Heavy Actinides Research Articles

Electronic structure and spectral properties of Am, Cm, and Bk: Charge-density self-consistentLDA+HIAcalculations in the FP-LAPW basis

We provide a straightforward and numerically efficient procedure to perform local-density-$\text{approximation}+\text{Hubbard}$ I approximation $(\text{LDA}+\text{HIA})$ calculations, including self-consistency over the charge density, within the full potential linearized augmented plane-wave (FP-LAPW) method. This implementation is all-electron, includes spin-orbit interaction, and makes no shape approximations for the charge density. The method is applied to calculate selected heavy actinides in the paramagnetic phase. The electronic structure and spectral properties of Am and Cm metals obtained are in agreement with previous dynamical mean-field theory $(\text{LDA}+\text{DMFT})$ calculations and with available experimental data. We point out that the charge-density self-consistent $\text{LDA}+\text{HIA}$ calculations predict the $f$ charge on Bk to exceed the atomic integer ${f}^{8}$ value by 0.22.

A carbon-cluster laser ion source for TRIGA-TRAP**This paper comprises partly the PhD theses of J Ketelaer and C Smorra.

A new laser ablation ion source was developed and tested for the Penning trap mass spectrometer TRIGA-TRAP in order to provide carbon-cluster ions for absolute mass calibration. Ions of different cluster sizes up to C+24 were successfully produced, covering the mass range up to the heavy actinide elements. The ions were captured in a Penning trap, and their time-of-flight cyclotron resonances recorded in order to determine their cyclotron frequency. Furthermore, the same ion source was used to produce GdO+ ions from a gadolinium target in sufficient amount for mass spectrometry purposes. The design of the source and its characteristics are presented.

Long-termα- and spontaneous fission measurement of aRf/Dbsample chemically prepared in aCa48onAm243experiment

Results from long-term measurements of a chemically separated Db/Rf sample prepared from the products of a $^{48}\mathrm{Ca}$ on $^{243}\mathrm{Am}$ irradiation are presented. The sample with the highest spontaneous fission activity out of eight samples produced in the course of chemical experiments performed in 2004 was selected for these measurements. We conclude that there is no evidence for SF-decay originating from heavy actinide isotopes in this sample. Hence, it is appropriate to assign the SF-events observed in this experiment to decay products of $^{288}115$.

Synthesis of superheavy nuclei: A search for new production reactions

Nuclear reactions leading to the formation of new superheavy (SH) elements and isotopes are discussed in the paper. ``Cold'' and ``hot'' synthesis, fusion of fission fragments, transfer reactions, and reactions with radioactive ion beams are analyzed along with their abilities and limitations. If the possibility of increasing the beam intensity and the detection efficiency (by a total of one order of magnitude) is found, then several isotopes of new elements with $Z=120\text{\ensuremath{-}}124$ could be synthesized in fusion reactions of titanium, chromium, and iron beams with actinide targets. The use of light- and medium-mass neutron-rich radioactive beams may help us fill the gap between the SH nuclei produced in the hot fusion reactions and the mainland. In these reactions, we may really approach the ``island of stability.'' Such a possibility is also provided by the multinucleon transfer processes in low-energy damped collisions of heavy actinide nuclei. The production of SH elements in fusion reactions with accelerated fission fragments looks less encouraging.

Relative longevity of odd-N nuclei in the heavy actinide region

It is shown that, contrary to the normal feature witnessed all across the periodic table that even–even nuclei are more abundant/longer-lived than their odd-A neighbours, many odd-mass heavy nuclei are seen to be much longer-lived than their even–even neighbours. In the trans-plutonium domain of even-Z actinides, all the predominantly alpha-decaying odd-N nuclei are longer-lived than their even-mass neighbours. Similar behaviour is observed for heavy odd-Z actinides as well. A semi-empirical criterion is stated to determine the relative longevity of the odd-N and even-N nuclei. This feature is sought to be explained by examining the respective alpha decay rates. A detailed discussion of α-decays of the β-stable curium (Z = 96) isotopic chain is presented to illustrate this phenomenon. Correspondence with a similar occurrence in spontaneously fissioning nuclei is briefly pointed out.

High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures

AbstractHigh-pressure studies have been performed on heavy rare earth metals Terbium (Tb) to 155 GPa and Holmium (Ho) to 134 GPa in a diamond anvil cell at room temperature. The following crystal structure sequence was observed in both metals hcp ⟶ Sm-type ⟶ dhcp ⟶ distorted fcc (hR-24) ⟶ monoclinic (C2/m) with increasing pressure. The last transformation to a low symmetry monoclinic phase is accompanied by a volume collapse of 5 % for Tb at 51 GPa and a volume collapse of 3 % for Ho at 103 GPa. This volume collapse under high pressure is reminiscent of f-shell delocalization in light rare earth metal Cerium (Ce), Praseodymium (Pr), and heavy actinide metals Americium (Am) and Curium (Cm). The orthorhombic Pnma phase that has been reported in Am and Cm after f-shell delocalization is not observed in heavy rare earth metals under high pressures.

