Fluoride Complexation of Element 104, Rutherfordium

Chemical characterization of heavy actinides and light transactinides - Experimental achievements at JAEA

The chemical characterization of the heaviest elements at the farthest reach of the periodic table (PT) and the classification of these elements in the PT are undoubtedly crucial and challenging subjects in chemical and physical sciences. The elucidation of the influence of relativistic effects on their outermost electronic configuration is also a critical and fascinating aspect. However, the heaviest elements with atomic numbers Z ≳ 100 must be produced at accelerators using nuclear reactions of heavy ions and target materials. Therefore, production rates for these elements are low, and their half-lives are as short as a few seconds to a few minutes; they are usually available in a quantity of only a few atoms at a time. Here, we review some highlighted studies on heavy actinide and light transactinide chemical characterization performed at the Japan Atomic Energy Agency tandem accelerator facility. We discuss briefly the prospects for future studies of the heaviest elements.

Co-precipitation behaviour of single atoms of rutherfordium in basic solutions

All superheavy elements (SHEs), with atomic numbers (Z) ≥104, have been artificially synthesized one atom at a time and their chemical properties are largely unknown. Because these heavy nuclei have short lifetimes as well as extremely low production rates, chemical experiments need to be carried out on single atoms and have mostly been limited to adsorption and extraction. We have now investigated the precipitation properties of the SHE Rf (Z = 104). A co-precipitation method with samarium hydroxide had previously established that the co-precipitation behaviour of a range of elements reflected these elements' tendency to form hydroxide precipitates and/or ammine complex ions. Here we investigated co-precipitation of Rf in basic solutions containing NH₃ or NaOH. Comparisons between the behaviour of Rf with that of Zr and Hf (lighter homologues of Rf) and actinide Th (a pseudo-homologue of Rf) showed that Rf does not coordinate strongly with NH₃, but forms a hydroxide (co)precipitate that is expected to be Rf(OH)₄.

From SRAFAP to ARCA and AIDA – developments and implementation of automated aqueous-phase rapid chemistry apparatuses for heavy actinides and transactinides

The development of automated rapid chemistry techniques and their application for batch-wise, chromatographic separations of heavy elements in the liquid-phase are outlined. Starting in the mid-1970s with manually performed separations using pressurized liquid-chromatography techniques, this development led to the first version of the Automated Rapid Chemistry Apparatus, ARCA, in the early 1980s. After a breakthrough to a much higher level of automation and miniaturization, the new apparatus ARCA II was built in the late 1980s. Based on it, the Automated Ion-exchange separation apparatus coupled with the Detection system for Alpha spectroscopy, AIDA, became operational in the late 1990s. In the context of technical and technological advancements, this article discusses the successful application of these instruments for (i) the search for superheavy elements, (ii) cross section measurements of actinide elements produced in multi-nucleon transfer reactions with actinide targets, (iii) chemical separation and characterization of the heavy actinides mendelevium, Md, and lawrencium, Lr, and (iv) studies of the transactinide elements rutherfordium, Rf, dubnium, Db, and seaborgium, Sg. Details of the separations are outlined together with the big advancements made over time and the limitations reached. For the transactinide elements, examples are given for their observed chemical behavior; often affected by an interplay between hydrolysis and complex formation. Influenced by relativistic effects, chemical properties of these elements sometimes deviated from those of their lighter homologs in the Periodic Table.

Extraction behavior of rutherfordium as a cationic fluoride complex with a TTA chelate extractant from HF/HNO3 acidic solutions

The aim of this study was to identify relevant Rf chemical species by using reversed-phase extraction chromatography with 2-thenoyltrifluoroacetone (TTA) resin as the stationary phase. Because TTA can be used to extract specific metal ions, the distribution ratios of the system enabled determination of the specific complex formation constant of Rf. We performed several experiments on chemical systems with Zr, Hf, No, and Rf, determined their adsorption coefficients, and deduced the K d values for Rf.

Coprecipitation experiment with Sm hydroxide using a multitracer produced by nuclear spallation reaction: A tool for chemical studies with superheavy elements

To establish a new methodology for superheavy element chemistry, the coprecipitation behaviors of 34 elements with samarium hydroxide were investigated using multitracer produced by a spallation of Ta. The chemical reactions were rapidly equilibrated within 10s for many elements. In addition, these elements exhibited individual coprecipitation behaviors, and the behaviors were qualitatively related to their hydroxide precipitation behaviors. It was demonstrated that the ammine and hydroxide complex formations of superheavy elements could be investigated using the established method.

