K. E. Gregorich

First Aqueous Chemistry with Seaborgium (Element 106)

For the first time, chemical separations of element 106 (Seaborgium, Sg) were performed in aqueous solutions. The isotopes Sg and Sg were produced in the Cm + Ne reaction at a beam energy of 121 MeV. The reaction products were continuously transported by a He(KCl)-jet to the computer-controlled liquid chromatography system ARCA. In 0.1 M HNO3/5 X ΙΟ -4 M HF, Sg was found to be eluted within 10 s from 1.6X8 mm cation-exchange columns (Aminex A6, 17.5±2 μπι) together with the hexavalent Moand W-ions, while hexavalent U-ions and tetravalent Zr-, Hf-, and element 104 ions were strongly retained on the column. Element 106 was detected by measuring correlated α-decays of the daughter isotopes 78-s 104 and 26-s 102. For the isotope Sg, we have evidence for a spontaneous fission branch. It yields a partial spontaneousfission half-life which is in agreement with recent theoretical predictions. The chemical results show that the most stable oxidation state of Sg in aqueous solution is +6, and that like its homologs Mo and W, Sg forms neutral or anionic oxoor oxohalide-compounds under the present condition. In these first experiments, Sg exhibits properties very characteristic of group 6 elements, and does not show U-like properties.

Gas phase chromatography of halides of elements 104 and 105

On-line isothermal gas phase chromatography was used to study halides of261104 (T1/2=65 s) and262,263105 (T1/2=34 s and 27 s) produced an atom-at-a time via the reactions248Cm(18O, 5n) and249Bk(18O, 5n, 4n), respectively. Using HBr and HCl gas as halogenating agents, we were able to produce volatile bromides and chlorides of the above mentioned elements and study their behavior compared to their lighter homologs in Groups 4 or 5 of the periodic table. Element 104 formed more volatile bromide than its homolog Hf. In contrast, element 105 bromides were found to be less volatile than the bromides of the group 5 elements Nb and Ta. Both 104 and Hf chlorides were observed to be more volatile than their respective bromides.

The hydration enthalpies of Md3+ and Lr3+

Lawrencium (3-min 260Lr) and lighter actinides were produced in the bombardment of a 249Bk target with 18O ions and loaded onto a cation exchange column in 0.05 M α-hydroxyisobutyrate solution at pH=4.85, together with the radioactive lanthanide tracers 166Ho, 171Er and 171Tm. In elutions with 0.12 M α=hydroxyisobutyrate solution (pH=4.85), trivalent Lr was eluted exactly together with the Er tracer and Md was eluted close to Ho. Lr elutes much later than expected based on the known elution positions of the lighter actinides and the expected analogy to the elution positions of the homologous lanthanides. From the measured elution positions, ionic radii were calculated for Lr3+ and Md3+. Semi- empiricalmodels allow the calculation of the heat of hydration from the ionic radii, resulting in ΔHhyd≈-−3654 kJ/mol for Md3+ and ΔHhyd≈ -−3685 kJ/mol for Lr3+.

Chemical Studies of Rutherfordium (Element 104) : Part II. Solvent Extraction into Tributylphosphate from HBr Solutions

The chemical properties of element 104, rutherfordium (Rf), and its group 4 homologs and other tetravalent cations were studied by solvent extraction into tributylphosphate (TBP) from various concentrations of HBr and HCl. Since bromide is a larger and softer (more polarizable) anion than chloride, the stability of Rfbromide complexes was expected to differ from the stability of Rf-chloride complexes, and a comparison of the two systems was undertaken. Our studies showed that the extraction trend decreased in the order Zr > Hf > Rf/Ti for HBr, compared to Zr > Hf > Rf > Ti for HCl, showing that Rf and Ti did not extract as well in either system, probably because they hydrolyzed more easily than Zr and Hf.

The heavy element volatility instrument (HEVI)

Abstract We have constructed the heavy element volatility instrument (HEVI), an on-line gas chromatography system which is used to continuously separate halides according to their volatility. Both gaseous HBr and HCl have been used as halogenating agents. The detailed design of the apparatus is described and some preliminary results are presented. The experimental results are compared with those from a Monte Carlo code simulation based on a microscopic model for gas thermochromatography in open columns with laminar flow of the carrier gas. There is good agreement between the experimental results and the simulation.

