Fluoride Complexation of Rutherfordium (Rf, element 104)

E. Strub ¹ , W. Brüchle ² B. Eichler ⁴ , H.W. Gäggeler ^4,6, J.P. Glatz ⁷ , A. Grund ⁴ , M. Gärtner ⁶ , E. Jäger ² , D. Jost ⁴ , U. Kirbach ³ , J.V. Kratz ¹ , A. Kronenberg ¹ , Y. Nagame ⁵ , M. Schädel ² , B. Schausten ² , E. Schimpf ² , D. Schumann ³ , P. Thörle ¹ , K. Tsukada ⁵ , A. Türler ⁴ , S. Zauner ¹

The first aqueous chemistry study of seaborgium (Sg, element 106)[1,2] was performed recently with the Automated Rapid Chemistry Apparatus ARCA II. ²⁶⁵Sg ( T_1/2 =7.4^+3.3_-2.7 s [3]) was eluted from a cation exchange column (Aminex A6) in 0.1 M HNO ₃/5·10^-4 M HF which is typical for a group-6 element. Due to the half-minute time span needed to prepare samples and to start counting[2], ²⁶⁵ Sg itself was not detected but correlated mother-daughter decays of its descendant ²⁶¹ Rf. Therefore, it was important that a HF concentration was selected[2] that was insufficient to elute group-4 elements. This had been demonstrated for Zr and Hf and was assumed for Rf. Due to the HSAB concept, Sg ⁶⁺ shows a higher affinity to F ^- ions than any 4+ ion. Thus, the tendency to form anionic fluoride complexes should increase from Rf to Sg. So even if Rf fluoride complexing is different from that of Zr and Hf and is more similar to that of the pseudo-homolog Th than to group 4 elements, it should stick to the column even at higher HF concentrations than Sg.

Nevertheless, it was of interest to verify experimentally that ²⁶¹Rf, under the condition of the seaborgium experiments, is not eluted from the cation exchange column and can only appear in the seaborgium sample as a result of the decay of ²⁶⁵Sg. Therefore, ²⁶¹ Rf was produced directly in the ²⁴⁸ Cm(¹⁸ O,5n) reaction at the Philips Cyclotron of the Paul Scherrer Institute. A 730 g/cm^{2 248}Cm target was bombarded with a 0.5 p A ¹⁸O⁵⁺ beam. The target contained 10 % Gd thus producing simultaneously short-lived Hf isotopes use to monitor on-line the behaviour of Hf and to perform yield checks.

For 0.1 M HNO ₃/5·10^-4 M HF, all ²⁶¹ Rf events were detected in sample 2. From this one can calculate a lower limit for the distribution coefficient K_d(5·10^-4 M HF )>148 . This proves unambiguously that the ²⁶¹ Rf decays detected in the Sg experiments[1,2] were originating with ²⁶⁵ Sg which had passed the cation exchange column.

There is evidence by D. Schumann et al.[4] that Rf shows a distinctly higher K_d in 0.1 M HCl/10^-2 M than Zr and Hf whereby Rf shows similarities to the pseudo homologue Th. Therefore, the Rf experiments were extended to 0.1 M HNO ₃ /10 ^-2 M HF. Also at this HF concentration, all ²⁶¹ Rf events were detected in sample 2, which defines a lower limit for the distribution coefficient K_d(10^-2 M HF )>312 (the higher limit with respect to 5·10^-4 M HF results from a higher number of detected events). The K_d values of Rf at 5 cdot10^-4 M HF and 10^-2 M HF (and an estimate at 10^-1 M HF) are shown in Fig. 1 together with the behaviour of Zr, Hf and Th in off-line experiments. Two observations can be made: i)the ²⁶¹ Rf results show similarities to the behaviour of Th and a sorption behaviour different from that of Zr and Hf, and ii)the behaviour of Hf produced on-line is in good agreement with data from off-line experiments.

Sorption of Zr, Hf, Th and Rf on a cation exchange resin (Aminex A6) at various HF concentrations. The on-line data for Hf and Th are compared to off-line data for Zr, Hf and Th. The aqueous solutions contained 0.1 M HNO₃.

References

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

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

[3] H.W. Gäggeler, Proc. Int. Conf. on Actinides, Baden-Baden, 21-26 September 1997, J. of Alloys and Compounds, in press

[4] D. Schumann et al., Labor für Radio- und Umweltchemie der Universität Bern und des Paul-Scherrer-Instituts, Annual Report 1996, p.37