Solvent extraction of Zr and Hf from hydrochloric acid using tributylphosphate for the extraction of element 104, rutherfordium

For the solvent extraction experiment on element 104 (Rf), solvent extraction of Zr and Hf as its homologues was performed in tributylphosphate (TBP)/hydrochloric acid (HCl) system using the carrier-free radiotracers88Zr and175Hf. Time dependences of the distribution ratios of Zr and Hf were investigated using 6.1 and 10.0M HCl and 0.5 and 2.0M TBP benzene solutions. The distribution ratios in equilibrium were determined for these elements as a function of HCl concentrations in the range of 4.1-10.2M. We found that the neutral chloride complexes of Zr and Hf were formed and extracted into the organic phase within 20 min in HCl with a concentration higher than 6 M. From the results, we propose that solvent extraction of Rf from 4-8M HCl into 2.0M TBP benzene is suitable for investigating the chloride complexation properties of Rf. In addition, for the development of the rapid liquid-liquid extraction apparatus, three types of microchannel devices, namely, a micro reactor, capillary tube and micro-chemical chip, were tested as a mixing-solution part in the apparatus. The chemical reactions of Zr and Hf in the extraction were found to be fast only when using the microchemical chip, which is important for investigations with the 68-s261Rf.

Sulfate complexation of element 104, Rf, in H2SO4/HNO3 mixed solution

The cation-exchange behavior of 261Rf (T 1/2= 78 s) produced in the 248Cm(18O, 5n) reaction was studied on a “one-atom-at-a-time” scale in 0.15–0.69 M H2SO4/HNO3 mixed solutions ([H+]=1.0 M) using an automated ion-exchange separation apparatus coupled with the detection system for alpha-spectroscopy (AIDA). It was found that adsorption probabilities ( decrease with an increase of [HSO4 −], showing a successive formation of Rf sulfate complexes. Rf exhibits a weaker complex formation tendency compared to the lighter homologues Zr and Hf. This is in good agreement with theoretical predictions including relativistic effects.

Aqueous-phase chemistry of the transactinides

The experimental techniques developed to perform rapid chemical separations of the heaviest elements in the aqueous phase are presented. In general, these include transport of the nuclear reaction products to a separation device by the gas-jet technique and dissolution in an aqueous solution containing inorganic ligands for complex formation. The complexes are chemically characterized by a partition method which can be liquid–liquid extraction, ion-exchange- or reversed-phase extraction chromatography. The separated fractions are quickly evaporated to dryness for the preparation of samples for α-particle spectroscopy. Comments are given on the special situation in which chemistry has to be studied with single atoms. Theoretical predictions of chemical properties are compared to the presently known chemical behaviour of rutherfordium, Rf (element 104), dubnium, Db (element 105), seaborgium, Sg (element 106), and hassium, Hs (element 108) and to that of their lighter homologs in the Periodic Table in order to assess the role of relativistic effects in the chemistry of the heaviest elements.

Oxidation of element 102, nobelium, with flow electrolytic column chromatography on an atom-at-a-time scale

We report here on the successful oxidation of element 102, nobelium (No), on an atom-at-a-time scale in 0.1 M alpha-hydroxyisobutyric acid (alpha-HIB) solution using a newly developed technique, flow electrolytic column chromatography. It is found that the most stable ion, No(2+), is oxidized to No(3+) within 3 min and that the oxidized No complex with alpha-HIB holds the trivalent state in the column above an applied potential of 1.0 V.

Chemical studies on rutherfordium (Rf) at JAERI

Chemical studies on element 104, rutherfordium (Rf), at JAERI (Japan Atomic Energy Research Institute) are reviewed. The transactinide nuclide261Rf has been produced in the reaction248Cm(18O, 5n) at the JAERI tandem accelerator with the production cross section of about 13 nb. On-line anion-exchange experiments on Rf together with the lighter homologues, group-4 elements Zr and Hf, in acidic solutions have been conducted with a rapid ion-exchange separation apparatus. From the systematic study of the anion-exchange behavior of Rf, it has been found that the properties of Rf in HCl and HNO3solutions are quite similar to those of Zr and Hf, definitely confirming that Rf is a member of the group-4 elements. However, we have observed an unexpected chemical behavior of Rf in HF solutions; the fluoride complex formation of Rf is significantly different from those of the homologues. Prospects of extending chemical studies on transactinide elements in the near future at JAERI are briefly considered.

Chemistry of Superheavy Elements

The number of chemical elements has increased considerably in the last few decades. Most excitingly, these heaviest, man-made elements at the far-end of the Periodic Table are located in the area of the long-awaited superheavy elements. While physical techniques currently play a leading role in these discoveries, the chemistry of superheavy elements is now beginning to be developed. Advanced and very sensitive techniques allow the chemical properties of these elusive elements to be probed. Often, less than ten short-lived atoms, chemically separated one-atom-at-a-time, provide crucial information on basic chemical properties. These results place the architecture of the far-end of the Periodic Table on the test bench and probe the increasingly strong relativistic effects that influence the chemical properties there. This review is focused mainly on the experimental work on superheavy element chemistry. It contains a short contribution on relativistic theory, and some important historical and nuclear aspects.