A significant enhancement of nuclear stability when approaching the closed spherical shells with Z = 114 (possibly 120, 122 and 126) and N = 184, following the doubly magic 208Pb nucleus, is expected for nuclei with large neutron excess. For this reason, for the synthesis of nuclei with Z = 114 and 116 we chose the reactions 244Pu, 248Cm + 48Ca, which are characterized by evaporation residues with a maximum number of neutrons.
Of greatest interest are the even-even isotopes because their a-decay energy is determined from the mass difference of the nuclei in their ground state and can serve as a direct test of theoretical models.
The formation and decay of even-even nuclei of elements 114 and 116 has been observed with the gas-filled recoil-nuclei separator at the U400 accelerator of the Flerov Laboratory of Nuclear Reactions. The new nuclides undergo sequential a-decay, which terminates by spontaneous fission. The total decay time of the nuclei is about 1.5 min.
Preliminary data from an experiment aimed at the synthesis of element 118 in the 249Cf + 48Ca reaction (performed this year in May-June) are also presented.
The results from these experiments, together with the formerly obtained data on the properties of the neighboring even-even isotopes of lighter elements, have been compared with calculations, performed within different microscopic nuclear models. They give evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay.
The possibilities of further investigations connected with the study of the chemical properties of superheavy atoms, mass-spectroscopy etc. will also be discussed.
The experiments have been 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).