The discovery of a new chemical element with atomic number Z=117 is reported. The isotopes ²⁹³117 and ²⁹⁴117 were produced in fusion reactions between ⁴⁸Ca and ²⁴⁹Bk. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with Z≥111, validating the concept of the long sought island of enhanced stability for superheavy nuclei.

Synthesis of a New Element with Atomic Number Z=117, Oganessian, Yu. Ts., Abdullin Sh. F., Bailey P. D., Benker D. E., Bennett M. E., Dmitriev S. N., Ezold J. G., Hamilton J. H., Henderson R. A., Itkis M. G., et al. , Phys. Rev. Lett., Apr/2010, Volume 104, Number 14, p.142502, (2010)

Abstract: A fundamental outcome of modern nuclear theory is the prediction of the "island of stability" in the region of hypothetical superheavy elements. A significant enhancement in nuclear stability at approaching the closed shells with Z = 114 (possibly 120 and 122) and N = 184 is expected for the nuclei with large neutron excess. For this reason, for the synthesis of nuclei with Z = 112-116 and 118, we chose the reactions ²³⁸U, ^242,244Pu, ²⁴³Am, ^245,248Cm, and ²⁴⁹Cf + ⁴⁸Ca, which are characterized by fusion products with a maximal neutron excess. The formation and decay properties of the heaviest nuclei were registered with the use of a gas-filled recoil separator installed at a 48Ca-beam of the heavy-ion cyclotron. The new nuclides mainly undergo sequential α-decay, which ends with spontaneous fission (SF). The total time of decay ranges from 0.5 ms to ~1 d, depending on the proton and neutron numbers in the synthesized nuclei. The atomic number of the new elements 115 and 113 was confirmed also by an independent radiochemical experiment based on the identification of the neutron-rich isotope ²⁶⁸Db (TSF ~ 30 h), the final product in the chain of α-decays of the odd-odd parent nucleus 288115. The comparison of the decay properties of 29 new nuclides with Z = 104-118 and N = 162-177 gives evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay. The investigations connected with the search for superheavy elements in Nature are also presented.

Synthesis and decay properties of superheavy elements, Oganessian, Yuri , Pure and Applied Chemistry, 2006, Volume 78, Issue 5, p.889 - 904, (2006)

Synthesis of the isotopes of elements 118 and 116 in the Cf249 and Cm245+Ca48 fusion reactions, Oganessian, Yu., Utyonkov V., Lobanov Yu., Abdullin F., Polyakov A., Sagaidak R., Shirokovsky I., Tsyganov Yu., Voinov A., Gulbekian G., et al. , Physical Review C, 10/2006, Volume 74, Issue 4, (2006)

Independent verification of the production of element 114 in the reaction of 244-MeV ⁴⁸Ca with ²⁴²Pu is presented. Two chains of time- and position-correlated decays have been assigned to ²⁸⁶114 and ²⁸⁷114. The observed decay modes, half-lives, and decay energies agree with published results. The measured cross sections at a center-of-target energy of 244 MeV for the ²⁴²Pu(⁴⁸Ca,3–4n)²⁸⁷,²⁸⁶114 reactions were 1.4(+3.2, -1.2) pb each, which are lower than the reported values.

Independent Verification of Element 114 Production in the Ca-48 + Pu-242 Reaction, Stavsetra, L., Gregorich K. E., Dvorak J., Ellison P. A., Dragojević I., Garcia M. A., and Nitsche H. , Physical Review Letters, Volume 103, Number 13, p.132502, (2009)