A short review is given on our current anion-exchange studies of element 104, rutherfordium (Rf), in HCI and HNO3 solutions. The distribution coefficients of the Rf homologues Zr, Hf, and Th(IV), and Pu(IV) on an anion-exchange resin were measured in 1.0-11.5 M HC1 and 1.1-13.1 M HNO3 with a batch method using the radiotracers Zr, Hf, Th, and Pu. In experiments for the short-lived Rf, the isotopes Zr, Hf, and Rf were produced in the O-induced reactions on Ge, Gd, and Cm targets, respectively, and their anion-exchange behavior in 4.0-11.5 M HCI and 8.0 M HNO3 was investigated using the Automated Ion-exchange separation apparatus coupled with the Detection system for Alpha spectroscopy (AIDA). In the HCl system, the percent adsorption of Rf on the anion-exchange resin increases steeply with increasing HCl concentration from 7.0 M to 11.5 M. This adsorption behavior is similar to that of the group-4 elements Zr and Hf, and is quite different from that of the pseudo-homologue Th(IV). The percent adsorption decreases in the order Rf>Zr>Hf.In the HNO3 system, Rf behaves like Zr and Hf in 8.0 M HNO3 but not like Pu(IV) and Th(IV), implying the formation of cationic or neutral species.
Fluoride complexation of element 104, rutherfordium (Rf), produced in the 248Cm(18O,5n)261Rf reaction has been studied by anion-exchange chromatography on an atom-at-a-time scale. The anion-exchange chromatographic behavior of Rf was investigated in 1.9-13.9 M hydrofluoric acid together with those of the group-4 elements Zr and Hf produced in the 18O-induced reactions on Ge and Gd targets, respectively. It was found that the adsorption behavior of Rf on anion-exchange resin is quite different from those of Zr and Hf, suggesting the influence of relativistic effects on the fluoride complexation of Rf.
Fission cross sections and fission fragment mass distributions were measured in the reactions of 40 Ca + 238 U and 48 Ca + 238 U at energies around the Coulomb barrier. Fusion probabilities were calculated based on the fluctuation dissipation model. The measured mass distributions for both reactions showed an asymmetric shape at low incident energies, whereas the distribution changed to a flat shape at higher energies. The variation of the mass distribution is explained by a change of the ratio between fusion and quasifission with nuclear orientation. The calculation reproduced the mass distributions and their energy dependence. The trajectories for fusion-fission were used to determine the fusion probability. Fusion probabilities for both reactions are identical as function of the center-of-mass energy (E c.m. ), but they differ when plotted as function of the excitation energy (E * ). Evaporation residue cross sections were calculated for the reaction 48 Ca + 238 U using a statistical model and the obtained fusion cross sections as input values. The results are compared to experimental data.
Fission-fragment mass distributions were measured for ^{237-240}U, ^{239-242}Np, and ^{241-244}Pu populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction ^{18}O+^{238}U at the Japan Atomic Energy Agency tandem facility. Among them, the data for ^{240}U and ^{240,241,242}Np were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.
Formation of anionic fluoride-complexes of element 104, rutherfordium, produced in the 248 Cm( 18 O, 5n) 261 Rf reaction was studied by anion-exchange on an atom-at-a-time scale. It was found that the hexafluoro complex of Rf, [RfF 6 ] 2− , was formed in the studied fluoride ion concentrations of 0.0005-0.013 M. Formation of [RfF 6 ] 2− was significantly different from that of the homologues Zr and Hf, [ZrF 6 ] 2− and [HfF 6 ] 2− ; the evaluated formation constant of [RfF 6 ] 2− is at least one-order of magnitude smaller than those of [ZrF 6 ] 2− and [HfF 6 ] 2− .
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