Volume 102, Issue 5

The neutron spectrum in the energy range of 0.1 to 5.85 MeV in the 0 ○ direction with respect to the incident proton beam accelerated in a Tandem Accelerator of the Wakasa Wan Energy Research Center (WERC), Japan, has been characterized using the multiple foil activation technique. Flux mapping of thermal and epithermal neutrons at 0 ○ , 20 ○ , 40 ○ and 60 ○ at downstream of the beam has also been carried out. The fast neutron spectrum averaged cross sections of several threshold reactions were measured. Two monitor reactions, 58 Ni( n ,  p ) 58 Co and 115 In( n ,  n ′,  γ ) 115m In, were used to determine the fast neutron flux. The use of fast neutron flux in validating some excitation functions is described.

Single crystals of the two new compounds [Am V O 2 (dipy)OOCCH 3 (H 2 O)] ( I ) and Cs 2 [(Am VI O 2 ) 2 (SO 4 ) 3 ] ( II ) have been synthesized and their structures have been determined by X-ray structural analysis. IR and optical spectra of polycrystalline compounds I and II were measured and interpreted. The structure I is formed by electroneutral complexes [Am V O 2 (dipy)OOCCH 3 (H 2 O)], in which dioxocations AmO 2 + are coordinated by two N atoms of a bidentate-chelating dipy ligand, two O atoms of a bidentate-chelating acetate anion and one O atom of water molecule. Due to a different nature of ligands, the distances and bond angles in the equatorial plane of AmO 2 + have a significant spread. The structure II is built of interlaced anionic and cationic layers. Anionic layers [(Am VI O 2 ) 2 (SO 4 ) 3 ] n 2 n − contain AmO 2 2+ groups and two types of SO 4 2− anions: tridentate- and tetradentate-bridging. Each AmO 2 2+ is coordinated in a monodentate way by three tridentate anions and two tetradentate ones. The coordination numbers of the outer Cs + cations are equal to 10 for Cs(1), and 16 for Cs(2). The dioxocations AmO 2 in both structures are linear and almost symmetric. Analysis of the obtained absorption spectra showed that the number and intensity of the absorption bands in them are dependent both on the equatorial ligand nature, and on the americium ion electronic configuration.

Wet chemistry experiments were carried out to investigate the effect of the redox state and aqueous speciation on the uptake of neptunium by titanium dioxide (TiO 2 ) and by calcium silicate hydrates (C-S-H) under alkaline conditions. TiO 2 was chosen as a reference sorbent to determine the surface complexation behaviour of neptunium under alkaline conditions. C-S-H phases are important constituents of cement and concrete. They may contribute significantly to radionuclide retention due to their high recrystallization rates making incorporation the dominating sorption mechanism for many radionuclides ( e.g. the actinides) on these materials. The sorption of neptunium on both solids was found to depend strongly on the degree of hydrolysis. On TiO 2 R d values for Np(IV), Np(V) and Np(VI) are identical at pH = 10 and decrease with progressing hydrolysis in case of Np(V) and Np(VI). On C-S-H phases, R d values for the three redox states are also identical at pH = 10. While the R d values for Np(VI) sorption on C-S-H phases decrease with progressing hydrolysis, the R d values for Np(IV) and Np(V) sorption are not affected by the pH. In addition to the effect of hydrolysis, the presence of Ca is found to promote Np(V) and Np(VI) sorption on TiO 2 whereas on C-S-H phases, the present wet chemistry data do not give unambiguous evidence. Thus, the aqueous speciation appears to have a similar influence on the sorption of the actinides on both types of solids despite the different sorption mechanism. The similar R d values for Np(IV,V,VI) sorption at pH = 10 can be explained qualitatively by invoking inter-ligand electrostatic repulsion between OH groups in the coordination sphere of Np(V) and Np(VI). This mechanism was proposed earlier in the literature for the prediction of actinide complexation constants with inorganic ligands. A limiting coordination number for each Np redox state, resulting from the inter-ligand electrostatic repulsion, allows the weaker sorption of the highest hydrolysed Np(V,VI) species to be explained.

