Abstract:The reactions 58Ni + 102pd-, 160W and 58Ni + 106Cd-~164Os were investigated to search for new decay data of neutron deficient nuclei. Excitation energies of the compound nuclei covered a range from 47 to 89 MeV. Velocity separation of the evaporation residues and position time correlations with the a decays of the implanted nuclei were used. The following new decay data were measured: 162Os (E~ = (6611 4-30) keV, T, A = (1.94-0.7) ms); 158W (Tv2 = (0,94-0.3) ms)" 158mW(E = 1.88MeV E_ = (82804-30) keV T,, = (0.01-1)

The experiment having led to the discovery of first isotopes of element 108, as published previously in Short Notes to this Journal [1, 2] are described and discussed in a final paper.Two isotopes of element 108 were produced by complete fusion of 2~ and 2~ respectively, with 58Fe. Both isotopes are e-emitters.For the isotope with mass 265 three e-decay sequences were observed. The e-decay energy is (10.36_+0.03)MeV, the half-life 1.8_017 ms. For the isotope with mass 264, which is the heaviest doubly even isotope known at present, the decay sequence of /7 + 364\ one atom was found. The measured half-life is ~ 6 36 ) gs.Our experimental results point towards an enhanced stability of the heaviest elements against spontaneous fission, which was already observed for element 106.

Recent experiments have shown that the heaviest elements are unexpectedly stable against spontaneous fission 1,2. For the heaviest known eveneven isotope 26~ a-decay has been observed, in variance with calculations predicting actinide fission halflives, 3,4. From the a-decay energies groundstate shell-effects were evaluated, indicating shell stabilization beyond element 104, 5. To further support this finding it is of special interest to investigate doubly even nuclei in the trans-actinide region. For these nuclei groundstate properties are determined most unambiguously. The investigation of 264108, separated by just one a-decay from 26~ would give information about the extension of stability towards the heavier elements. From the observation of a (6C~) ms spontaneous fission acitivity in irradiations of Z~ with 58Fe performed at Dubna it was speculated that 264108, formed in a complete fusion reaction with subsequent evaporation of one neutron, may undergo a-decay 6. The observed activity tentatively was assigned to the fission of the grand-daughter 256104 with 7 ms halflife. The ExperimentIn our experiment we used the same reaction. The beam energy of 5.04 MeV/ u corresponded to an excitation energy of (19_+2) MeV in the compound system. The beam of 2xlO 12 particles/s on the average was obtained from ion sources of isotopically enriched SSFe with an enrichment of 25 at. %. The integral particle dose after 250 h beam on target was 1.2xlO 18. We used 2~ targets with a purity of 92.4 % (2~ %, 2~ %) of an average thickness of 410 Ng/ cm 2. The target material was evaporated onto 35 Ng/cm 2 carbon foils and covered with 15 Ng/cm 2 carbon. The evaporation residues recoiling from the target were separated in-flight by the velocity filter SHIP and after passing two time-of-flight pickups implanted into position sensitive surface barrier detectors, where their decay was measured as described in Ref. 2. The efficiency of our setup for residues from fusion and neutron evaporation is 25 %, for those from a-evaporation it is reduced by a factor of about I0. The residues are implanted close to the detector surface at a distance corresponding to I0 % of the range of II MeV a-particles. Therefore the chance to detect a decay from an implanted residue with full energy is 55 %. For a-particles emitted into the backward hemisphere only the energy loss in the detector is registered, which is at least 0.7 MeV. In the case of fission at least one fragment is detected with full energy, the second fragment also may escape from the detector. The energy resolution of the silicon detectors, which were cooled to 263 K, is 35 keV FWHM. The position resolution is 0.3 mm FWHM for full-energy alphas and decreases by a factor of about two for low energy escapes. From time-of-flight and implantation energy a rough mass determination with a resolution of 30 % at FWHM is possible, which allows to discriminate between the three groups of particles impinging on the detector: Scattered projectiles with a rate 50/s on the average, target-like recoil...

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