Superheavy Elements Research Articles

DGFRS-2—A gas-filled recoil separator for the Dubna Super Heavy Element Factory

The main goal of development of the new Dubna Gas-Filled Recoil Separator (DGFRS-2) is to sufficiently improve the efficiency of studies on heavy and superheavy nuclei at the Super Heavy Element Factory of the Flerov Laboratory of Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research. The use of beams with the intensity up to 6×1013 s−1 (10 pμA) delivered by the DC280 cyclotron requires an effective setup providing high suppression of background reaction products. The described gas-filled separator is optimized for synthesis and study of heavy isotopes produced in complete fusion reactions of massive nuclei. Basic characteristics of DGFRS-2, as well as the results of the first test experiments, are presented. In comparison to the DGFRS-1, the transmission efficiency was doubled, and the background was reduced by a factor 200.

Superconducting Linac Booster for Super-Heavy Element Experiments at RIKEN Radioactive Isotope Beam Factory

The Rikagaku Kenkyūjo (RIKEN) heavy-ion linac was upgraded by introducing a new super-conducting linac-booster to advance the super-heavy elements synthesis program beyond nihonium at the RIKEN Radioactive Isotope Beam Factory (RIBF). The total acceleration voltage was upgraded from 25 MV with 12 room temperature drift-tube-linacs (DTLs) to 39 MV by introducing a superconducting linac booster, SRILAC. The upgrade of the beam intensity is realized by a newly constructed superconducting electron-cyclotron resonance ion source (SC-ECRIS). The construction of SRILAC and the SC-ECRIS started in FY2017. After the hardware installation and commissioning, the first beam acceleration test was successfully conducted at the end of FY2019, and user beam service was started in FY2020. This article provides an overview of the upgrade and its present performance.

A few ECRIS developments at LBNL

In addition to the ongoing investigation of bremsstrahlung radiation produced by energetic electrons in the superconducting Electron Cyclotron Resonance (ECR) ion source VENUS, the LBNL ion source group has pursued several other research topics since 2015 to explore potential advancements of ECR ion sources. Three of these activities are investigations into the benefits of non-cylindrical plasma chambers, the development of new high-temperature ovens that will efficiently produce the intense, metallic ion beams necessary for super heavy element production at the 88-Inch Cyclotron facility, and the finalization of the conceptual and engineering design of the superconducting NbTi magnet structure for the 4th generation, 45 GHz ECR ion source MARS-D. This article summarizes and briefly discusses the highlights of these ECRIS developments.

New Coulomb barrier scaling law with reference to the synthesis of superheavy elements

The synthesis of superheavy elements based on fusion reactions and multinucleon transfer reactions is sensitive to the reaction energies, where the Coulomb barrier plays a crucial role as it must be overcome for the projectile and target to contact each other. The Coulomb barrier cannot be measured directly, and the synthesis of superheavy elements is sensitive to it. In this study, we systematically extract the barrier information from the experimental fusion excitation functions by the empirical cross section formula, which is based on a single-Gaussian distribution of fusion barrier heights. A total of 403 sets of experimental data are fitted, among which 243 sets have good results with ${\ensuremath{\chi}}^{2}/pt$ less than one. The extracted fusion barrier is a dynamical barrier, which includes the overall effect of coupled channels. Different from the prediction of the WKJ formula, the new scaling law proposed in this work is almost identical to the CW76 Coulomb barrier at the $z=170--300$ region and reproduces well the centroid barrier extracted from quasielastic scattering. The comparison to the other 14 bare potential models and exploration of the very large $z$ region is also discussed. It is also remarkable to find that the predictions of the DP2015 potential, including of the contributions of the macroscopic and shell correction terms, are highly consistent with the extracted results in this work and the CW76 potential. The numerical results demonstrated that the dynamical fusion barrier and the centroid barrier extracted from the quasielastic reaction (the reaction threshold) could be the same physical quantity for superheavy reactions. This study could provide important references for the synthesis of superheavy nuclei based on fusion reactions and very heavy multinucleon transfer reactions.

