The discovery of element 117 (INVITED)

In November 2016, four new chemical elements, nihonium (Nh, Z=113), moscovium (Mc, Z=115), tennessine (Ts, Z=117), and oganesson (Og, Z=118) joined the periodic table. Elements 117 and 118 are the highest atomic numbers Z reached to date, and the A=294 isotopes of these elements are the heaviest nuclei ever synthesized. The existence of these elements, together with a consistent trend of increasing lifetimes for superheavy isotopes as neutron number N increases, provide strong evidence for the long sought 'island of stability' for superheavy nuclei as the nuclear shell closure at N=184 is approached. Mc, Ts, and Og were discovered using the hot fusion process, bombarding actinide targets with intense beams of doubly-magic ⁴⁸Ca ions at the Dubna Gas-Filled Recoil Separator at the Joint Institute for Nuclear Research in Russia. For element 117, a berkelium target from Oak Ridge National Laboratory (ORNL) was required. The berkelium was produced by intense neutron irradiation at ORNL's High Flux Isotope Reactor and chemically separated at the adjoining Radiochemical Engineering Development Center. The ²⁴⁹Bk + ⁴⁸Ca reaction produced two isotopes of element 117 with atomic weights A=293 and 294. These isotopes decayed into ten additional heaviest isotopes of elements 115, 113, 111, 109, 107, and 105, all closer to the predicted island of stability than previously achieved. The discovery of element 117 will be described together with results of other recent experiments in superheavy element research in the context of the critical importance of actinide target materials, implications for the periodic table and the island of stability, and opportunities to synthesize even heavier nuclei, including heavy isotopes of element 118 and new elements 119 and 120. (author)