The results of an attempt aimed at the synthesis of element 118 in the reaction 249Cf (48Ca,3n)294118 are reported. The experiment was performed employing the Dubna Gas-Filled Recoil Separator and the U-400 heavy-ion cyclotron at FLNR (JINR, Dubna). In the course of a 2300-hour irradiation of an enriched 249Cf target (0.23 mg/cm2) with a beam of 245-MeV 48Ca ions, we accumulated a total beam dose of 25•1019 ions. We detected two events that may be attributed to the formation and decay of nuclei with Z = 118. For one event, we observed -decay chain of two correlated -decays with corresponding energies and correlation times of E1= (11.65 ±0.06) MeV, tl=255ms and E2 = (10.71± 0.17)MeV, t2=42.1 ms and, finally, a spontaneous fission with the sum of the kinetic energies of the fission fragments Etot = 207 MeV (TKE - 230 MeV) and tSF = 0.52 s. In the second event chain, the recoil nucleus decayed into two fission fragments with Etot =223MeV (TKE ~245 MeV) 3.16 ms later, without intervening -decays. The probabilities that these events were caused by the chance correlations of unrelated signals are negligible. Both events were observed at an excitation energy of the compound nucleus 297118 of E* = (30.0 ±2.4) MeV, close to the expected maximum of the 3n-evaporation channel. The relationship between the decay energy Q and decay period T shows that sequential -transitions in the first event correspond to the decay chain with Z = 118 --> 116-->114. Decay characteristics of the newly observed nuclides are compared with radioactive decay properties of the even-even isotopes with Z= 116, 114 and 112 previously produced in the reactions 244Pu; 248Cm + 48Ca and with calculations made in various nuclear models.|
The investigation has been performed at the Flerov Laboratory of Nuclear Reactions, JINR.