E. Jäger

Chemical investigation of hassium (element 108).

The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes 269Hs (refs 8, 9) and 270Hs (ref. 10) in the fusion reaction between 26Mg and 248Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO4, for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO4. These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table.

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.

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.

Chromatographic Study of Rutherfordium (Element 104) in the System HCl/Tributylphosphate (TBP)

Liquid-liquid extractions of Zr and Hf from various concentrations of HCl were conducted using tributylphosphate (TBP) in toluene (benzene), and undiluted TBP. Quantitative extraction of both Zr and Hf into TBP in toluene (benzene) was observed from > 8 M HCl and into undiluted TBP from > 10M HCl. Extraction from 8 M HCl into undiluted TBP was 80% for Zr and 20% for Hf. Based on these results, a chromatographic separation of Hf from Zr with the Automated Rapid Chemistry Apparatus ARCA II was performed on 1.6X8 mm columns containing undiluted TBP on an inert support. This separation was also studied on-line with short-lived Hf produced in the Gd(0,xn) reaction and with 78-s Rf produced in the Cm(O,5n) reaction at the Philips Cyclotron of the Paul Scherrer Institute. From the distribution of α-decays of Rf between the Hfand Zr-fraction, the R v a l u e for Rf in 8 M HCl was determined, and the following extraction trend was established: Zr > Rf > Hf. This is at variance with earlier results where Rf seemingly did not extract as well as Hf which was tentatively attributed to increased hydrolysis of Rf.

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.

Gas Chromatographic Studies of Oxide and Hydroxide Species of Tungsten - Model Experiments with Respect to the Physico-Chemical Characterization of Seaborgium (Element 106)

The adsorption/desorption behavior of carrier-free W on quartzglass surfaces in dry and moist oxygen was studied by off-line thermochromatography in the temperature ränge 330—1480 K. In dry oxygen W could not be volatilized. In moist oxygen the deposition temperature varies significantly with the humidity of the carrier-gas. The transport can be described by W02(0H)2(g) W03(.d,) + HzOjg) analogous to the lighter homologue Mo. Evaluation of the experimental data was carried out by means of the new Computer code TECPROF which allows to consider real temperature profiles. Up to an admixture of 60 ng MoOj-carrier the formation of heteropolynuclear complexes was not observed. However, the W was deposited at higher temperatures compared to experiments without a homologous carrier. Isothermal on-line gas chromatography experiments were carried out with short-hved W isotopes to model future Sg experiments and to optimize the experimental conditions.

Modeling a Nielsbohrium (Element 107) On-Line Gas Phase Separation Procedure with Rhenium

As a basis for future heavy dement experiments off-line tracer studies with rhenium have been carried out to model the chemical behavior of nielsbohrium (element 107) in thermochromatographic separations with oxygen as a reactive gas. A specific Version of the On-Line Gas Phase Separation Apparatus, OLGA, has been developed to perform on-line chemical separations of short-lived products from heavy ion nuclear reactions, and to enable a-spectroscopic measurements of the separated products after condensation on thin foils.

Chromatographic studies of Rf (element 104) with tributylphosphate (TBP)

Abstract The extraction behaviour of Rutherfordium (element 104, Rf), carrier-free Zirconium, and Hafnium in the system HCl–TBP (tributylphosphate) was examined. The 65-s α-emitting isotope 261 Rf was produced via the reaction 248 Cm( 18 O,5 n ). The recoil products were transported with a He(KCl) jet to the Automated Rapid Chemistry Apparatus (ARCA) where 1140 separations were performed. In contrast to previous data showing a seemingly low extraction of Hf and Rf we find much higher distribution coefficients. The extraction increases in the order Hf

Search for the missing α-decay branch in 239Cm

Summary A search for the α-decay of 239Cm was carried out via the fusion reaction 232Th(12C,5n) 239Cm at GSI, applying nuclear chemistry techniques. The 239Cm α-decay would provide a link between known nuclear masses and α-decay energies measured in the region of the superheavy elements. Three different 12C beam energies were chosen in the present experiments. The α peaks of 238,240Cm were observed clearly at a 12C energy of 74 MeV, while 238Cm disappeared at 70 MeV to provide an undisturbed α-spectrum for a sensitive search of 239Cm decay. No α-events between 6.34 and 6.50 MeV, which could be assigned to 239Cm with confidence, were observed in this experiment. The upper limit of the α-branch of 239Cm was less than 1.0×10−5.

Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A.MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the238U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV208Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the208Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.