Volume 91, Issue 8 -

We present results of the second experiment on the chemical identification of element 112. Similar to the first test in 2000, we aimed at the production of the spontaneously fissioning 283 112 nuclei with T 1/2 ≈3min. A nat U 3 O 8 (with some Nd) target, 2mg of U/cm 2 thick, was bombarded with 233-MeV 48 Ca ions (the energy in the middle of the target layer). The nuclei recoiling from the target were thermalized in flowing helium. The target chamber was connected through a 25m long capillary to detectors of α -particles and fission fragments. All the equipment and detectors were kept at ambient temperature. According to the test experiments, of all the heavy elements produced in the bombardment, only Hg, Rn and At could be transported to the detectors. The first detecting device was similar to that used earlier – an assembly of 8 pairs of PIPS detectors coated with gold. Here one would observe the decay of element 112 atoms if they like Hg adsorbed on gold. The atoms which were not retained and freely passed through the PIPS detectors entered a new, flow-through ionization chamber, 5000 cm 3 in volume, optimized for detecting fission fragments. The PIPS detectors and the ionization chamber were placed inside a large assembly of 3 He – filled neutron counters to detect prompt neutrons from the fission events. In 22.5 days, a beam dose of 2.8×10 18 ions was accumulated. More than 95 of the simultaneously produced α -active 185 Hg ( T 1/2 =49 s) were found deposited already on the first pair of PIPS detectors; meanwhile, all the PIPSs did not detect any fission event. In the ionization chamber, eight fission events were observed in coincidence with neutron counts while the expected background was insignificant. Hence, the spontaneous fissions of the volatile activity can be conclusively attributed to the decay of element 112 produced in the fusion reaction 48 Ca+ 238 U, and formerly observed in Dubna physical experiments. Evaluation of the experimental data in terms of the adsorption enthalpies indicates much weaker interaction of element 112 with Au than that of Hg. One can conclude that in the given chemical environment, element 112 behaves like Rn rather than like Hg. The formation cross section of 283 112 estimated from the data amounts to several pb. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions at JINR in November–December 2001.

The first chemical studies of the monopositive einsteinium ion, Es + , in the gas phase have been carried out, and its behavior compared to those for other actinide ions, with a particular focus on Bk + . The yield of laser-ablated EsO + indicates that the Es + -O bond energy is significantly smaller than that of Bk + -O. Fluorination of Es + and Bk + through F-abstraction from hexafluoropropene demonstrated clearly the stability of the divalent state of Es: whereas by this process both BkF + and BkF 2 + are produced, only the "divalent" EsF + product is formed. The reaction of Es + with several different alkenes produced only small yields of adducts ( e.g. , EsC 4 H 8 + ). These adducts are the first organoeinsteinium complexes to be identified. Whereas Es + was inert towards 1,5-cyclooctadiene, in contrast Bk + dehydrogenated this substrate to give BkC 8 H 8 + , demonstrating the greater activation activity of Bk + than Es + towards alkenes. The chemical behaviors observed in these studies are consistent with the predicted electronic structure and energetics of Es + , and the results obtained are discussed in the context of systematic trends across the actinide series.

The chemical properties of americium chlorides have been studied in the fused eutectic LiCl-KCl as a function of temperature ranging from 743K to 943K. Titration of Am(III) chlorocomplex by O 2- demonstrated the existence of soluble AmO + species as well as the precipitation of AmOCl and Am 2 O 3 solid compounds. The corresponding solubility products of AmOCl and Am 2 O 3 have been determined. Calculated Am(II) disproportionation equilibrium constant indicates that the stability of Am(II) chlorocomplex increases with the increase of temperature.

In the field of nuclear waste storage, knowledge of the sorption behavior of radionuclides onto mineral surfaces is of fundamental importance in order to evaluate environmental impacts. We have studied uranyl ion sorption mechanisms onto zirconium silicate and zirconium oxide with a double approach, coupling both macroscopic and structural investigations. The spectroscopic results, reported in a previous paper, allowed a complete determination of the different species involved in the studied sorption reactions. This paper presents the results of the macroscopic part of the study. Surface characterizations of the solids were first carried out: determination of the pH of the point of zero charge, of the surface site densities and of the surface acidity constants. The experimental retention data were then interpreted in terms of a constant capacitance model using the FITEQL code. Results from structural investigation were used to constrain the modeling. This coupled approach led to accurate reactions stoichiometry and associated sorption constants values, since they were determined with respect to different experimental results, both macroscopic and microscopic.