Alpha and cluster decays of superheavy elements and 2p radioactivity of medium nuclei

The alpha decay, the cluster radioactivity and the heavy particle emission half-lives of known and some still unknown superheavy nuclei have been investigated within the original Generalized Liquid Drop Model and analytic formulas. The Q value has been calculated mainly from the recent NUBASE2020 tables. The agreement between the experimental data and the theoretical predictions of the alpha decay half-lives of the superheavy nuclei is correct and some predictions are provided for unknown nuclei. For some emissions of heavy particles by superheavy nuclei the half-life is comparable to the alpha decay half-lives or even smaller. The 76−80Zn, 78Ga, 72,74−76Cu, 69,71Ni and 47K nuclei seem to be the best probable candidates at an emission by superheavy nuclei, their daughter nuclei being the 205,206Hg, 206,207Tl, 208Pb, 209,210Bi, 211Po and 210At nuclei. The GLDM allows to reproduce roughly the very sparse data on 2p radioactivity.

Investigations on 48Ca and 58Fe-induced heavy ion fusion reactions

In this paper, we have studied all possible [Formula: see text] and [Formula: see text]-induced fusion reactions for the synthesis of superheavy nuclei [Formula: see text]. The semi-empirical formula for fusion barriers of [Formula: see text] and [Formula: see text] is constructed. Fusion and evaporation residue cross-sections are studied using advance statistical model (ASM). The fusion and evaporation residue cross-sections of this work are compared with that of experiments. We have also predicted suitable targets and production cross-sections for the synthesis of superheavy elements using the [Formula: see text] and [Formula: see text]-induced fusion reactions. The [Formula: see text]-induced fusion reactions can be used to synthesize superheavy elements [Formula: see text] and 124 using the targets [Formula: see text]Cm, [Formula: see text]Bk and [Formula: see text]Cf have half-lives greater than one year and produce the cross-sections in terms of pb.

Post-irradiation characterization of a high burnup mixed oxide fuel rod with minor actinides

The Advanced Fuels Campaign performed a series of irradiation tests of minor actinide-bearing mixed oxide fuel (MA-MOX), the so-called AFC-2C&D experiments, to investigate the transmutation of long-lived transuranic actinide isotopes contained in spent nuclear fuel via fast reactor technology at burnups exceeding 10 % fission of initial metallic atoms. This manuscript reports the test results derived from one of the five MA-MOX rodlets taken to higher burnup in the AFC-2D irradiation. This includes both non-destructive investigations, such as gamma and neutron spectrometry, and destructive investigations, such as fission gas release, ceramography, and chemical burnup analysis. In addition, the microstructure of the fuel was investigated using advanced electron microscopy techniques including electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was observed with EBSD that the pellet had subdivision of the grains and the TEM observed migration of cladding material into the 5 metal precipitates in the fuel which could have been from the higher than desired oxygen/metal ratio. The TEM also showed an enrichment of Cr in fuel clad chemical interaction (FCCI) layer.

Quasifission in Kr84,86 -induced reactions populating superheavy elements

Background: Quasifission plays a detrimental role in the synthesis of superheavy elements. Understanding the dynamics of quasifission helps in selecting the optimum target projectile combinations for the synthesis of new superheavy elements.Purpose: The influence of various entrance channel parameters on the quasifission dynamics was explored. Three reactions, $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$, $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$, and $^{86}\mathrm{Kr}+^{197}\mathrm{Au}$, expected to be dominated by quasifission, were studied near the Coulomb barrier energies.Methods: The binary fission fragments from the three reactions were detected using the double arm time-of-flight spectrometer CORSET. The mass and total kinetic energy distributions, which are sensitive to the quasifission dynamics, were measured and analyzed for the three reactions.Results: The mass and total kinetic energy distributions of all the three reactions have the characteristics that show the dominance of quasifission. For the reaction $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$, symmetric quasifission has been found to be relatively more dominant compared to $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$ reaction. The results have been compared with $^{48}\mathrm{Ca}$- and $^{52}\mathrm{Cr}$-induced reactions, populating nearby nuclei, to understand the evolution of quasifission. An analysis of the timescales in these reactions using a dynamical calculation shows that $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$ takes a longer time to reach a particular dinuclear shape compared to the other two reactions. The driving potential at the barrier radius also shows a shallower pocket for $^{84}\mathrm{Kr}+^{198}\mathrm{Pt}$ compared to $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$ and $^{86}\mathrm{Kr}+^{197}\mathrm{Au}$ reactions.Conclusion: The entrance channel isospin difference is found to influence the quasifission dynamics. A higher entrance channel isospin difference may have a larger evolution time, thus driving the systems toward a more symmetric mass split.