One-electron physics of the actinides

We present a detailed analysis of the one-electron physics of the actinides. Various linear muffin-tin orbital basis sets are analyzed in order to determine a robust bare Hamiltonian for the actinides. The hybridization between $f$ and $spd$ states is compared with the $f\text{\ensuremath{-}}f$ hopping in order to understand the Anderson-like and Hubbard-like contributions to itineracy in the actinides. We show that both contributions decrease strongly as one moves from the light actinides to the heavy actinides, while the Anderson-like contribution dominates in all cases. A real-space analysis of the band structure shows that nearest-neighbor hopping dominates the physics in these materials. Finally, we discuss the implications of our results to the delocalization transition as a function of atomic number across the actinide series.

Preparation of a novel macroporous silica-based 2,6-bis(5,6-diisobutyl-1,2,4-triazine-3-yl)pyridine impregnated polymeric composite and its application in the adsorption for trivalent rare earths

A novel macroporous silica-based 2,6-bis(5,6-diisobutyl-1,2,4-triazine-3-yl)pyridine (iso-Bu-BTP), a neutral chelating agent having several softatom nitrogen, polymeric composite (iso-Bu-BTP/SiO2-P) was synthesized. It was done through impregnation and immobilization of iso-Bu-BTP molecule into the pores of SiO2-P particles with 40–60 μm of bead diameter and 0.6 μm of mean pore size. The effective impregnation resulted from the intermolecular interaction of iso-Bu-BTP and co-polymer inside the SiO2-P particles by a vacuum sucking technique. To understand the possibility of applying iso-Bu-BTP in the MAREC process developed, the adsorption behavior of a few representative rare earths (REs) such as Ce(III), Nd(III), Gd(III), Dy(III), Er(III), Yb(III), and Y(III) towards iso-Bu-BTP/SiO2-P was investigated at 298 K. The influence of the HNO3 concentration in a wide range of pH 5.52–3.0M and a few chelating agents such as formic acid, citric acid, and diethylenetriaminepentaacetic acid (DTPA) on the adsorption of RE(III) was examined. It was found that in the presence of chelating agent, the adsorption ability of the tested RE(III) towards iso-Bu-BTP/SiO2-P decreased due to two competition reactions of RE(III) with iso-Bu-BTP/SiO2-P and chelating agents. In a 0.01M HNO3 solution containing 1M formic acid or 1M citric acid, light RE(III) showed lower adsorption towards iso-Bu-BTP/SiO2-P than that of the heavy one. This makes the separation of light RE(III) from the heavy one possible. Based on the similarity of minor actinides and heavy RE(III) in chemical properties and the results of column separation experiments, chromatographic partitioning of light RE(III) from a simulated high level liquid waste solution composed of the heavy RE(III) and minor actinides in MAREC process is promising.

Study of heavy nuclei at FLNR (Dubna)

Experiments are described and main results presented on the synthesis and decay properties of superheavy nuclides, produced in fusion reactions induced by a 48Ca-beam on heavy actinide targets. In such reactions neutron-rich nuclei are formed. For them, according to theory, an abrupt enhancement of stability due to nuclear shell effects is expected. The decay properties of the new nuclides are compared with calculations of theoretical models, which predict the existence of “islands of stability” in the region of hypothetical superheavy elements.

Theoretical description of metal–ligand bonding within f-element complexes: A successful and necessary interplay between theory and experiment

Abstract The quantum chemical study presented here shows various aspects of the bonding of lanthanide (La 3+ , Gd 3+ ) and actinide (U 3+ , Am 3+ , Cm 3+ ) ions with N -heterocyclic ligands (polyazines, BTP: bis(1,2,4-triazinyl)-2,6-pyridine). Several families of complexes, differing by their coordination sphere, have been examined. Clearly, the lanthanide complexes always show a purely ionic bonding. The behaviour of U(III) is also well defined with a more or less strong backbonding interaction whatever the complex is. In contrast, the heavy actinides (Am 3+ and Cm 3+ ) are changeable, with a weak covalent character, going from donation to backdonation, depending on the coordination sphere of the complex.

