Physics Division Seminars bring us speakers on a variety of physics related subjects. Usually these are held in the Building 6008 large Conference Room, at 3:00 pm on the chosen day, but times and locations may vary. For more information, contact our seminar chairman,
Tel (Office): (865) 574-6124 (FAX): (865) 574-1268
A fundamental outcome of modern nuclear shell theory is the prediction of the “islands of stability” in the region of hypothetical superheavy elements. A significant enhancement in nuclear stability at approaching the closed spherical shells with Z=114 (possibly 120 and 122) and N=184 which follow the doubly magic 208Pb nucleus is expected for the nuclei with large neutron excess. For this reason, for the synthesis of nuclei with Z=112-116 and 118 we chose the reactions 238U, 242,244Pu, 243Am, 245,248Cm and 249Cf + 48Ca which are characterized by evaporation residues with a maximal number of neutrons.
The formation and decay of the nuclei with Z=112-116 and 118 were registered with the use of the Gas Filled Recoil Separator installed at a heavy ion beam of the U-400 cyclotron. The new nuclides mainly undergo sequential alpha-decays which are ended with spontaneous fission (SF). The total time of decays ranges from 0.1 s to ~1d depending on the proton and neutron number in the synthesized nuclei. The neutron-rich isotope 268Db (TSF ~ 30h), the final product in the chain of alpha-decays of the odd-odd parent nucleus 288115, was also separated with the use of radio-chemical method, and the characteristics of its spontaneous fission were subsequently measured.
The mechanism of formation of superheavy nuclei in the fusion reactions between actinide nuclei and 48Ca ions was studied separately. From the measured excitation functions it follows that the evaporation products in these reactions are formed in the 2-5n evaporation channels, the maximal formation cross section for the heaviest nuclei substantially depends on the neutron number in the compound nucleus.
The properties of the 29 new nuclides with Z=104-118 and N=162÷177 are now being compared with the predictions of microscopic nuclear models.
The comparison gives evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay. The prospects for future investigations connected with the study of chemical properties of superheavy atoms, mass spectroscopy and search for SHE in Nature will be discussed.
The experiments were carried out at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna) in collaboration with the Analytical & Nuclear Chemistry Division of the Lawrence Livermore National Laboratory (USA).
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