Conceptualized Transmutation Reactor based on Cold Fusion Mechanism (Possibility of Transmutation to Superheavy Element by Cold Fusion Mechanism)

Conceptualized Transmutation Reactor based on Cold Fusion Mechanism (Possibility of Transmutation to Superheavy Element by Cold Fusion Mechanism)

Chemical conversions in lead thin films induced by heavy-ion beams at Coulomb barrier energies

Superheavy elements are produced via fusion reactions, by bombarding thin target foils with intense beams of energetic ions with energies around the Coulomb barrier. Currently, production of targets of rare isotopes relies mainly on the molecular plating (MP) method, which yields thin films of the desired target material on a backing foil. Intense heavy-ion beams are known to modify the film structure, which leads to deterioration of the targets over irradiation time. To gain a better understanding of the processes at a microscopic scale, comparative tests were carried out using Coulomb barrier heavy-ion beams provided from the UNILAC accelerator at GSI Darmstadt, Germany. For this purpose, targets of lead were prepared using MP and Drop-on-Demand inkjet-printing. Lead targets were baked-in and analysed by digital optical microscopy, scanning electron microscopy and confocal Raman spectroscopy.

Analysis of the fusion mechanism in the synthesis of superheavy element 119 via the Cr54+Am243 reaction

The combined dinuclear system (DNS) and statistical model implanted in KEWPIE2 have been used to study the prospects for the synthesis of a superheavy element (SHE) with $Z=119$ in the $^{54}$Cr+$^{243}$Am fusion reaction. The method of calculation has been verified by description of the evaporation residue cross sections measured for the $^{48}$Ca+$^{243}$Am reaction. The calculated results of the partial and total cross sections for the complete fusion, quasifission, fast fission and evaporation residues formation for both reactions are discussed.

Possibility to form Z=120 via the Ni64+U238 reaction using the dynamical cluster-decay model

According to different theoretical studies, the next magicity for the proton number should occur at $Z=114$, 120, or 126 and for the neutron number $N=184$. Superheavy nuclei of interest for the forthcoming synthesis of the isotopes with $Z=119$, 120 are investigated. Many reactions have yet to be studied in order to find the possible incoming channels for the formation of $Z=120$. In this work, synthesis of superheavy element 120 in terms of the fission and quasifission cross sections via $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction is evaluated and discussed. We have explored the possibility of formation of $Z=120$ via $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction, using the experimental data from Kozulin , Phys. Lett. B 686, 227 (2010). Some experimental efforts have been made to synthesize $^{302}120$, via Ni-induced and Cr-induced reaction, i.e., $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction at five ${E}^{*}$'s (excitation energies) and the $^{54}\mathrm{Cr}+^{248}\mathrm{Cm}$ reaction at only one energy, respectively. In this work, we have studied the Ni-induced reaction at given energies by including higher multipole deformations ${\ensuremath{\beta}}_{\ensuremath{\lambda}i}$ $(\ensuremath{\lambda}=2,3,4$; $i=1,2)$, and compact orientations ${\ensuremath{\theta}}_{ci}$ using the coplanar degree of freedom $(\mathrm{\ensuremath{\Phi}}={0}^{\ensuremath{\circ}})$ within the framework of the dynamical cluster-decay model (DCM). The neck-length parameter is the only one, which is fixed in reference to the observed data for fission cross section $({\ensuremath{\sigma}}_{\mathrm{ff}})$ calculated for mass region $A/2\ifmmode\pm\else\textpm\fi{}20$, and quasifission cross section $({\ensuremath{\sigma}}_{\mathrm{qf}})$ for the incoming channel of $^{64}\mathrm{Ni}+^{238}\mathrm{U}$ reaction. Our calculations for Ni-induced reaction have shown a good concurrence with the experimental data for quasifission and fission cross sections along with the DCM-calculated estimated and predicted cross sections for evaporation residues (ERs), which can be used for future references. Our results demonstrate the insignificant compound nucleus formation possibility ${P}_{\text{CN}}$ $(\ensuremath{\ll}1)$ for $Z=120$ from Ni-induced reaction because of the very low order of evaporation residues; thus, this work is not supporting this incoming channel for the formation of $Z=120$.

