O. N. Malyshev

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.

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.

GABRIELA: A new detector array for γ-ray and conversion electron spectroscopy of transfermium elements

With the aid of the Geant4 Monte Carlo simulation package a new detection system has been designed for the focal plane of the recoil separator VASSILISSA situated at the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. GABRIELA (Gamma Alpha Beta Recoil Investigations with the ELectromagnetic Analyser VASSILISSA) has been optimised to detect the arrival of reaction products and their subsequent radioactive decays involving the emission of α- and β-particles, fission fragments, γ- and X-rays and conversion electrons. The new detector system is described and the results of the first commissioning experiments are presented.

Modernization of the detector system at the recoil separator Vassilissa

Abstract Since the past 10 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of experimental work in the region of the elements with 92⩽Z⩽94 , 14 new isotopes have been identified by the parent–daughter correlations. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z⩾110 ) with intensive 48 Ca extracted beams, improvements in the focal plane detector system have been made. As a result, energy and position resolutions of the detectors have been significantly improved, the detection efficiency for the α -particles, emitted from the implanted into the focal plane detector recoil nuclei, has been increased to 85%. The decay properties of the new isotope of element 112 and the isotope of the new element 114 with mass 287 have been measured in two-nuclei decays.

In-beam separation and mass determination of superheavy nuclei. Part I

Within the past 15 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in complete fusion reactions induced by heavy ions. The study of decay properties and formation of cross-sections of the isotopes of elements 110, 112 and 114 was performed using high-intensity Ca beams and Th; U; Pu targets. For further experiments aimed at the synthesis of the superheavy element isotopes ðZX110Þ with the use of intense Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made. The results from the test reactions and new results for the isotope 112 are presented. r 2003 Elsevier B.V. All rights reserved. PACS: 23.60.+e; 25.85.Ca; 29.30.Cm; 29.40.Pe

The upgrade of the kinematic separator VASSILISSA - Experimental results and plans

Within the past twelve years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of the experimental work, fourteen new isotopes have been identified by the parent-daughter correlations in the region of elements with 92≤Z≤94. The study of the decay properties and formation cross sections of the isotopes of elements 110, 112, and 114 was performed using high-intensity 48Ca beams and 232Th, 238U, and 242Pu targets. At the beam energies corresponding to the calculated cross-section maxima of the 3n evaporation channels, the isotopes 277110, 283112, and 287114 were produced and identified. For further experiments aimed at the synthesis of the superheavy element isotopes (Z≥110) with the use of intensive 48Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made.

Monte-Carlo optimization of the transmission of recoil separators

Abstract For the optimization of the transmission of recoil separators a Monte-Carlo computer code was developed. A special subroutine generates a set of “real” particles, which will be stored dynamically in the computers memory. The mean program varies the parameters of the focusing system of the separator in steps, calculates the transmission for each parameter set by transporting a large number of “particles” using the first-order ion optics and searches the best combination of the variables over the permitted ranges of values.

Decay widths of highly excited Ra compound nuclei

Abstract Excitation functions were obtained for the x n, p x n and αx n decay channels of 216,218,220 Ra compound nuclei produced in the reactions 22 Ne+ 194,196,198 Pt in the excitation energy range of 40–160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed cold products after the termination of evaporation cascades of a different numbers of stages. Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios. Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6–0.7 and the ratio of the level density parameters, a f a v = 1.0 ) reproduces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter a f a v , i.e. are very suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data — the absolute cross section values and cross section ratios — we were able to independently estimate the values of two principal parameters of the model, C and a f a y . Our calculations showed that a large part of the pre-fission neutrons is evaporated by hot pre-actinide compound nuclei before they reach the saddle point (50 to 100% of the total number of pre-fission neutrons are pre-saddle).

Cross sections of 102 element isotopes formation in the reactions of22Ne+236U and26Mg+232Th

The production cross sections of the isotopes252102,253102, and254102 were measured for the heavy ion fusion reactions of22Ne+236U and26Mg+232Th by using the kinematic separator VASSILISSA. The obtained excitation functions and the maximum production cross sections are compared with the ones for more asymmetric reactions leading to the same compound nucleus258102. The experimental cross sections and the results of statistical model calculations are compared and discussed.