. They were moved across the beam with a velocity of 2 cm/ms. The targets were monitored by Rutherford scattering. The evaporation residues recoiling from the target were separated in-flight from projectiles and products of other nuclear reactions by the velocity filter SHIP /5/. Then they passed two thin, large area time-of-flight detectors and an absorber of 200 Ng/cm ~ Hostaphan and were finally implanted into an array of seven position sensitive surface barrier detectors where their a-decay or spontaneous fission could be measured /6/. From time-of-flight and implantation energy a rough mass determination is possible. The efficiency of our setup for residues from prompt neutron emission is 25 %, for those from prompt ~-emission it is reduced by a factor of about lO. The implantation depth of the evaporation residues is lO % of the range of the a-particles. Therefore we have only 55 % probability to observe the full a-decay energy. The mass resolution of our setup is 13 % FWHM. The energy resolution of the detector array, which was cooled to 258 K is 32 keV FWHM for a-particles, the position resolution 0.3 mm FWHM. The calibration error is estimated to be + ]5 keV for a-energies between 8.5 and lO MeV, ~nd + 25 keV for a-energies above lO MeV. The lower-time limit of detection given by the flight time through SHIP is about 1.5 Ns, double events with a spacing in excess of 3 Ns were fully resolved by the electronic setup. Experimental ResultsThree chains of particular interest were observed in the array detectors. They are shown with their characteristics in Fig. l, together with our interpretation in terms of the a-decay of the isotope 26si08 following its pro...

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