Theoretical evidence for neutron magic number 184 from cluster radioactivity studies

The existence of neutron magic number 184 is demonstrated through the cluster decay studies. The alpha and black cluster radioactivity have been studied in the superheavy region [Formula: see text] using modified generalized liquid drop model (MGLDM). The values obtained from this work were compared with that of experiments. During the study of black cluster radioactivity, surprisingly it is observed that the shortest half-lives of cluster decay in all superheavy elements are observed for daughter neutron number 184 and nearly equal to that. This is a clear evidence for the existence of the neutron magic number [Formula: see text]. This study is useful in the understanding of superheavy element nuclei structure.

Application of the Relativistic Electron Localization Function to Study the Electronic Structure of Superheavy Elements

A formula for calculating the relativistic electron localization function (RELF) within the framework of the Dirac-Fock method is obtained. An approach similar to that used earlier in [A.D. Becke and K.E. Edgecombe, The Journal of Chemical Physics 92, 5397 (1990)] in deriving an expression for the nonrelativistic electron localization function (ELF) is applied. It is demonstrated that the expression for RELF differs from the expression for ELF with replacement of the nonrelativistic electron density by its relativistic counterpart. Relativistic calculations of ELF and RELF for a number of superheavy elements are performed and the results are compared. By several examples it is shown that the ELF value equal to 0.5 does not necessarily correspond to the distribution density of homogeneous electron gas. Keywords: relativistic electron localization function, Dirac-Fock method, superheavy elements, electron gas.

Reactivity of superheavy elements Cn and Fl and of their oxides in comparison with homologous species of Hg and Pb, respectively, towards gold and hydroxylated quartz surfaces.

Adsorption energies, Eads, and other properties of atoms and oxides of the superheavy elements (SHEs) Cn and Fl, as well as of the homologous species of Hg and Pb, on Au(111) and fully hydroxylated quartz surfaces are predicted on the basis of 2c-DFT calculations and a periodic slab model using BAND software. The ambition of the work is to interpret the outcome of "one-atom-at-a-time" gas-phase chromatography experiments on the reactivity/volatility of SHEs. The present results with an improved (dispersion corrected) exchange-correlation functional show that, in agreement with our earlier predictions and experimental results on Pb, Hg and Cn, the sequence of the Eads values of the atoms on the gold surface should be Pb ≫ Hg > Fl > Cn, with rather moderate Eads values smaller than 90 kJ mol-1 (except for that for Pb). Oxides of Hg, Cn and Fl should be much more reactive with the gold surface than the corresponding atoms, with Eads values of about 200 kJ mol-1. A striking difference in the geometry of the deposited oxides was found between group 12 and group 14. An analysis of the Eads values for M and MO (M = Hg/Cn and Pb/Fl) on the hydroxylated α-quartz surface enables one to conclude that atoms of Hg, Cn and Fl should not interact with such a surface at room temperature, while Pb should adsorb on it. Oxides of these elements, on the contrary, should strongly adsorb on quartz with Eads ≥ 100 kJ mol-1. The present theoretical data agree with the experimental results on the elemental species of Hg, Cn and Fl.

Semi-empirical fission model for r-process based on the recent experiments and three-dimensional Langevin approach

Nuclear fission of superheavy elements can affect the r-process nucleosynthesis via a fission cycling process. In that process, the identification of the nuclide of a fission fragment and its abundance is essential for a precise evaluation of their contribution as seed nuclei of the r-process. We have investigated the nuclear fission of SHEs from the proton-rich side to the neutron-rich side using our three-and four-dimensional Langevin models [1, 4]. This model can reproduce the experimental data on nuclear fission of actinides and SHEs, while it can only provide various quantities to each fission fragment, we also developed a semi-empirical charge distribution model based on abundant experimental data of actinides [3]. A new semi-empirical nuclear fission model for the r-process is made by combining these two models.

Ionization energy and valence electron orbital binding energy of superheavy element Og(<i>Z</i> = 118) and its homologs

