The decay properties of 290

We have studied the dependence of the production cross sections of the isotopes 282,283 112 and 286,287 114 on the excitation energy of the compound nuclei 286 112 and 290 114. The maximum cross section values of the xn-evaporation channels for the reaction 238 U͑ 48 Ca, xn͒ 286−x 112 were measured to be 3n = 2.5 −1.1 +1.8 pb and 4n = 0.6 −0.5 +1.6 pb; for the reaction 242 Pu͑ 48 Ca, xn͒ 290−x 114: 2n ϳ 0.5 pb, 3n = 3.6 −1.7 +3.4 pb, and 4n = 4.5 −1.9 +3.6 pb. In the reaction 233 U͑ 48 Ca,2-4n͒ 277-279 112 at E * = 34.9± 2.2 MeV we measured an upper cross section limit of xn ഛ 0.6 pb. The observed shift of the excitation energy associated with the maximum sum evaporation residue cross section ER ͑E * ͒ to values significantly higher than that associated with the calculated Coulomb barrier can be caused by the orientation of the deformed target nucleus in the entrance channel of the reaction. An increase of ER in the reactions of actinide targets with 48 Ca is consistent with the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N = 184. In the present work we detected 33 decay chains arising in the decay of the known nuclei 282 112, 283 112, 286 114, 287 114, and 288 114. In the decay of 287 114͑␣͒ → 283 112͑␣͒ → 279 110͑SF͒, in two cases out of 22, we observed decay chains of four and five sequential ␣ transitions that end in spontaneous fission of 271 Sg ͑T ␣/SF = 2.4 −1.0 +4.3 min͒ and 267 Rf ͑T SF ϳ 2.3 h͒, longer decay chains than reported previously. We observed the new nuclide 292 116 ͑T ␣ =18 −6 +16 ms, E ␣ = 10.66± 0.07 MeV͒ in the irradiation of the 248 Cm target at a higher energy than in previous experiments. The observed nuclear decay properties of the nuclides with Z = 104-118 are compared with theoretical nuclear mass calculations and the systematic trends of spontaneous fission properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z = 104-118 and N = 163-177. The experiments were performed with the heavy-ion beam delivered by the U400 cyclotron of the FLNR ͑JINR, Dubna͒ employing the Dubna gas-filled recoil separator.

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The results of two experiments designed to synthesize element 115 isotopes in the 243 Am + 48 Ca reaction are presented. Two new elements with atomic numbers 113 and 115 were observed for the first time. With 248-MeV 48 Ca projectiles, we observed three similar decay chains consisting of five consecutive α decays, all detected in a total time interval of 30 s. Each chain was terminated by a spontaneous fission (SF) with a high-energy release and a lifetime of about a day. With 253-MeV 48 Ca projectiles, we registered a different decay chain of consecutive α decays detected in a time interval of 0.5 s, also terminated by spontaneous fission, but after 1.8 h. The decay properties of the eleven new αand SF-decaying nuclei are consistent with expectations for consecutive α decays originating from the parent isotopes 288 115 and 287 115, produced in the 3n-and 4n-evaporation channels, respectively. Support for the assignment of the atomic numbers of all of the nuclei in the 288 115 decay chain was obtained in an independent experiment in which a long-lived spontaneous fission activity, 268 Db (15 events), was found to be chemically consistent with the fifth group of the periodic table. The odd-odd isotope 288 115 was observed with largest cross section of about 4 pb. In the SF decay of 268 Db, a total kinetic energy of 230 MeV and a neutron multiplicity per fission of 4.2 were measured. The decay properties of the 11 new isotopes with Z = 105-115 and the production cross sections are in agreement with modern concepts of the role of nuclear shells in the stability of superheavy nuclei. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research.