Synthesis Of New Superheavy Elements Research Articles

Influence of «FLAT–TOP» of the resonant system of the DC-280 accelerator on the beam of charged particles

Experiments on the synthesis of superheavy elements (SHE) which were done at Flerov Laboratory of Nuclear Reactions (FLNR) measure that the probability of occurrence of one SHE event is ranged from one event per month to one event per week. Nowadays, Z = 118 (Oganesson) is the most used chemical element which is synthesized with 48Ca. The synthesis of new SHEs requires heavier bombarding particles: 50Ti, 54Cr, 58Fe, etc. In the experiments with heavier particles, the SHE isotope production cross section is expected to be lower than in the case of using 48Ca. It follows that it is necessary to increase the intensity of bombarding ion beams in order to collect experimental statistics in a reasonable time. Thus, for the synthesis of superheavy elements in the FLNR at the Joint Institute for Nuclear Research (JINR), the Factory of Superheavy Elements was created. The basic equipment of the SHE Factory is the DC-280 cyclotron complex. The accelerator provides high intensity of ion beams. As the intensity in the beam increases, the energy spread increases. That to minimize this process, the DC-280 cyclotron is equipped with an additional FLAT–TOP accelerating system.

Examination of promising reactions with Am241 and Cm244 targets for the synthesis of new superheavy elements within the dinuclear system model with a dynamical potential energy surface

Two actinide isotopes, $^{241}$Am and $^{244}$Cm, produced and chemically purified by the HFIR/REDC complex at ORNL are candidates for target materials of heavy-ion fusion reaction experiments for the synthesis of new superheavy elements (SHEs) with $Z>118$. In the framework of the dinuclear system model with a dynamical potential energy surface (DNS-DyPES model), we systematically study the $^{48}$Ca-induced reactions that have been applied to synthesize SHEs with $Z=112$--118, as well as the hot-fusion reactions with $^{241}$Am and $^{244}$Cm as targets which are promising for synthesizing new SHEs with $Z=119$--122. Detailed results including the maximal evaporation residue cross section and the optimal incident energy for each reaction are presented and discussed.

Research program of superheavy elements and nuclides based on HIAF

Atomic nuclei are many-body quantum systems composed of protons and neutrons. The nuclides, whose existence is attributed to the nuclear shell effects, are referred to as superheavy nuclides. The exploration of the limit to the existence of nuclides with regard to the proton and mass numbers is at the frontier of nuclear physics research. So far, superheavy elements with an atomic number of up to 118 and hundreds of superheavy nuclides have been experimentally synthesized and identified in various laboratories, resulting in the completion of the seventh period of the periodic table of elements. The High Intensity heavy-ion Accelerator Facility (HIAF) is now under construction and is scheduled to be commissioned in 2025. The study on superheavy elements and nuclei is one of the main research goals of HIAF. Intense heavy-ion beams can be extracted from around the middle point of linac, and the beam energies can be finely adjusted around the Coulomb barriers of nuclear reactions. Therefore, HIAF will provide great opportunities for the synthesis of new superheavy elements and to explore the superheavy island of stability, which was theoretically predicted long ago via fusion and multi-nucleon transfer reactions, respectively. This paper presents the research program of superheavy elements and nuclides based on HIAF, provides an overview of the research on superheavy nuclei, and describes the electromagnetic spectrometers and experimental measurement systems associated with fusion and multi-nucleon transfer reactions. The HIAF physics goals, including the synthesis of new superheavy elements and nuclides, the study on the nuclear structure of superheavy nuclei, the reaction mechanism of producing superheavy nuclides, and the atomic structure and chemical properties of the heaviest elements, are illustrated in detail. Lastly, the future research plan for superheavy elements and nuclides is summarized.

Zeptosecond contact times for element Z=120 synthesis

The synthesis of new superheavy elements beyond oganesson (Z=118) requires fusion reactions with projectile nuclei with proton numbers larger than that of 48Ca (Z=20), which has been successfully employed for the synthesis of elements with Z=112-118. In such reactions, fusion is drastically hindered by fast non-equilibrated dynamical processes. Attempts to produce nuclei with Z=120 using the 64Ni+238U, 58Fe+244Pu, 54Cr+248Cm, and 50Ti+249Cf reactions have been made, which all result in larger Coulomb forces than for 48Ca-induced reactions, but no discovery has been confirmed to date. In this work, mass and angle distributions of fission fragments from these reactions have been measured with large angular coverage to aid in selection of the most promising projectile-target combination that would favor fusion. The results yield information on reaction contact times, with the longest exhibited by 50Ti+249Cf.