Ionization potentials of neutral atoms and positive ions in the limit of large atomic number

The many-electron Schr\"odinger equation allows one to make predictions for the nonrelativistic ionization potentials of both highly charged positive atomic ions and neutral atoms in the limit of large atomic number $Z$. Beginning with theoretical configuration interaction data on both Li- and Be-like positive atomic ions by K.T. Chung et al. [Phys. Rev. A 45, 7766 (1992); Phys. Rev. A 47, 1740 (1993)], their nonrelativistic first ionization potentials $I$ are plotted in the form of $I∕{Z}^{2}$ versus $1∕Z$. In these two sequences, it is then clear that ${\mathrm{lim}}_{Z\ensuremath{\rightarrow}\ensuremath{\infty}}(I∕{Z}^{2})$ remains finite and that this limit is approached essentially linearly. Interpretation is afforded by means of the so-called $1∕Z$ expansion of atomic theory. The question of the behavior of the nonrelativistic ionization potentials for large $Z$ is then addressed for neutral atoms. We perform density functional theory (DFT) calculations using the local density approximation with Dirac exchange and neglecting Coulomb correlation for the heavy actinides as well as for the rare gas and alkali atoms from $Z=10$ up to $Z=291$. Our results suggest that there is a nonzero value for ${\mathrm{lim}}_{Z\ensuremath{\rightarrow}\ensuremath{\infty}}I$ for both the noble gases and the alkalis, the second one implying that there is a lower bound of about $2.56\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to the ionization potential of any nonrelativistic neutral atom. We also explore an alternative approach via the one-body potential $V(r)$ of DFT by calculating the eigenenergies of the highest occupied atomic orbital in neutral atoms. Finally, relativistic corrections needed to compare measured ionization potentials to the predictions from the Schr\"odinger equation are briefly considered.

Study of Heavy Nuclei at FLNR (Dubna)

Experiments are described and main results presented on the synthesis and decay properties of superheavy nuclides, produced in fusion reactions induced by a 48Ca-beam on heavy actinide targets. In such reactions neutron-rich nuclei are formed. For them, according to theory, an abrupt enhancement of stability due to nuclear shell effects is expected. The decay properties of the new nuclides are compared with calculations of theoretical models, which predict the existence of “islands of stability” in the region of hypothetical superheavy elements.

Production and Characterization of Siderophores and its Application in Arsenic Removal from Contaminated Soil

Siderophores are small molecular weight extracellular organic compounds secreted by microorganisms under iron-starved conditions, used by them to chelate and solubilize iron. Though they are specific ferric iron chelator, but is reported that they bind other metals also, such as divalent heavy metals and actinides because of potentially high metal-siderophore stability constants. Thus metal contaminant fate and transport in subsurface environment can be heavily influenced by siderophores. This approach can be successfully used in removing many toxic metals off the soil which poses a serious health threat. Our research focuses on the correlation between cell growth and siderophore production and chemical characterization of the siderophore type. Its also documents the development of an assay method for the screening of different metals for complexation with siderophores based on the Chrome Azurol S (CAS) assay. The present research aims at batch scale mobilization of arsenic from arsenic contaminated soils using siderophore produced by P. azotoformans and thus evaluating its efficiency as compared to Ethylene Diamine Tetra Acetic Acid (EDTA), Citric Acid (CA) for the same. FT-IR spectroscopic studies were carried out to determine the interaction between soil, arsenic and siderophore. Results have shown that the cell growth and siderophore production are inversely related. Characterization of siderophore produced by P. azotoformans has revealed that it is of mixed-type catecholate and hydroxamate. Siderophore was found to complex with heavy metals like Cadmium, Lead, Nickel, Arsenic (III, V), Aluminium, Magnesium Zinc, Copper, Cobalt, Strontium other than Iron. Five washings by siderophore, EDTA, CA removed almost 92.8%, 77.3%, 70.0% arsenic respectively as compared to only 33.8% removal by control. Washing of arsenic contaminated soil with tap water revealed that ≈ 65.8% of arsenic in contaminated soil is in freely available or weakly bound form. The IR spectra revealed that hydrogen bonding exists between siderophore, arsenic and soil. Encouraging results of arsenic removal by biomolecule-siderophore can lead to an emerging tool brimming with opportunities for environmental clean up.

A simple model for the f–d transition of actinide and heavy lanthanide ions in crystals

The original simple model for 4 f –5 d transitions [C.K. Duan, M.F. Reid, G.W. Burdick, Phys. Rev. B 66 (2002) 155108; C.K. Duan, M.F. Reid, J. Solid State Chem. 171 (2003) 299], which assumes a stronger exchange Coulomb interaction between 4 f orbitals and 5 d orbitals than the 4 f spin–orbit interaction, is extended to the case where the effect of the f (4 f for heavy lanthanide and 5 f for actinide ions) spin–orbit interaction is stronger than that of the exchange Coulomb interaction. The extended model is used to discuss the 4 f –5 d spectra of heavy lanthanide and actinide ions in crystals.

Extension of a Simple Model for f−d Transition andApplications to Reassignment of Excitation Spectra of Ho3+-DopedCrystals

We present an extension of the original simple modelfor f–d transition proposed by Duan and co-workers[Phys. Rev. B 66 (2002) 155108; J. Alloys Compound. 366 (2004) 34] tothe case applicable to heavy lanthanide and actinide ions in crystalsand apply it to the interpretation of the low-temperaturef–d excitation spectra of Ho3+ doped in crystals LiYF4 andCaF2. A consistent assignment of all the transition bands isobtained.

