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 Bk-249 + Ca-48 reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of (293)117 and (294)117 isotopes was measured at five excitation energies of the (297)117 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 (281)Rg competing with spontaneous fission (alpha/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope (277)Mt (T-SF approximate to 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 alpha energies and half-lives of odd-Z isotopes observed in the (293)117 and (294)117 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.