The decay properties of 290

The discovery of a new chemical element with atomic number Z=117 is reported. The isotopes (293)117 and (294)117 were produced in fusion reactions between (48)Ca and (249)Bk. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with Z > or = 111, validating the concept of the long sought island of enhanced stability for superheavy nuclei.

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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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