Experimental studies of the 249Bk + 48Ca reaction including decay properties and excitation function for isotopes of element 117, and discovery of the new isotope 277Mt

Studies of superheavy nuclei produced in the ²⁴⁹Bk + ⁴⁸Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of ²⁹³117 and ²⁹⁴117 isotopes was measured at five excitation energies of the ²⁹⁷117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1_-0.6^+1.2 pb for the 3n and 2.4_-1.4^+3.3 pb for the 4n reaction channels. Alpha emission from ²⁸¹Rg competing with spontaneous fission (α/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope ²⁷⁷Mt (T_SF≈5 ms). The measured decay properties are in good agreement with those expected based on the properties of neighboring even-Z and odd-Z nuclei. The α energies and half-lives of odd-Z isotopes observed in the ²⁹³117 and ²⁹⁴117 decay chains together with results obtained for lower-Z superheavy nuclei demonstrate enhanced stability with increasing neutron number toward the predicted new magic number N=184.