Cross-section limits for the Pb-208 (Kr-86, n) 118-293 reaction

Abstract.In April-May, 2001, the previously reported experiment to synthesize element 118 using the 208Pb(86Kr,n)293118 reaction was repeated. No events corresponding to the synthesis of element 118 were observed with a total beam dose of 2.6 x 1018 ions. The simple upper-limit cross-sections (1 event) were 0.9 and 0.6 pb for evaporation residue magnetic rigidities of 2.00 Tm and 2.12 Tm, respectively. A more detailed cross-section calculation, accounting for an assumed narrow excitation function, the energy loss of the beam in traversing the target and the uncertainty in the magnetic rigidity of the Z = 118 recoils is also presented. Re-analysis of the primary data files from the 1999 experiment showed the reported element 118 events are not in the original data. The current results put constraints on the production cross-section for synthesis of very heavy nuclei in cold-fusion reactions.

Gas chemical investigation of hafnium and zirconium complexes with hexafluoroacetylacetone using preseparated short-lived radioisotopes

Abstract Volatile metal complexes of the group 4 elements Zr and Hf with hexafluoroacetylacetonate (hfa) have been studied using short-lived radioisotopes of the metals. The new technique of physical preseparation has been employed where reaction products from heavy-ion induced fusion reactions are isolated in a physical recoil separator – the Berkeley Gas-filled Separator in our work – and made available for chemistry experiments. Formation and decomposition of M(hfa)4 (M=Zr, Hf) has been observed and the interaction strength with a fluorinated ethylene propylene (FEP) Teflon surface has been studied. From the results of isothermal chromatography experiments, an adsorption enthalpy of -ΔHa=(57±3) kJ/mol was deduced. In optimization experiments, the time for formation of the complex and its transport to a counting setup installed outside of the irradiation cave was minimized and values of roughly one minute have been reached. The half-life of 165Hf, for which conflicting values appear in the literature, was measured to be (73.9±0.8) s. Provided that samples suitable for α-spectroscopy can be prepared, the investigation of rutherfordium (Rf), the transactinide member of group 4, appears possible. In the future, based on the studies presented here, it appears possible to investigate short-lived single atoms produced with low rates ( e.g. , transactinide isotopes) in completely new chemical systems, e.g. , as metal complexes with organic ligands as used here or as organometallic compounds.

Chemical Studies of Rutherfordium (Element 104) : Part III. Solvent Extraction into Triisooctylamine from HF Solutions

The extraction of element 104, rutherfordium (Rf), and its group 4 homologs into 0.25 M triisooctylamine (TIOA) in o,m,p-xylene from various concentrations of hydrofluoric acid was studied. This extraction system was of particular interest because all group 4 cations studied formed complexes with the same coordination number (CN) of 6. Therefore, species extracted were similar and the extractability depends on the radii of the fluoride complexes. The studies showed that the extractability for the group 4 elements in this system decreases in the order Ti > Zr ~ Hf > Rf, in inverse order from the decrease in ionic radii Rf > Zr ~ Hf > Ti. This order is different than the order in the hydrolysis, TBP-HC1, and TBP-HBr extraction systems where Rf, due to decreasing CN, behaves more similarly to Ti than to Zr and Hf.

Lawrencium chemistry: no evidence for oxidation states lower than 3+ in aqueous solution

Lawrencium (3-min 260Lr) together with other actinides, was produced in the bombardment of a 249Bk target with 18O ions. There was no sign of a reduction of Lr3+ in dilute hydrochloric acid by V2+ or Cr2+, although in the same experiments, Md3+ was reduced to Md2+ (E°=−0.2 V). The resulting limit for the reduction potential of the Lr3+/Lr1(2)+ couple is E° < −0.44 V.

Electron-capture delayed fission properties of242Es

Electron-capture delayed fission was observed in {sup 244}Es produced via the {sup 237}Np({sup 12}C,5n){sup 244}Es reaction at 81 MeV (on target) with a production cross section of 0.31{+-}0.12 {micro}b. The mass-yield distribution of the fission fragments is highly asymmetric. The average preneutron-emission total kinetic energy of the fragments was measured to be 186{+-}19 MeV. Based on the ratio of the number of fission events to the measured number of {alpha} decays from the electron-capture daughter {sup 244}Cf (100% {alpha} branch), the probability of delayed fission was determined to be (1.2{+-}0.4) x 10{sup -4}. This value for the delayed fission probability fits the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron-capture.