Yu. Ts. Oganessian

Synthesis of nuclei of the superheavy element 114 in reactions induced by 48Ca

The stability of heavy nuclides, which tend to decay by α-emission and spontaneous fission, is determined by the structural properties of nuclear matter. Nuclear binding energies and lifetimes increase markedly in the vicinity of closed shells of neutrons or protons (nucleons), corresponding to ‘magic’ numbers of nucleons; these give rise to the most stable (spherical) nuclear shapes in the ground state. For example, with a proton number of Z = 82 and a neutron number of N = 126, the nucleus 208Pb is ‘doubly-magic’ and also exceptionally stable. The next closed neutron shell is expected at N = 184, leading to the prediction of an ‘island of stability’ of superheavy nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2). The heaviest known nuclei have lifetimes of less than a millisecond, but nuclei near the top of the island of stability are predicted to exist for many years. (In contrast, nuclear matter consisting of about 300 nucleons with no shell structure would undergo fission within about 10−20 seconds.) Calculations indicate that nuclei with N > 168 should already benefit from the stabilizing influence of the closed shell at N = 184. Here we report the synthesis of an isotope containing 114 protons and 173 neutrons, through fusion of intense beams of 48Ca ions with 242Pu targets. The isotope decays by α-emission with a half-life of about five seconds, providing experimental confirmation of the island of stability.

Chemical characterization of element 112

The heaviest elements to have been chemically characterized are seaborgium (element 106), bohrium (element 107) and hassium (element 108). All three behave according to their respective positions in groups 6, 7 and 8 of the periodic table, which arranges elements according to their outermost electrons and hence their chemical properties. However, the chemical characterization results are not trivial: relativistic effects on the electronic structure of the heaviest elements can strongly influence chemical properties. The next heavy element targeted for chemical characterization is element 112; its closed-shell electronic structure with a filled outer s orbital suggests that it may be particularly susceptible to strong deviations from the chemical property trends expected within group 12. Indeed, first experiments concluded that element 112 does not behave like its lighter homologue mercury. However, the production and identification methods used cast doubt on the validity of this result. Here we report a more reliable chemical characterization of element 112, involving the production of two atoms of 283112 through the alpha decay of the short-lived 287114 (which itself forms in the nuclear fusion reaction of 48Ca with 242Pu) and the adsorption of the two atoms on a gold surface. By directly comparing the adsorption characteristics of 283112 to that of mercury and the noble gas radon, we find that element 112 is very volatile and, unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element 112 as a typical element of group 12, and its successful production unambiguously establishes the approach to the island of stability of superheavy elements through 48Ca-induced nuclear fusion reactions with actinides.

New insights from studies of spontaneous fission with large detector arrays

Abstract The new insights into a) the rapidly varying structures of neutron-rich nuclei up to spins as high as 20+ and 19−, b) the spontaneous fission process itself, and c) modes of cluster radioactivity that have come out of studies of spontaneous fission of 242Pu, 248Cm and 252Cf with large detector arrays are presented. The studies include γ-γ-γ coincidences with the second generation (14–20 Compton-suppressed Ge arrays) and new, third generation (35–45 Compton-suppressed Ge arrays, Gammasphere and Eurogam) gamma-ray detector arrays. The neutron-rich nuclei observed span the full range from superdeformed (β2 ≥ 0.4) double-magic ground states to spherical double-magic nuclei and competing shapes in between. New structure insights include the following: New regions of identical ground state bands to spins of 10+ to 16+ were discovered; one at the sudden onset of large deformation at N=60,62 in both even- and odd-A 98–101Sr, one at midshell N = 64,66 in 108,110Ru, one for N = 88–90 144,146Ba, one for N = 92–94 152,154Nd, and at high spin in 156Nd and also in excited bands in these regions. New shape coexisting structures in both even- and odd-A for A = 96–102 Sr and Zr nuclei are found. Evidence is found in both even-even and odd-A nuclei for a new region of octupole deformation around Z = 56 and N = 86–88. Rapid changes in moments of inertia are observed with changes in Z and N of two units with sudden changes in the moments of inertia associated with band crossings. The levels in many neutron-rich nuclei are observed for the first time. Earlier, plunger studies yielded lifetimes of low spin states and recently the Doppler Shift Line Shape Analysis yielded lifetimes of high spin states. For the first time, direct measurements of yields and neutron multiplicities have been made for five correlated pairs of fission fragments of SrNd, ZrCe, MoBa, RuXe and PdTe nuclei. Neutron multiplicities from 0–10 v emission (10v for the first time) were observed in MoBa correlated pairs with the 0 and 7–10 neutron-emission yields enhanced compared to gross yields for all fragments. The MoBa data provide evidence for two different fission modes. By unfolding the observed MoBa yields, the masses and excitation energies and mass distributions at scission were extracted. These data revealed a new mode involving the high neutron multiplicities that occurs essentially through one pair, 108Mo144Ba, 107Mo145Ba, or 106Mo146Ba or some combination where the 144Ba, 145Ba and/or 146Ba at scission are hyperdeformed with a long-to-short axis ratio of 3:1. The zero-neutron emission channels provide new examples of the cold rearrangements of nucleons in a new type of cluster radioactivity. The cluster radioactivity model predicted the observed enhancements of the zero-neutron channels for odd-A-odd-A nuclei as well as the observed fine structure, that is, the spin distributions of the two fragments. The measurements of intensities and γ-γ-γ coincidences in SF with large detector arrays open a new era in the determination of previously inaccessible properties of neutron-rich nuclei and the fission process.

