Study of neutron-deficient isotopes of Fl in the ²³⁹Pu, ²⁴⁰Pu + ⁴⁸Ca reactions

The results of the experiments aimed at the synthesis of Fl isotopes in the ²³⁹Pu + ⁴⁸Ca and ²⁴⁰Pu + ⁴⁸Ca reactions are presented. The experiment was performed using the Dubna gas-filled recoil separator at the U400 cyclotron. In the ²³⁹Pu+⁴⁸Ca experiment one decay of spontaneously fissioning ²⁸⁴Fl was detected at 245-MeV beam energy. In the ²⁴⁰Pu+⁴⁸Ca experiment three decay chains of ²⁸⁵Fl were detected at 245 MeV and four decays were assigned to ²⁸⁴Fl at the higher ⁴⁸Ca beam energy of 250 MeV. The α-decay energy of ²⁸⁵Fl was measured for the first time and decay properties of its descendants ²⁸¹Cn, ²⁷⁷Ds, ²⁷³Hs, ²⁶⁹Sg, and ²⁶⁵Rf were determined more precisely. The cross section of the ²³⁹Pu(⁴⁸Ca,3n)²⁸⁴Fl reaction was observed to be about 20 times lower than those predicted by theoretical models and 50 times less than the value measured in the 244Pu+⁴⁸Ca reaction. The cross sections of the ²⁴⁰Pu(⁴⁸Ca,4-3n)²⁸⁴,²⁸⁵Fl at both ⁴⁸Ca energies are similar and exceed that observed in the reaction with lighter isotope ²³⁹Pu by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate rapid decrease of stability of superheavy nuclei with departing from the neutron number N=184 predicted to be the next magic number.