Experiments on the synthesis of superheavy nuclei Fl 284 and Fl 285 in the Pu 239,240 + Ca 48 reactions

Irradiations of Pu239 and Pu240 targets with Ca48 beams aimed at the synthesis of Z=114 flerovium isotopes were performed at the Dubna Gas Filled Recoil Separator. A new spontaneously fissioning (SF) isotope Fl284 was produced for the first time in the Pu240+Ca48 (250 MeV) and Pu239+Ca48 (245 MeV) reactions. The cross section of the Pu239(Ca48,3n)Fl284 reaction channel was about 20 times lower than predicted by theoretical models and about 50 times lower than the maximum fusion-evaporation cross section for the 3n and 4n channels measured in the Pu244+Ca48 reaction. In the Pu240+Ca48 experiment, performed at 245 MeV in order to maximize the 3n-evaporation channel, three decay chains of Fl285 were detected. The α-decay energy of Fl285 was measured for the first time and decay properties of its descendants Cn281, Ds277, Hs273, Sg269, and Rf265 were determined with higher accuracy. The assignment of SF events observed during the irradiation of the Pu240 target with a 250 MeV Ca48 beam to Fl284 decay is presented and discussed. The cross sections at both Ca48 energies are similar and exceed that observed in the reaction with the lighter isotope Pu239 by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate a rapid decrease of stability of superheavy nuclei as the neutron number decreases from the predicted magic neutron number N=184.