.
Nuclear fission is one of the radioactive decay modes of heavy nuclei, and is a limiting factor in the existence of the superheavy elements. For decades, both experimental and theoretical works ...devoted to the study of fission have been performed, and are still ongoing. Among them, the so-called
β
(electron capture)-delayed fission is of special interest. In this work, all known cases of electron-capture delayed fission in heavy nuclei were used to establish a semi-empirical approach that could quantitatively estimate the probability of the process. This framework was constructed based on a qualitative discussion of the fission-barrier shape, in contrast to a parabolic shape. The results show a strong effect of the fission-barrier shape on the delayed fission process. The predictions for yet unmeasured cases of electron-capture delayed fission in nuclei with atomic numbers in the wide range (
79
≤
Z
≤
119
) including the superheavy nuclei are given. It is suggested that
β
-delayed fission manifests itself as one of the main decay modes of the superheavy nuclei, if
β
decay takes place.
One of the most important atomic properties governing an element's chemical behavior is the energy required to remove its least-bound electron, referred to as the first ionization potential. For the ...heaviest elements, this fundamental quantity is strongly influenced by relativistic effects which lead to unique chemical properties. Laser spectroscopy on an atom-at-a-time scale was developed and applied to probe the optical spectrum of neutral nobelium near the ionization threshold. The first ionization potential of nobelium is determined here with a very high precision from the convergence of measured Rydberg series to be 6.626 21±0.000 05 eV. This work provides a stringent benchmark for state-of-the-art many-body atomic modeling that considers relativistic and quantum electrodynamic effects and paves the way for high-precision measurements of atomic properties of elements only available from heavy-ion accelerator facilities.
The spontaneous fission half-lives in superheavy nuclei are predicted by theoretical approaches from the order of few years up to the age of the earth. In this work, all known experimental ...spontaneous fission half-lives of nuclei with Z>95 have been used for the extraction of the effective widths of the parabolic fission barriers with heights from two different theoretical models: FRLDM and ETFSI. The results are discussed with consideration on the shape of the fission barriers for the particular models. By extrapolating the extracted widths of the parabolic barrier, and the calculated fission-barrier heights from the two models, the spontaneous fission half-lives for nuclei with Z=98−120 were calculated within the presently suggested semi-empirical approach. Two theoretical models results in completely different scenarios for the shape of fission barrier in superheavy nuclei. Interestingly, the results with FRLDM predict that spontaneous fission in superheavy nuclei and in particular Z=120, may compete with α decay, and thus the spontaneous fission branches of these nuclei are significant enough such that they should be considered during experimental studies.
Fission is the one of the primary radioactive decay modes for the heaviest nuclei and ultimately determines the existence of the heaviest elements on a macroscopic time scale, e.g.,
≥
10
-
14
s. The ...present experimental data on the decay properties of the heaviest nuclei with proton numbers 102–118 and/or of neutron numbers up to 177 show that fission occurs occasionally. This confirms that shell structure plays an essential role for their stability against fission. The shell effect on fission manifests in both collective and single-particle ways, which can experimentally be studied in decays of even–even, odd-A and odd–odd nuclei. At the same time, high-
K
states formed in couplings of quasiparticles are also known to be stable against fission. However, detailed knowledge and theoretical descriptions on a retardation effect/strength of high-
K
quantum number on fission are still scarce. In the present work, fission from high-
K
states are discussed and described within the semi-empirical approach. Fission half-lives are calculated for various high-
K
states, which have been theoretically predicted to exist in Fm-Rf (
Z
= 100–104) and Hs-Ds (
Z
= 108–112). The results are found to be in line with the available experimental findings, and also leading to different intriguing predictions, e.g., high-
K
states in superheavy nuclei tend to be more stable against fission compared to their ground states.
Experimental investigations of transactinoide elements provide benchmark results for chemical theory and probe the predictive power of trends in the periodic table. So far, in gas-phase chemical ...reactions, simple inorganic compounds with the transactinoide in its highest oxidation state have been synthesized. Single-atom production rates, short half-lives, and harsh experimental conditions limited the number of experimentally accessible compounds. We applied a gas-phase carbonylation technique previously tested on short-lived molybdenum (Mo) and tungsten (W) isotopes to the preparation of a carbonyl complex of seaborgium, the 106th element. The volatile seaborgium complex showed the same volatility and reactivity with a silicon dioxide surface as those of the hexacarbonyl complexes of the lighter homologs Mo and W. Comparison of the product’s adsorption enthalpy with theoretical predictions and data for the lighter congeners supported a Sg(CO)₆ formulation.
Until recently, ground-state nuclear moments of the heaviest nuclei could only be inferred from nuclear spectroscopy, where model assumptions are required. Laser spectroscopy in combination with ...modern atomic structure calculations is now able to probe these moments directly, in a comprehensive and nuclear-model-independent way. Here we report on unique access to the differential mean-square charge radii of ^{252,253,254}No, and therefore to changes in nuclear size and shape. State-of-the-art nuclear density functional calculations describe well the changes in nuclear charge radii in the region of the heavy actinides, indicating an appreciable central depression in the deformed proton density distribution in ^{252,254}No isotopes. Finally, the hyperfine splitting of ^{253}No was evaluated, enabling a complementary measure of its (quadrupole) deformation, as well as an insight into the neutron single-particle wave function via the nuclear spin and magnetic moment.
Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam ...energies from below-barrier to 25% above, for the reactions of ^{48}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P_{CN} (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ^{50}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.
In deep inelastic multinucleon transfer reactions of 48Ca + 248Cm we observed about 100 residual nuclei with proton numbers between Z=82 and Z=100. Among them, there are five new neutron-deficient ...isotopes: 216U, 219Np, 223Am, 229Am and 233Bk. As separator for the transfer products we used the velocity filter SHIP of GSI while the isotope identification was performed via the α decay chains of the nuclei. These first results reveal that multinucleon transfer reactions together with here applied fast and sensitive separation and detection techniques are promising for the synthesis of new isotopes in the region of heaviest nuclei.
A nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z=114). An upgraded TASISpec decay station was placed behind the gas-filled ...separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ^{48}Ca+^{242}Pu and ^{48}Ca+^{244}Pu provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ^{286}Fl and ^{288}Fl, respectively. A prompt coincidence between a 9.60(1)-MeV α particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ^{282}Cn. Spectroscopy of ^{288}Fl decay chains fixed Q_{α}=10.06(1) MeV. In one case, a Q_{α}=9.46(1)-MeV decay from ^{284}Cn into ^{280}Ds was observed, with ^{280}Ds fissioning after only 518 μs. The impact of these findings, aggregated with existing data on decay chains of ^{286,288}Fl, on the size of an anticipated shell gap at proton number Z=114 is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.
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.
PDF
