J. Krier

Superheavy Element Flerovium (Element 114) Is a Volatile Metal

The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d(10)7s(2) and 6d(10)7s(2)7p1/2(2), respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Experimental gas-solid chromatography studies on Cn have, indeed, revealed a metal-metal bond formation with Au. In contrast to this, for Fl, the formation of a weak bond upon physisorption on a Au surface was inferred from first experiments. Here, we report on a gas-solid chromatography study of the adsorption of Fl on a Au surface. Fl was produced in the nuclear fusion reaction (244)Pu((48)Ca, 3-4n)(288,289)Fl and was isolated in-flight from the primary (48)Ca beam in a physical recoil separator. The adsorption behavior of Fl, its nuclear α-decay product Cn, their lighter homologues in groups 14 and 12, i.e., Pb and Hg, and the noble gas Rn were studied simultaneously by isothermal gas chromatography and thermochromatography. Two Fl atoms were detected. They adsorbed on a Au surface at room temperature in the first, isothermal part, but not as readily as Pb and Hg. The observed adsorption behavior of Fl points to a higher inertness compared to its nearest homologue in the group, Pb. However, the measured lower limit for the adsorption enthalpy of Fl on a Au surface points to the formation of a metal-metal bond of Fl with Au. Fl is the least reactive element in the group, but still a metal.

Synthesis and detection of a seaborgium carbonyl complex

A carbonyl compound that tips the scales Life is short for the heaviest elements. They emerge from high-energy nuclear collisions with scant time for detection before they break up into lighter atoms. Even et al. report that even a few seconds is long enough for carbon to bond to the 106th element, seaborgium (see the Perspective by Loveland). The authors used a custom apparatus to direct the freshly made atoms out of the hot collision environment and through a stream of carbon monoxide and helium. They compared the detected products with theoretical modeling results and conclude that hexacarbonyl Sg(CO)6 was the most likely structural formula. Science, this issue p. 1491; see also p. 1451 A special apparatus enables synthesis of a compound with carbon bonds to a short-lived element produced via nuclear reaction. [Also see Perspective by Loveland] 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.

Rapid synthesis of radioactive transition-metal carbonyl complexes at ambient conditions.

Carbonyl complexes of radioactive transition metals can be easily synthesized with high yields by stopping nuclear fission or fusion products in a gas volume containing CO. Here, we focus on Mo, W, and Os complexes. The reaction takes place at pressures of around 1 bar at room temperature, i.e., at conditions that are easy to accommodate. The formed complexes are highly volatile. They can thus be transported within a gas stream without major losses to setups for their further investigation or direct use. The rapid synthesis holds promise for radiochemical purposes and will be useful for studying, e.g., chemical properties of superheavy elements.

New Short-Lived Isotope 221U and the Mass Surface Near N=126

Two short-lived isotopes ^{221}U and ^{222}U were produced as evaporation residues in the fusion reaction ^{50}Ti+^{176}Yb at the gas-filled recoil separator TASCA. An α decay with an energy of E_{α}=9.31(5)  MeV and half-life T_{1/2}=4.7(7)  μs was attributed to ^{222}U. The new isotope ^{221}U was identified in α-decay chains starting with E_{α}=9.71(5)  MeV and T_{1/2}=0.66(14)  μs leading to known daughters. Synthesis and detection of these unstable heavy nuclei and their descendants were achieved thanks to a fast data readout system. The evolution of the N=126 shell closure and its influence on the stability of uranium isotopes are discussed within the framework of α-decay reduced width.

High intensity target wheel at TASCA: target wheel control system and target monitoring

At GSI Darmstadt, the gas-filled recoil separator transactinide separator and chemistry apparatus (TASCA) is in operation for experiments with superheavy elements. It is optimized for hot-fusion reactions with actinide targets. The small cross sections of such reactions require the capability to accept highest beam intensities. The limited availability of some of the exotic actinide isotopes limits the size of target systems. To maintain target integrity during long experiments, automated target monitoring and control is necessary. Here, the TASCA target wheel system and the on-line target monitoring are described.

In-situ formation, thermal decomposition, and adsorption studies of transition metal carbonyl complexes with short-lived radioisotopes

Abstract We report on the in-situ synthesis of metal carbonyl complexes with short-lived isotopes of transition metals. Complexes of molybdenum, technetium, ruthenium and rhodium were synthesized by thermalisation of products of neutron-induced fission of 249Cf in a carbon monoxide-nitrogen mixture. Complexes of tungsten, rhenium, osmium, and iridium were synthesized by thermalizing short-lived isotopes produced in 24Mg-induced fusion evaporation reactions in a carbon monoxide containing atmosphere. The chemical reactions took place at ambient temperature and pressure conditions. The complexes were rapidly transported in a gas stream to collection setups or gas phase chromatography devices. The physisorption of the complexes on Au and SiO2 surfaces was studied. We also studied the stability of some of the complexes, showing that these start to decompose at temperatures above 300 ℃ in contact with a quartz surface. Our studies lay a basis for the investigation of such complexes with transactinides.

Decomposition studies of group 6 hexacarbonyl complexes. Part 1: Production and decomposition of Mo(CO)6 and W(CO)6

Abstract Chemical studies of superheavy elements require fast and efficient techniques, due to short half-lives and low production rates of the investigated nuclides. Here, we advocate for using a tubular flow reactor for assessing the thermal stability of the Sg carbonyl complex – Sg(CO)6. The experimental setup was tested with Mo and W carbonyl complexes, as their properties are established and supported by theoretical predictions. The suggested approach proved to be effective in discriminating between the thermal stabilities of Mo(CO)6 and W(CO)6. Therefore, an experimental verification of the predicted Sg–CO bond dissociation energy seems to be feasible by applying this technique. By investigating the effect of 104,105Mo beta-decay on the formation of 104,105Tc carbonyl complex, we estimated the lower reaction time limit for the metal carbonyl synthesis in the gas phase to be more than 100 ms. We examined further the influence of the wall material of the recoil chamber, the carrier gas composition, the gas flow rate, and the pressure on the production yield of 104Mo(CO)6, so that the future stability tests with Sg(CO)6 can be optimized accordingly.