Batch sorption experiments have been carried out to understand the interaction of different metal cations such as Am(III), Eu(III), Sr(II), and Cs(I) with bentonite clay at varying pH (1–9). The effects of other experimental parameters such as ionic strength (0.01–1 M (NaClO 4 )), clay to metal ion concentration ratio, and the presence of complexing anions such as oxalic acid (ox), carbonate (CO 3 2− ), ethylenediaminetetraacetic acid (EDTA), and humic acid (HA) on Eu(III) sorption have also been investigated. The sorption of Eu(III) has been found to be invariant with the change in ionic strength suggesting inner-sphere complexation on the bentonite surface. Near quantitative sorption of Eu(III) and Am(III) has been observed in the entire pH range and there is marginal influence of the presence of 1 · 10 – 4  M of ox and CO 3 2− on the sorption profile. However, the presence of 1 · 10 – 4  M EDTA suppresses the sorption of Eu(III) ion onto bentonite. Desorption studies of Eu(III) loaded onto bentonite using varying concentrations of HClO 4 (0.01–1.0 M) solutions reveal that higher acidity favors the process. The sorption of Eu(III) on bentonite followed the Langmuir isotherm suggesting monolayer sorption process. The data fitting to D-R isotherm suggested that the Eu(III) sorption on bentonite follows ion exchange mechanism. The sorption capacity of bentonite clay was determined to be 3.8(± 0.1) × 10 −4  moles/g using Langmuir and D-R isotherms.

The ability of Euglena mutabilis cells – a unicellular protozoan with a flexible pellicle, which is typically found in acid mine drainage (AMD) environments – to bioaccumulate uranium under acid conditions was studied in batch sorption experiments at pH 3 and 4 using Na 2 SO 4 and NaClO 4 as background media. It was found that axenic cultures of Euglena mutabilis Schmitz were able to bioaccumulate in 5 days 94.9 to 99.2% of uranium from a 1 · 10 – 5  mol/L uranium solution in perchlorate medium and 95.1 to 95.9% in sodium sulfate medium, respectively. The speciation of uranium in solution and uranium bioaccumulated by Euglena mutabilis cells, were studied by laser induced fluorescence spectroscopy (LIFS). The LIFS investigations showed that the uranium speciation in the NaClO 4 systems was dominated by free uranyl(VI) species and that the UO 2 SO 4 species was dominating in the Na 2 SO 4 medium. Fluorescence spectra of the bioaccumulated uranium revealed that aqueous uranium binds to carboxylic and/or (organo)phosphate groups located on the euglenid pellicle or inside the Euglena mutabilis cells. Reduced uranium immobilization rates of 0.93 – 1.43 mg uranium per g Euglena mutabilis biomass were observed in similar experiments, using sterile filtrated AMD waters containing, 4.4 · 10 – 5  mol/L uranium. These lower rates were attributed to competition with other cations for available sorption sites. Additional LIFS measurements, however, showed that the speciation of the bioaccumulated uranium by the Euglena mutabilis cells was found to be identical with the uranium speciation found in the bioaccumulation experiments carried out in Na 2 SO 4 and NaClO 4 media. The results indicate that Euglena mutabilis has the potential to immobilize aqueous uranium under acid condition and thus may be used in future as promising agent for immobilizing uranium in low pH waste water environments.

The sorption behavior of arsenic on minerals from a lateritic weathering has been investigated to evaluate its potential for the decontamination of arsenic ions from aqueous media. Various physicochemical parameters such as pH, equilibration time, adsorbent dose, concentration of adsorbate, effect of diverse ions and temperature were studied in order to simulate the best conditions under which this material can be used as an adsorbent, employing batch method and radiotracer technique. Maximum adsorption was observed in buffer solutions having pH of 2.0 and 8–9, using 0.5 g of adsorbent for 6.674 · 10 – 5  mol L – 1 arsenic concentration in 10 min equilibration time. The sorption data followed the pseudo-second order reaction kinetics. The adsorption data fit both the Freundlich and Dubinin-Radushkevich isotherm equations over the range of 3.195 · 10 – 5  to 3.195 · 10 – 3  mol L – 1 arsenic concentration. The characteristic Freundlich constants i.e. 1/ n  = 0.427 ± 0.009 and K  = 3.04 × 10 −4  ± 1.46 · 10 – 5  mol g – 1 have been computed for the sorption system. The sorption mean free energy from the Dubinin-Radushkevich isotherm is 12.00 ± 0.38 kJ mol – 1 indicating ion-exchange mechanism of chemisorption. The uptake of arsenic increases with the rise in temperature (278.16–323.16 K). Thermodynamic quantities i.e. ΔG , ΔS and ΔH have also been calculated for the system. The sorption process was found to be endothermic.

