Yuri Oganessian

Heavy Ions and Nuclear Fission

The discovery of neutron-induced fission of uranium nuclei in the experiments performed by Hahn and Strassmann (Ha 39, Me 39) has signified an observation of nuclear transmutations of a radically new type—the processes of large-scale rearrangement of finite amounts of nuclear matter. As a matter of fact, the fission is a result of an irreversible deformation, that grows in time, in the course of which an original, nearly spherical heavy nucleus undergoes drastic changes in its topology and is transformed into two fragments of comparable, yet generally unequal, mass. The time evolution of a fissioning system is accompanied by multiple redistribution of available energy between various degrees of freedom, and culminates in the release of a vast amount of energy in the form of kinetic energy of the fission fragments and energy (mass) of accompanying radiation, i.e., prompt neutrons and γ rays. To initiate such radical rearrangement, an extremely weak external perturbation proved to suffice—the capture of a slow neutron causing only tiny changes in the total number of nucleons as well as in the total energy of the original nucleus.

Nuclear physics. Sizing up the heavyweights.

Colliding a heavy projectile with an even heavier target nucleus only occasionally produces superheavy elements. Analyses of the processes that prevent fusion suggest that projectile size is one of the problems.

Synthesis and decay properties of the heaviest nuclei

The formation and decay properties of the heaviest nuclei with Z=112–116 and 118 were studied in the reactions 238U, 242,244Pu, 243Am, 245,248Cm and 249Cf + 48Ca. The new nuclides mainly undergo sequential α-decay, which ends with spontaneous fission. The total time of decay ranges from 0.5 ms to ~1 day, 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 and prospects of superheavy element research are also presented.The experiments were carried out at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna) in collaboration with the Analytical and Nuclear Chemistry Division of the Lawrence Livermore National Laboratory (USA).

At the End of the Nuclear Map

Theoretical predictions about appearances of new shells and the “islands of stability” close to the nuclear mass limits have been recently confirmed experimentally. The talk is devoted to the synthesis and study of the decay properties of heaviest elements. Further progress is directly associated with the development of heavy ion accelerator facilities, new setups and more sophisticated detectors. New experimental potentialities are presented also.

Synthesis and Properties of Heaviest Nuclei with Z = 114 - 116 in 48Ca Induced Reactions

The paper presents part of the results on the synthesis of superheavy nuciides with Z = 114 and 116 in the fusion reactions with 4 8 Ca ions obtained in 2000-2001. In the irradiation of targets made from enriched 2 4 4 Puand 2 4 5 Cm isotopes with beam doses of 1.5 . 10 1 9 and 2.3 . 10 1 9 , respectively, the detector array situated in the focal plane of the gas-filled separator registered heavy atoms of new elements undergoing sequential α-decays terminated by spontaneous fission. The time of the decay chains is approximately one min. Decay properties of the synthesized nuclei are consistent the consecutive α-decays originating from the parent nuclides 2 8 8 114 and 2 9 6 116 produced in the 4n-evaporation channels with the cross section of about 0.5 picobarn. Comparison of T S F and T α values for the nuclei with Z = 110 and 112 with those obtained earlier for more light isotopes of these elements points to an enhanced stability of heavy nuclei with an increase in the neutron number. The α-decay energies Q a measured experimentally in the chains 116 - α 1 - 114 - α 2 - 112 - α 3 - 110 are compared with theoretical predictions of different nuclear models. This comparison shows that the difference between the experiment and theory is in the range of ′0.5 MeV. From this it follows that the theoretical models predicting the decisive influence of the nuclear structure on the stability of superheavy elements are well-founded not only qualitatively but in some sense also quantitatively. The prospects of further investigations in the field of superheavy nuclei are discussed briefly.

Synthesis and study of properties of superheavy atoms. Factory of Superheavy Elements

The state-of-the-art of studies dealing with the synthesis and properties of new superheavy elements is scrutinized. The experiments intended to identify and study the chemical properties of superheavy elements, as well as the facilities used for this purpose, are described. Special attention focuses on the theoretical assessment of the relativistic effects on the properties of these elements. The prospects of these investigations have been considered in the light of the creation of the worlds first Factory of Superheavy Elements in Russia.The bibliography includes 48 references.

Heaviest elements (synthesis and decay properties)

For the 60 years that have passed after the discovery of the first artificial elements Np and Pu, the investigations of the properties of new elements have become one of the fundamental and quickly developing fields of 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). The theoretical description of the masses and fission barriers of the new nuclei led in the mid-1960s 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 the enhanced stability of nuclei, close to the deformed shells Z=108 and N=162, relative to different decay modes and also the reactions of their synthesis are discussed from the point of view of advancing into an u...

Conference on low energy nuclear dynamics

Abstract The 15th EPS Nuclear Physics Divisional Conference on Low Energy Nuclear Dynamics (LEND-95) was held in St. Petersburg (Russia) from April 18 to 22, 1995. It was organized by the European Physical Society, the Russian Academy of Sciences, the Joint Institute for Nuclear Research (Dubna), the V.G. Khlopin Radium Institute (St. Petersburg) and Hahn-Meitner-Institute (Berlin). The conference was sponsored by the European Commission DGXII for Science, R&D, the Russian Foundation for Fundamental Research and the Joint Institute for Nuclear Research (Dubna). Approximately 150 scientists from 19 countries (including the U.S., Japan and Mexico) and from the Joint Institute for Nuclear Research participated in LEND-95.