W. Paulus¹, R. Günther¹, W. Brüchle², K. Eberhardt¹, K. E. Gregorich³, E. Jäger², J.V. Kratz¹, U. Kirbach⁴, B. Kubica⁵, R. Misiak⁵, Y. Nagame⁶, M. Schädel², B. Schausten², E. Schimpf², A. Seibert¹, E. Strub¹, D. Schumann⁴, P.Thörle¹, N. Trautmann¹, A. Waldek¹, G. Wirth², S. Zauner¹

1Institut für Kernchemie, Universität Mainz;²GSI, Darmstadt;³Lawrence Berkeley National Laboratory, Berkeley; ⁴Technische Universität Dresden; ⁵Institute of Nuclear Physics, Krakow; ⁶Japan Atomic Energy Research Institute, Tokai

We have previously reported on the first study of chemical properties of element 106, seaborgium, in aqueous solutions [1-4]. Seaborgium was rapidly eluted from a cation exchange column together with its homologues molybdenum and tungsten in 0.1 M HNO₃/5x10^-4 M HF. Its chemical form was presumably a neutral or anionic oxygen containing fluoride. However, species containing no fluoride could not be excluded. In order to verify that fluoride complexing played a role in our previous study of seaborgium chemistry, we performed another series of cation exchange separations in which 0.1 M HNO₃ without HF was used as eluent. A target of 691 µg/cm^{2 248}Cm containing 22 µg/cm^{2 152}Gd was bombarded with ²²Ne⁵⁺ ions at 123 MeV at an average intensity of 0.4 particle µA. The ¹⁵²Gd in the target produced ¹⁶⁹W on-line which was used to monitor the chemical yield of tungsten by -ray spectroscopy. The reaction products were transported with a He(KCl)-gas jet to the automated chemistry apparatus ARCA. The collection and cycle time was 45 s. The activity was dissolved in 0.1 M HNO₃ and eluted from 1.6x8 mm cation exchange columns (Aminex A6) within 8 s. For an improved decontamination from Bi/Po activities, the effluent passed a 0.6x5 mm column filled with finely grained Pd/H₂ where Bi/Po were electrochemically deposited. The effluent was sprayed onto 400-500 µg/cm² Ti foils mounted on Al frames and evaporated to dryness by IR-light. The Ti foils were inserted between pairs of PIPS detectors and assayed for -particles. The energy resolution was 60 keV on the average and 550 keV for -particles penetrating the Ti foil. The detection efficiency was thus doubled as compared to the previous experiments. For correlated pairs of mother-daughter decays this increases the detection efficiency by a factor of 4.

4575 experiments were performed. Only one correlated pair (8.26 MeV-8.18 MeV) of -particles was registered. A Monte Carlo simulation yields 0.49 random correlations which means that the observed correlation has a 30% probability to be random. From the number of beam particles, the target thickness, the jet transportation yield, the detection efficiency and the chemical yield of tungsten (thereby assuming that the chemical yield of seaborgium is identical to the chemical yield of tungsten, i.e. 59% on the average), 4.7 (+3.7/-2.5) (68 % conf. level) correlations were expected. The probability distribution around this expectation value leaves a probability of 14 % for the experimental observation to be compatible with the expected number of correlations. From the lack of seaborgium events we conclude that seaborgium in the absence of HF sorbs on the cation exchange resin.

This non-tungsten like behaviour of seaborgium under the given conditions may be attributed to its weaker tendency to hydrolyze:

M(H₂O)₆⁶⁺   <-->   M(OH)(H₂O)₅⁵⁺ + H⁺

M(OH)₅(H₂O) ⁺   <-->   M(OH)₆ + H⁺

M(OH)₆   <-->   MO₂(OH)₂ + 2H₂O

MO₂(OH)₂   <-->   MO₄^2- + 2H⁺

For Mo and W, the sequence of subsequent hydrolysis reactions in diluted HNO₃ reaches the neutral spezies MO₂(OH)₂ [5]. A weaker tendency to hydrolyze for seaborgium would stop this sequence earlier, e.g. with M(OH)₅(H₂O)⁺, which sorbs on a cation exchange resin.

A decreasing tendency to hydrolyze (Nb>Ta>105>Pa) is reported [6] to determine the extraction of the group-5 chlorides into aliphatic amines, thus, a similar behaviour in the neighbouring group-6 is conceivable, but needs to be verified.

In the presence of fluoride ions having a strong tendency to replace OH^--ligands, the formation of neutral or anionic fluoride species is favoured:

M(OH)₆ + 2 HF   <-->   MO₂F₂ + 4H₂O

MO₂F₂ + F^-   <-->  MO₂F₃^-

Thus, in the previous experiments with seaborgium in the presence of fluoride ions, neutral or anionic fluoride complexes, e.g. MO₂F₂ or MO₂F₃^-, were likely to be formed and were eluted from the cation exchange columns [1-4].

[1] M. Schädel et al., GSI Scientific Report 1995, GSI 96-1, 10 (1996)

[2] M. Schädel et al., Nature 388, 55 (1997)

[3] M. Schädel et al., Radiochim. Acta 77, 149 (1997)

[4] M. Schädel, J.V. Kratz, Phys. Bl. 53, 865 (1997)

[5] C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, John Wiley 1976

[6] V. Pershina, Radiochim. Acta, in press