Experiments on the synthesis of superheavy nuclei<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Fl</mml:mi><mml:mprescripts/><mml:none/><mml:mn>284</mml:mn></mml:mmultiscripts></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Fl</mml:mi><mml:mprescripts/><mml:none/><mml:mn>285</mml:mn></mml:mmultiscripts></mml:math>in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi>Pu</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>239</mml:mn><mml:mo>,</mml:mo><mml:mn>240</mml:mn></mml:mrow></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mi>Ca</mml:mi><mml:mprescripts/><mml:none/><mml:mn>48</mml:mn></mml:mmultiscripts></mml:mrow></mml:math>reactions

Irradiations of $^{239}\mathrm{Pu}$ and $^{240}\mathrm{Pu}$ targets with $^{48}\mathrm{Ca}$ 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 $^{284}\mathrm{Fl}$ was produced for the first time in the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ (250 MeV) and $^{239}\mathrm{Pu}+^{48}\mathrm{Ca}$ (245 MeV) reactions. The cross section of the $^{239}\mathrm{Pu}(^{48}\mathrm{Ca},3n)^{284}\mathrm{Fl}$ 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 $^{244}\mathrm{Pu}+^{48}\mathrm{Ca}$ reaction. In the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ experiment, performed at 245 MeV in order to maximize the $3n$-evaporation channel, three decay chains of $^{285}\mathrm{Fl}$ were detected. The $\ensuremath{\alpha}$-decay energy of $^{285}\mathrm{Fl}$ was measured for the first time and decay properties of its descendants $^{281}\mathrm{Cn}, ^{277}\mathrm{Ds}, ^{273}\mathrm{Hs}, ^{269}\mathrm{Sg}$, and $^{265}\mathrm{Rf}$ were determined with higher accuracy. The assignment of SF events observed during the irradiation of the $^{240}\mathrm{Pu}$ target with a 250 MeV $^{48}\mathrm{Ca}$ beam to $^{284}\mathrm{Fl}$ decay is presented and discussed. The cross sections at both $^{48}\mathrm{Ca}$ energies are similar and exceed that observed in the reaction with the lighter isotope $^{239}\mathrm{Pu}$ 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 $N=184$.

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