Experimental Identification of Spin-Parities and Single-Particle Configurations inNo257and Itsα-Decay DaughterFm253

alpha-gamma and alpha-electron coincidence spectroscopy for a short-lived heavy actinide nucleus (257)No (T(1/2) = 24.5 s) has been performed using a gas-jet transport system and an on-line isotope separator. Spin-parities of excited states in (253)Fm fed by the alpha decay of (257)No have been identified on the basis of the measured internal conversion coefficients. The nu3/2(+)[622] configuration has been assigned to the ground state of (257)No as well as to the 124.1 keV level in (253)Fm. It was found that the ground-state configuration of (257)No is different from that of lighter N = 155 isotones.

Study of the f → d transition of heavy lanthanide and actinide ions in crystals using the simple model

AbstractIn this paper we extend the original simple model for f → d transition proposed by Duan and co‐workers [Phys. Rev. B 66, 155108 (2002); J. Solid State Chem. 171, 299 (2003)] to the more general cases applicable to heavy lanthanides and actinides. The model is successfully applied to the interpretation of low‐temperature f–d excitation spectra of Er3+ doped in crystals LiYF4 and CaF2. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of Copper Binding in the Ternary System Cu2+/Humic Acid/Goethite at Neutral to Acidic pH

Binding of heavy metal and actinide ions to natural colloids, such as humic substances (HSs) and metal (hydr)-oxides, plays an important role in the ecotoxicological behavior of these ions. Several thermodynamic models have been constructed to predict the speciation of these ions in metal/HS or metal/oxide binary systems. However, in natural environments the adsorption of HSs on oxides can influence the binding of target metals, leading to deviation from the additivity of calibrated binary models. In this study binding of copper (Cu2+) to the purified Aldrich humic acid (PAHA)/goethite complex in the neutral to acidic pH region was investigated by measuring Cu2+ binding isotherms. The measured isotherms were compared with the results obtained for the binary systems under similar conditions. The comparison revealed that Cu2+ binding in the ternary system is enhanced with respect to the sum of Cu2+ binding in the corresponding binary systems. From the analysis of the charging behavior of the adsorbed PAHA as well as the smeared-out potential profile near the PAHA/goethite interface, the increase of Cu2+ binding to the complex was mainly attributed to the decrease of proton competition to the functional groups of the adsorbed PAHA and the change of the electrostatic potential in the vicinity of the goethite surface.

Quantum Chemical Calculations Show that the Uranium Molecule U2 Has a Quintuple Bond.

Covalent bonding is commonly described by Lewis's theory1, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description2 for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for actinide–actinide bonds is restricted to the matrix-isolation3 of uranium hydrides, including H2U–UH2, and the gas-phase detection4 and preliminary theoretical study5 of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each—so all known covalent bonding types are contributing.

Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides

In solution thermodynamics, and more recently in surface chemistry, it is well established that relationships can be found between the free energies of formation of aqueous or surface metal complexes and thermodynamic properties of the metal ions or ligands. Such systematic dependencies are commonly termed linear free energy relationships (LFER). A 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model has been used to model “in house” and literature sorption edge data for eleven elements: Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) to provide surface complexation constants for the strong sites on montmorillonite. Modelling a further 4 sets of sorption isotherms for Ni(II), Zn(II), Eu(III) and U(VI) provided complexation constants for the weak sites. The protolysis constants and site capacities derived for the 2SPNE SC/CE model in previous work were fixed in all of the calculations. Cation exchange was modelled simultaneously to provide selectivity coefficients. Good correlations between the logarithms of strong S K x−1 and weak W1 K x−1 site binding constants on montmorillonite and the logarithm of the aqueous hydrolysis constants OH K x were found which could be described by the following equations: Strong (≡S SOH) sites: log S K X − 1 = 8.1 ± 0.3 + ( 0.90 ± 0.02 ) log OH K X Weak (≡S W1OH) sites: log W 1 K X − 1 = 6.2 ± 0.8 + ( 0.98 ± 0.09 ) log OH K X where x is an integer. Sorption data for heavy metals and actinides such as Pd(II), Pb(II), Pu(III), Zr(IV), U(IV), Np(IV), Pu(IV) and Pa(V), are important in safety studies for radioactive waste repositories and are either very poorly known or not available at all. The LFER approach was used to estimate surface complexation constants for these radionuclides on both site types. The surface protolysis constants, site capacity values, selectivity coefficients and surface complexation constants given in this work, coupled with the LFERs established for the strong and weak sites on montmorillonite, are considered to form a sound basis for a thermodynamic sorption database.