High intensity vanadium beam for synthesis of new superheavy elements with well-controlled emittance by using "slit triplet".

To provide a very powerful vanadium (V) beam with an intensity of at least 6 particle μA for synthesizing a new superheavy element (SHE) with atomic number Z = 119, we have developed a high-temperature oven (HTO) system to evaporate the metallic V powder inside the new superconducting (SC) electron cyclotron ion source. We successfully extracted a V13+ beam with a maximum beam intensity of 600 eμA with 2.8-kW microwave power and 900-W heating power of the HTO. Furthermore, from a systematic study of the dependence of the beam intensity on the microwave power and the HTO power, we successfully produced a V13+ beam of 300 eμA at a consumption rate of 3 mg/h, allowing a one-month duration continuous beam to carry out the SHE synthesis. In addition, to avoid serious damage to newly introduced SC acceleration cavities by beam losses, the beam should be transported with a well-controlled emittance. To efficiently limit the beam emittance, we employed a slit triplet consisting of three pairs of slits installed around the focus point of the low-energy beam transport. The first result of the emittance reduction was observed by a pepper-pot type emittance meter as a function of the acceptance of the slit triplet.

Systematics of production cross sections and predictions for the synthesis of new superheavy elements

A systematic analysis of hot fusion reactions with a comparison to experimental cross sections related to the synthesis of superheavy elements is carried out using the hivap code. A single parameter set is found which when implemented in the hivap computer code, describes experimental production cross sections for reactions using a $^{48}\mathrm{Ca}$ beam on actinide targets reasonably well. A simple scaling based on the mass asymmetry of the projectile-target system is introduced for projectiles lighter than $^{48}\mathrm{Ca}$. It is seen that only three parameters scaled by a constant factor are required to describe cross-section data obtained using lighter projectiles. The projectile dependence on the production cross section is also studied. From these investigations, new reactions with $^{48}\mathrm{Ca}$ incident on various actinide targets are proposed for the production of unknown heavier isotopes of $Z=116$ and 118 and for the synthesis of new superheavy elements with $Z=119$ and 120.

Systematic research on [formula omitted]-decay rates of spherical and deformed nuclei

α decay is of significant interest for the study of nuclear structure and for the synthesis of new superheavy elements. Theoretical approaches in recent α-decay studies are reviewed. After a brief review of the systematic laws of α-decay half-lives, various expressions for the decay width are discussed together with the α preformation factor. These expressions are separately derived within the semiclassical approximation, direct method, distorted wave approach, (modified) two-potential approach, and coupled-channel approach. Within the coupled-channel representation, two techniques to deal with the coupling interaction matrix are discussed for precise descriptions of the α-decay fine structure. Various α-decay calculations within all these different approaches are presented and compared. It is found that both semiclassical and quantum-mechanical calculations well reproduce the experimental α-decay half-lives within a mean factor of about two. However, the semiclassical formalism fails in describing the α transitions to high-lying members of one rotational band, while the coupled-channel approach including enough decay channels gives a precise description of the α-decay fine structure observed in heavy deformed nuclei.

High-power radioactive targets as one of the key problems in further development of the research program on synthesis of new superheavy elements

Over the past decade, numerous isotopes of superheavy elements with Z = 112–118 have been synthesized. Production cross sections for these species are in the range of a few picobarns (1 pb = 10−36 cm2) or less. To get access to heavier nuclides and carry out a detailed study on their properties, a sufficient increase of the overall sensitivity of experiments is needed. The direct solution consist in increasing of the beam intensity and development of separators providing the necessary background suppression. Therefore, the design of high power radioactive targets becomes a key problem. In this report, we consider specific requirements to the development of targets for the synthesis of superheavy elements.

