Towards the “islands of stability” of superheavy elements

Abstract

For the 60 yr that have passed since the discovery of the first artificial elements Np and Pu investigations of the properties of new elements have become one of the most quickly developing fields of fundamental nuclear physics and nuclear chemistry. The transition from the traditional method of producing transuranium elements, where continuous and pulsed neutron fluxes have been used, to nuclear reactions induced by heavy ions has made it possible to synthesize 12 new elements heavier than fermium ( Z=100). In the mid-1960s, the theoretical description of the masses and fission barriers of the new nuclei led to the prediction of “islands of stability” for the very heavy and superheavy nuclides in the vicinity of the closed proton and neutron shells. The experimental data that demonstrate enhanced stability of nuclei, relative to different decay modes, close to the deformed shells Z=108 and N=162, and also the reactions for their synthesis are discussed from the point of view of advancing into the unexplored region of heavier (superheavy) and significantly longer-lived nuclides, situated close to the spherical shells Z=114 and N=184. First results on the synthesis of superheavy nuclei in 48Ca-induced reactions are presented. The observed decay chains of individual atoms consisting of sequential α-decays and terminated by spontaneous fission, as well as the energies and half-lives, are in agreement with the predictions of theoretical models describing the structure of heavy nuclei. They are considered as a first evidence of the existence of the hypothetical region of stability of superheavy elements. The experiments were carried out at the FLNR heavy ion accelerator in the framework of a large collaboration with LLNL (Livermore), GSI (Darmstadt), RIKEN (Saitama), the Institute of Physics and Department of Physics of the Comenius University (Bratislava) and the Department of Physics of the University in Messina.