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In 54Cr ion bombardments of targets prepared from enriched lead isotopes the spontaneous fission probability for the isotopes of element 106 formed in the reactions Pb(54Cr, 1,2n)106 was investigated. The spontaneous fission half-life of the doubly-even isotope 26~has been shown to be TSF>~5ms. The half-lives of neighbouring oddmass isotopes have also been estimated: TSF(259106)~>0.1S and Tsv(261106)~>0.4s. The results of the experiments are discussed in comparison with the latest theoretical calculations.In connection with experiments to synthesize element 108 some data are necessary on properties of isotopes with Z=106 which can be produced as a result of the e-decay of the Z=108 nuclides. The nuclear stability itself of element 106 against spontaneous fission (lifetimes TSF ) presents also interest in view of a sharp change in the TSF systematics for Z = 104. For comparison with theory, doubly-even isotopes with Z> 104 (primarily Z= 106) are most informative since for these nuclei the uncertain hindrance factor for Tsv due to oddness is absent. The Tsv data for isotopes of element 106 are rather scarce, they concern only the odd-mass isotopes 259106[1] and 263106 [2]. In our experiments to search for spontaneously fissioning isotopes with Z=106, carried out in 1974, we used the "cold fusion" reactions Pb+54Cr which are most suitable because of the low excitation energy of the compound nucleus (Emi,~lgMeV). In bombardments of the lead isotopes 2~176176 by 54Cr ions of wide range energies up to 290 MeV we detected two spontaneous fission activities with sharply different half-lives: in millisecond (4-10ms) [1] and second (~l.5s) [3] ranges. As a result of a series of control experiments and cross bombardments it was unambiguously established that the activities are both due to isotopes of element 106. The long-lived activity was identified to be the isotope 255104 (the spontaneous fission probability bsv~50%) formed as a result of 2s9106 edecay, and this allowed one to establish the considerable e-decay probability for this isotope of element 106 [3]. The short-lived activity might be due to the isotopes 259106 or 26~ However, the theoretically predicted [4] lifetime TsF (26o106) had to be two orders of magnitude smaller than the known value of TsF 258 (154104) [5], i.e. to be ~0.1ms, which could not be observed in our experiments. The then available data on the probability of neutron emission in "cold fusion" experiments also indicated the most probable value A = 2591 With the construction of the U400 accelerator, the intensity of Ti, Cr, Mn and Fe ion beams increased by several tens of times. Having improved the experimental technique to adjust it to the increased beam intensity and having at our disposal lead isotopes with higher enrichment we turned again to the study of the reactions Pb + 5~Cr with the aim of determining in more detail the properties of both the oddmass and doubly-even isotopes of element 106. The performance of the experiments was similar to that described earlier, and it allowed one to re...

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Elements Z= 104-109ISynthesis/Spontaneousfission/Alpha decay ICold fusion reactions AbsttactThe formation and radioactive properties of "cold fusion" reaction products with Z = 104-109 have been investigated using a highly sensitive technique for detecting spontaneous fission ("online") and a-decay ("off-line"). It has been shown that all the isotopes with Z = 106 -109 produced in these experiments, including the doubly-even 106 and 108, undergo mainly a-decay. Possibilities of synthesizing the heavier elements with Z > 109 are discussed.

In bombarding 2~ and 2~176 targets by 55Mn and SaFe ions the yields of "cold fusion" reactions have been determined using a sensitive technique for detecting T~> 1 ms spontaneous fission and the e-decay of heavy actinide elements. It has been shown that the A=263-265 isotopes of element 108, including the even-even isotope 264108, undergo mainly e-decay. The obtained results, together with the known data on the properties of the isotopes of elements 104 and 106, provide evidence for the enhanced stability of the Z = 108 nuclei against spontaneous fission.

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