Synthesis of new superheavy elements using the Dubna Gas-Filled Separator: The complex of technologies

All the new isotopes of superheavy elements in the reactions with 48Ca ions and actinide targets, synthesized at the Flerov Laboratory of Nuclear Reactions (FLNR), have been obtained at the Dubna Gas-Filled Recoil Separator (DGFRS). Success was achieved with the use of several techniques, algorithms and approaches with record performance in their area. The most sophisticated of them are rare event detection technology, methods of measuring the effective equilibrium charges of heavy ions and nuclei in hydrogen at low pressure, modified approaches to evaluation of probabilistic characteristics of detected multichain events, beam-free test methods of detection equipment, and some other ones. The author analyzes the approaches that have led ultimately to a real breakthrough and an advance in experiments on the synthesis of superheavy elements. The development of detection systems also attracts the authors’ attention. Contributions from some facilities—such as SHIP, TASCA, and GARIS (RIKEN)—to synthesis experiments are partially noted. Under consideration are methods of automation of similar experiments, including the development of control and shielding systems for operation with highly active actinide targets. Some techniques for detecting α decay of heavy nuclei that came before the current experiments with electromagnetic separators are reviewed briefly. The methods, algorithms and hardware presented cover a more than twenty-year period of commissioning and application in long-term experiments at FLNR DGFRS.

Main restrictions in the synthesis of new superheavy elements: Quasifission and/or fusion fission

The synthesis of superheavy elements stimulates the effort to study the peculiarities of the complete fusion with massive nuclei and to improve theoretical models in order to extract knowledge about reaction mechanism in heavy ion collisions at low energies. We compare the theoretical results of the compound nucleus (CN) formation and evaporation residue (ER) cross sections obtained for the $^{48}$Ca+$^{248}$Cm and $^{58}$Fe+$^{232}$Th reactions leading to the formation of the isotopes A=296 and A=290, respectively, of the new superheavy element Lv (Z=116). The ER cross sections, which can be measured directly, are determined by the complete fusion and survival probabilities of the heated and rotating compound nucleus. That probabilities can not be measured unambiguously but the knowledge about them is important to study the formation mechanism of the observed products. For this aim, the $^{48}$Ca+$^{249}$Cf and $^{64}$Ni+$^{232}$Th reactions have been considered too. The use of the mass values of superheavy nuclei calculated in the framework of the macroscopic-microscopic model by Warsaw group leads to smaller ER cross section for all of the reactions (excluding the $^{64}$Ni+$^{232}$Th reaction) in comparison with the case of using the masses calculated by Peter M\"oller {\it et al}.

Preparation of 48Ca and its use in experiments on synthesis of new superheavy elements

A procedure was developed for preparing metallic 48Ca in a high yield. The starting 48CaCO3 was first calcined to 48CaO and then mixed with Al powder. The charge was fired at 1200–1300°C in a specially designed reactor. The vapor of the released 48Ca was condensed in the reactor zone cooled to room temperature. The process was monitored with 47Ca tracer. Much attention was given to recovery of expensive 48Ca from products of the aluminothermic process. The metallic 48Ca obtained was used for generating a beam of 48Ca5+ ions on an accelerator complex of FLNR JINR in experiments on synthesis of new superheavy elements.

Peculiarities of Nuclear Fusion in Synthesis of Superheavy Elements

The small probabilities of synthesis of new superheavy elements at GSI (Darmstadt, Germany), Joint Institute for Nuclear Research (Dubna, Russia), and RIKEN (Wako, Japan) during the last decade stimulate the experimental and theoretical studies of the nuclear reaction mechanism. 1– 4 In preparation of these experiments, the main aim is to reach maximum cross sections of the yield of evaporation residues (ER) as a result of the de-excitation of the heated compound nucleus which is formed in complete fusion of the projectile and target nuclei. Because the ER excitation function in the synthesis of superheavy elements has very narrow width for “cold fusion” reactions (5–10 MeV) with 208 Pb and 209 Bi targets 5 and the width of the “hot fusion” reactions with 48 Ca projectile on actinide tar

Treatment of competition between complete fusion and quasifission in collisions of heavy nuclei

A model of competition between complete fusion and quasifission channels in fusion of two massive nuclei is extended to include the influence of dissipative effects on the dynamics of nuclear fusion. By using the multidimensional Kramers-type stationary solution of the Fokker-Planck equation, the fusion rate through the inner fusion barrier in mass asymmetry is studied. Fusion probabilities in symmetric 90Zr+ 90Zr, 100Mo+ 100Mo, 110Pd+ 110Pd, 136Xe+ 136Xe, almost symmetric 86Kr+ 136Xe and 110Pd+ 136Xe reactions are calculated. An estimation of the fusion probabilities is given for asymmetrical 62Ni+ 208Pb, 70Zn+ 208Pb, 82Se+ 208Pb, and 48Ca+ 244Pu reactions used for the synthesis of new superheavy elements.