The aqueous dissolution of synthetic brannerite (UTi 2 O 6 ) in an open atmosphere has been investigated. Previous data in the literature have been combined with new experimental work, dealing with the release of uranium from brannerite as a function of solution pH and aqueous carbonate species, in oxygenated solutions. From these data we have developed a conceptual model for uranium release from brannerite consisting of two reaction steps: oxidation of surface uranium(IV) atoms, and subsequent detachment of U(VI) atoms into solution, which is catalysed by surface coordination with protons (acidic media) or carbonate species (alkaline media in equilibrium with the atmosphere). A kinetic rate law is derived for this simple reaction mechanism and fitted to experimental data. The resulting predictive equation for uranium release qualitatively describes the pH-dependent behaviour observed in experiment, and quantitatively gives an upper limit for uranium release from brannerite over a range of conditions and experiment types.

The limiting transport properties, i.e . the limiting ionic conductivity (λ°) and the limiting diffusion coefficient ( D °), of lanthanide and actinide ions at 298.15K have been calculated by means of the microscopic version of the Stokes–Einstein law involving (i) the effective charge and the ionic radius of the ions and (ii) the ion micro-viscosity, i.e . the viscosity of the hydrated water molecules in the vicinity of the ion. The latter quantity was derived from the variation of the dynamic properties of the water molecules in the first hydration shell with the surface charge density of common mono-atomic cations of various charges. The obtained results were found to be consistent with the experimental data given in the literature.

A method for simultaneous quantitative determination of Mo(VI), Re(VII), Ru(III) and NO 3 - ions in aqueous solutions, using capillary zone electrophoresis (CZE) technique with direct UV detection, was developed. Electrophoretic mobility of Ru(III) ions in 0.1M NaHCO 3 solution was determined as 6.0×10 -8 m 2 V -1 s -1 at 25 °C. The best analytical characteristics of the developed method were achieved with a 100mM carbonate buffer solution containing 0.3mM TTAB, at a pH value of 9.8 and at an applied voltage of 10kV. Under these conditions, a linear relationship between the measured absorbance (λ=230nm) and the concentration of the species under study were observed in the range 5×10 -6 –10 -3 M for Mo(VI), 7×10 -6 –10 -3 M for Re(VII) and 10 -5 – -4 M for Ru(III). The minimum detectable concentrations were determined as 4.1×10 -6 M for NO 3 - , 5.2×10 -6 M for Mo(VI), 7.4×10 -6 M for Re(VII) and 1.0×10 -5 M for Ru(III). The accuracy of quantitative determination was estimated as 3%–5% for NO 3 - and 5%–6% for Mo(VI), Re(VII) and Ru(III) ions. The presence of 0.06M NaNO 3 (HNO 3 ) and of 10 -3 M U(VI) in the sample affected neither the detection limits of the species under study nor the selectivity of the method.

Nuclear microprobe analysis has been used to study the aqueous sorption of cadmium and selenium at different solution pH values onto hydroxyapatite. By coupling Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering (RBS) microanalyses, we have quantified the concentration and modeled the depth distribution of Cd and Se in particles of calcium hydroxyapatites (HAP). No cadmium precipitation was observed for those experiments in which the initial concentration was greater than the solubility of Cd(OH) 2 and CdCO 3 at solution pH=9 and 12. The observation of the solid by transmission and scanning electron microscopies, SEM and TEM, showed the polycrystalline nature of the particles; these particles were constituted by aggregated nanocrystals. This additional information allowed us to interpret the incorporation of Cd and Se into the apatitic substrate as a penetration in the intra-particle porosity generated by this aggregation of crystals and probably not as diffusion in the solid structure.