Search for 28O and study of neutron-rich nuclei near the N = 20 shell closure

Abstract A search for 28 O with a 78 AMeV beam of the neutron-rich isotope 36 S has been performed for the first time. Evidence for the unbound character of 28 O was obtained. In the same experiment the half-lives of the very neutron-rich isotopes 27,29 F and 30 Ne were measured and those for 28,29 Ne and 30,31 Na reexamined. The results are compared to shell-model predictions and conclusions drawn regarding the extent of the region of deformation around N = 20.

Indication for a volatile element 114

Abstract Recently, the chemical investigation of element 112 revealed a highly volatile, noble metallic behaviour, as expected for the last group 12 member of the periodic table. The observed volatility and chemical inertness were ascribed to the growing influence of relativistic effects on the chemical properties of the heaviest elements with increasing nuclear charge. Here, we report for the first time on gas phase chemical experiments aiming at a determination of element 114 properties. This element was investigated using its isotopes 287114 and 288114 produced in the nuclear fusion reactions of 48Ca with 242Pu and 244Pu, respectively. Identification of three atoms of element 114 in thermochromatography experiments and their deposition pattern on a gold surface indicates that this element is at least as volatile as simultaneously investigated elements Hg, At, and element 112. This behaviour is rather unexpected for a typical metal of group 14.

Experiments on the production of fermium neutron-deficient isotopes and new possibilities of synthesizing elements with Z > 100

Abstract Experimental and theoretical data on the production of the neutron-deficient isotopes 244 Fm and 246 Fm in the bombardment of lead and bismuth isotopes with 40 Ar and 37 C1 ions are presented. By using different lead isotopes, the cross sections for the ( 40 Ar, x n) reactions with x = 1, 2, 3 and 4 have been measured. It is shown that when a target of the “magic” 208 Pb nucleus or its neighbours is bombarded with ions of mass

Synthesis of the heaviest elements in 48Ca-induced reactions

= 40 , the compound nuclei formed appear to be weakly excited and to de-excite by emitting only 2 or 3 neutrons. New possibilities of synthesizing elements with atomic number Z > 100 are being discussed in the framework of the experimental data obtained.

TWO-NEUTRON EXCHANGE OBSERVED IN THE 6HE + 4HE REACTION. SEARCH FOR THE DI-NEUTRON CONFIGURATION OF 6HE

Abstract The observation of atomic numbers Z that are 40% larger than that of Bi, the heaviest stable element, is an impressive extension of nuclear survival. Although the super heavy nuclei (SHN) are at the limits of Coulomb stability, shell stabilization lowers the ground-state energy, creates a fission barrier, and thereby enables the SHE to exist. The fundamentals of the modern theory concerning the mass limits of nuclear matter have been experimentally verified.

Experiments on the synthesis of neutron-deficient kurchatovium isotopes in reactions induced by 50Ti Ions

Differential cross sections for the elastic scattering of 6 He exotic nuclei from a gaseous helium target has been measured in a wide angular range in the CM system at a 6 He beam energy of 151 MeV. The large cross-sections obtained at backward angles are discussed in terms of a two-neutron exchange process. The results of DWBA calculations show that this effect can account for the cross sections obtained between 1208 and 1608 assuming the spectroscopic factor to be about 1 for the di-neutron cluster as was predicted by theory for 6 He. q 1998 Published by Elsevier Science B.V. All rights reserved.

New cold and ultra hot binary and cold ternary spontaneous fission modes for 252Cf and new band structures with gammasphere

Abstract By bombarding Pb isotopes with 50 Ti ions, two new isotopes of element 104 (kurchatovium) with mass numbers 255 and 256 have been synthesized. The isotope 256 Ku has been formed in the reaction 208 Pb ( 50 Ti, 2n) and has a spontaneous fission half-life of about 5 msec. The odd isotope 255 Ku has been formed with a maximum cross section in the reaction 207 Pb( 50 Ti, 2n) and has a half-life of several seconds. The experimental results obtained, together with the available data on the properties of kurchatovium, substantially change the understanding of the stability of heavy nuclei with respect to spontaneous fission. In this connection the influence of the structure of the fission barrier on the properties of heavy nuclei is discussed. On the basis of the experimental data on the synthesis of Fm and Ku neutron-deficient isotopes in reactions induced by ions heavier than argon, the possibilitie s of synthesizing elements with Z≧106 are investigated.