The ionization energy of the superheavy element Og (<i>Z</i> = 118) and its homolog elements Ar, Kr, Xe, Rn, and their ions are systematically calculated by using the GRASP2K program based on the multi-configuration Dirac-Hartree-Fock (MCDHF) method, taking into account relativistic effects, electron correlation effects between valence shell electrons, quantum electrodynamics effects, and Breit interaction. To reduce the uncertainty of the ionization energy derived from electron correlation effects which are not fully considered, the ionization potential of the superheavy element Og<sup>0–2+</sup> and its homolog element Rn<sup>0–2+</sup> are extrapolated by the extrapolation method. The ionization energy of extrapolated Rn<sup>0–5+</sup> and Og<sup>5+</sup> coincide well with experimental and other theoretical values. These results can be used to predict the unknown physical and chemical properties of the atoms and compounds of the superheavy element Og. In addition, the calculation results of the electron orbital binding energy of the atomic valence shell of the superheavy element Og and its homolog elements Ar, Kr, Xe, and Rn under relativistic and non-relativistic conditions show that owing to the relativistic effect, there occur strong orbital contraction phenomena in the 7s orbital and 7p<sub>1/2</sub> orbital and strong splitting phenomena in the 7p<sub>1/2 </sub> orbital and 7p<sub>3/2</sub> orbital of Og, which may cause the physical and chemical properties of the superheavy element Og to differ from those of other homologs.

Ionization energy and valence electron orbital binding energy of the superheavy element Og(Z=118) and its homologs

The ionization energy of the superheavy element Og (Z=118) and its homolog elements Ar, Kr, Xe, Rn, and their ions were systematically calculated using the GRASP2K program based on the multi-configuration Dirac-Hartree-Fock (MCDHF) method, taking into account relativistic effects, electron correlation effects between valence shell electrons (VV), quantum electrodynamics (QED) effects, and Breit interaction. To reduce the uncertainty of the ionization energy derived from electron correlation effects which are not fully considered, the ionization potential of the superheavy element Og<sup>0-2+</sup> and its homolog element Rn<sup>0-2+</sup> are extrapolated by the extrapolation method. The ionization energy of extrapolated Rn<sup>0-5+</sup> and Og<sup>5+</sup> coincides well with experimental and other theoretical values. These results can be used to predict the unknown physical and chemical properties of the atoms and compounds of the superheavy element Og. In addition, the calculation of the electron orbital binding energy of the atomic valence shell of the superheavy element Og and its homolog elements Ar, Kr, Xe, and Rn under relativistic and non-relativistic conditions shows that due to the relativistic effect, there is a strong orbital contraction phenomenon in the 7s and 7p<sub>1/2</sub> orbitals and a strong splitting phenomenon occurs in the 7p<sub>1/2</sub>and 7p<sub>3/2</sub> orbitals of Og, which may cause the physical and chemical properties of the superheavy element Og to be different from other homologs.

Application of a novel gas phase synthesis approach to carbonyl complexes of accelerator-produced 5d transition metals

Abstract In 2014 the first synthesis of a transactinide carbonyl complex – seaborgium hexacarbonyl – was reported. This was achieved in gas-phase chemical experiments in a beam-free environment behind the recoil separator GARIS. Extending this work to heavier elements requires more efficient techniques to synthesize carbonyl complexes as production rates of transactinide elements drop with increasing atomic number. A novel approach was thus conceived, which retains the benefit of a beam-free environment but avoids the physical preseparation step. The latter reduces the yields for products of asymmetric reactions such as those used for the synthesis of suitable isotopes of Sg, Bh, Hs and Mt. For this a series of experiments with accelerator-produced radioisotopes of the lighter homologues W, Re and Os was carried out at the tandem accelerator of JAEA Tokai, Japan. A newly developed double-chamber system, which allows for a decoupled recoil ion thermalization and chemical complex formation, was used, which avoids the low-efficiency physical preseparation step. Here, we demonstrate the feasibility of this newly developed method using accelerator-produced short-lived radioisotopes of the 5d homologues of the early transactinides.

Empirical formula for heavy cluster decay in the superheavy region

In this paper, an empirical formula to describe the heavy cluster decay with [Formula: see text] ([Formula: see text] is the charge number of the emitted cluster) from superheavy elements is proposed. The predictions of our formula are compared with the predictions of Coulomb and Proximity Potential Model (CPPM), the predictions of Analytical Super Asymmetric Fission (ASAF) model and the predictions by UDL formula of Qi et al. Excellent agreement is found between the predictions of CPPM and ASAF models and those of the present formula. Using our formula, we have calculated the decay half-lives of various heavy cluster emission from superheavy elements leading to doubly magic [Formula: see text]Pb and compared with the predictions of CPPM. The predictions of UDL formula deviate considerably from the predictions of this formula, CPPM and ASAF model, giving much lower decay half-lives, as the order of decay half-life increases. Possible heavy cluster emissions from [Formula: see text] and 122 nuclei leading to [Formula: see text]Pb and [Formula: see text]Bi are also predicted.