Results from the first sup 2 sup 4 sup 9 Cf + sup 4 sup 8 Ca experiment

The results of an attempt aimed at the synthesis of element 118 in the reaction sup 2 sup 4 sup 9 Cf( sup 4 sup 8 Ca, 3n) sup 2 sup 9 sup 4 118 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 sup 2 sup 4 sup 9 Cf target (0.23 mg/cm sup 2) with a beam of 245-MeV sup 4 sup 8 Ca ions, we accumulated a total beam dose of 2.5 centre dot 10 sup 1 sup 9 ions. We detected two events that may be attributed to the formation and decay of nuclei with Z = 118. For one event, we observed a decay chain of two correlated alpha-decays with corresponding energies and correlation times of E subalpha sub 1 = (11.65 +- 0.06) MeV, t subalpha sub 1 = 2.55 ms and E subalpha sub 2 = (10.71 +- 0.17) MeV, t subalpha sub 2 = 42.1 ms and, finally, a spontaneous fission with the sum of the kinetic energies of the fission fragments E sub t sub o sub t = 207 MeV (TKE approx 230 MeV) and t sub S sub F = 0.52 s. In the second event chain, the recoil nucleus decayed into two fission fragments with E sub t sub o sub t = 223 MeV (TKE approx 245 MeV) 3.16 ms later, without intervening alpha-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 sup 2 sup 9 sup 7 118 of E* = (30.0 +- 2.4) MeV, close to the expected maximum of the 3n-evaporation channel. The relationship between the decay energy Q subalpha and decay period T subalpha shows that sequential alpha-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 sup 2 sup 4 sup 4 Pu, sup 2 sup 4 sup 8 Cm + sup 4 sup 8 Ca and with calcula various nuclear models.