ALBEGA: A Decay Spectroscopy Setup for Chemically Separated Samples

Di Nitto, A.; Yakushev, A.; Düllmann, C. E.; Khuyagbaatar, J.; Krier, J.; Ballof, J.; Bar, J.; Budzynski, T.; Cox, D. M.; Derkx, X.; Dormand, J.; Despotopulos, J. D.; Eberhardt, K.; Even, J.; Grabiec, P.; Harkness-Brennan, L.; Herzberg, R. D.; Hübner, A.; Jäger, E.; Judson, D.; Kindler, B.; Klos, H.; Kratz, J. V.; Kulawik, J.; Kurz, N.; Lens, L.; Lommel, B.; Moody, K.; Panas, A.; Prokaryn, P.; Rudolph, Dirk; Runke, J.; Rusanov, I.; Scharrer, P.; Schausten, B.; Shaughnessy, D.; Szmigiel, D.; Ward, A. J.; Wegrzecki, M. Published in: GSI Report

Online chemical adsorption studies of Hg, Tl, and Pb on SiO2 and Au surfaces in preparation for chemical investigations on Cn, Nh, and Fl at TASCA

Abstract Online gas-solid adsorption studies with single-atom quantities of Hg, Tl, and Pb, the lighter homologs of the superheavy elements (SHE) copernicium (Cn, Z=112), nihonium (Nh, Z=113), and flerovium (Fl, Z=114), were carried out using short-lived radioisotopes. The interaction with Au and SiO2 surfaces was studied and the overall chemical yield was determined. Suitable radioisotopes were produced in fusion-evaporation reactions, isolated in the gas-filled recoil separator TASCA, and flushed rapidly to an adjacent setup of two gas chromatography detector arrays covered with SiO2 (first array) and Au (second array). While Tl and Pb adsorbed on the SiO2 surface, Hg interacts only weakly and reached the Au-covered array. Our results contribute to elucidating the influence of relativistic effects on chemical properties of the heaviest elements by providing experimental data on these lighter homologs.

Identification of Reaction Products in 50Ti+249Cf Reactions at TASCA

The neutron-rich nuclei 33P and 33S in the upper sd-shell were investigated by means of the 26Mg(13C,npa) and 26Mg(13C,2na) fusion-evaporation reactions. Excited states with intermediate and high spins have been populated. The level schemes of both nuclei have been considerably extended. Utilizing the gammagamma-angular correlation method the spin-parity assignment of the new excited states in 33P has been investigated. The experimentally determined energy levels as well as the known reduced transition probabilities (i.e. B(M1) and B(E2) values) from both nuclei were compared to 0hbaromega and 1hbaromega truncated p-sd-pf shell-model calculations using the PSDPF interaction. For the energy levels a very good agreement between experiment and theory was shown for both 33P and 33S. However, for B(M1) and B(E2) values the calculated values cannot reproduce the experimental results with satisfying agreement for all transitions. In some places the discrepancy between experiment and theory is even large, which requires further experimental as well as theoretical investigation of this thesis for these nuclei. The second part was focused on the upgrade and commissioning tests of the Lund- York-Cologne CAlorimeter (LYCCA). As a key device of the High resolution In-flight SPECtroscopy (HISPEC) campaign of the FAIR/NUSTAR collaboration, LYCCA was designed to identify the reaction products after the secondary target, as well as to track the particle trajectory event by event. After the successful employment of the precursor LYCCA-0 in the PreSPEC campaign, the electronic as well as mechanic components of the LYCCA system were upgraded by STFC Daresbury Laboratory. Using the high integrated AIDA Front-End electronics with ASICs the signals from more than thousand DSSSD-channels were pre-amplified and processed. Since 2016, the new LYCCA setup is located at the Cologne tandem accelerator. Triple-Alpha tests and in-beam experiments of elastic scattering were carried out to check the specifications of the system after the upgrade. The obtained results allow first important conclusions about energy resolution and efficiency of the calorimeter at low energies for future NUSTAR experiments.Di Nitto, A.; Khuyagbaatar, J.; Ackermann, D.; Adamczewski-Musch, J.; Andersson, LiseLotte; Badura, E.; Block, M; Brand, H.; Cox, D. M.; Düllmann, Ch. E.; Dvorak, J.; Eberhardt, K.; Ellison, P. A.; Esker, N. E.; Even, J.; Fahlander, Claes; Forsberg, Ulrika; Gates, J.M.; Golubev, Pavel; Gothe, O.; Gregorich, K.E.; Hartmann, W.; Herzberg, R.-D.; Heßberger, F. P.; Hoffmann, J.; Hollinger, R.; Hübner, A.; Jäger, E.; Jeppsson, J.; Kindler, B.; Klein, S.; Kojouharov, I.; Kratz, J.V.; Krier, J.; Kurz, N.; Lahiri, S.; Linev, S.; Lommel, B.; Maiti, M.; Mändl, R.; Merchán, E.; Minami, S.; Mistry, A. K.; Mokry, Ch.; Nitsche, H.; Omtvedt, J. P.; Pang, G.; Pysmenetska, I.; Renisch, D.; Rudolph, Dirk