Fast progressing immuno -PET asks to explore new radionuclides. One of the promising candidates is 90 Nb. It has a half-life of 14.6 h that allows visualizing and quantifying biological processes with medium and slow kinetics, such as tumor accumulation of antibodies and antibodies fragments or drug delivery systems and nanoparticles. 90 Nb exhibits a positron branching of 53% and an average kinetic energy of emitted positrons of E mean  =0.35 MeV. Currently, radionuclide production routes and Nb V labeling techniques are explored to turn this radionuclide into a useful imaging probe. However, efficient separation of 90 Nb from irradiated targets remains in challenge. Ion exchange based separation of 90 Nb from zirconium targets was investigated in systems AG 1 × 8 – HCl/H 2 O 2 and UTEVA-HCl. 95 Nb ( t 1/2  = 35.0 d), 95 Zr ( t 1/2  = 64.0 d) and 92 m Nb ( t 1/2  = 10.15 d) were chosen for studies on distribution coefficients. Separation after AG 1 × 8 anion exchange yields 99% of 90/95 Nb. Subsequent use of a solid-phase extraction step on UTEVA resin further decontaminates 90/95 Nb from traces of zirconium with yields 95% of 90/95 Nb. A semi-automated separation takes one hour to obtain an overall recovery of 90/95 Nb of 90%. The amount of Zr was reduced by factor of 10 8 . The selected separation provides rapid preparation (< 1 h) of high purity 90 Nb appropriate for the synthesis of 90 Nb-radiopharmaceuticals, relevant for purposes of immuno -PET. Applying the radioniobium obtained, 90/95 Nb-labeling of a monoclonal antibody (rituximab) modified with desferrioxamine achieved labeling yields of >90% after 1 h incubation at room temperature.

Erbium-169 [ T 1/2  = 9.4 d, E β (   max   )  = 342 keV (45%) and 351 keV (55%), E γ  = 110.5 KeV (0.0014%)] is the most preferred radionuclide for radiation synovectomy (RSV) of digital joints. Radiolabeled particulates of appropriate size, wherein the radionuclide is irreversibly attached to the preformed particles, offer distinct advantages as the radiopharmaceuticals for use in RSV over other radiotherapeutic agents in terms of minimal leakage of radioactivity from the joint cavity. Hyroxyapatite (HA) particles, regarded as one of the most suitable carrier moiety for radiosynoviortheses applications, have been envisaged for labeling with 169 Er with an aim to developing an agent for RSV of digital joints. Erbium-169 was produced with a specific activity of 415 ± 23 MBq/mg and radionuclidic purity of 99.22 ± 0.17% (both at the end of bombardment) by thermal neutron irradiation of enriched (98.2% in 168 Er) erbium target at a flux of ∼ 8 · 10 13  n/cm 2  s for 21 d. HA particles (1–10 micrometer size), synthesized in-house, were labeled with 169 Er in high yield and radiochemical purity (> 99%) using 5 mg of HA particulates at pH 7.5. The radiolabeled particulates showed excellent in vitro stability in normal saline and human serum. Biodistribution and imaging studies (using 171 Er-HA) carried out in normal Wistar rats following intra-articular injection of 169 Er-HA particulates in the knee joint showed near-complete retention of injected radioactivity within the synovial cavity with almost no activity leaching out to the other organs/tissue.

The radiation stability of an unsymmetrical diglycolamide, di-2-ethylhexyl-di-octyl diglycolamide (DEHDODGA) has been evaluated under various process conditions. The influence of diluent ( n -dodecane), phase modifier ( N , N -dihexyl octanamide, DHOA), and nitric acid on the radiolysis of DEHDODGA was investigated. The distribution ratio ( D ) of Am(III), Eu(III), and Sr(II) in the irradiated solvent was determined as a function of absorbed dose of γ -radiation. The distribution ratios decreased with increase of absorbed dose, indicating the radiolytic degradation of DEHDODGA with increase of absorbed dose. The stripping behavior of Am(III) from the degraded organic phase was studied. Quantitative back extraction of trivalent metal ions from the irradiated organic phase was achieved in a single contact using the stripping formulation composed of dilute nitric acid and citric acid. Our studies indicated the robust nature of DEHDODGA with respect to radiolysis, and innocuous nature of degradation products.

Polyaminecarboxilic ligands like diethylenetriaminepentaacetic acid form stable complexes with many heavy metal ions, excelling as cation chelants especially in the field of radiopharmacy. The aim of this work is to characterize, by using the Time Differential Perturbed Angular Correlations (TDPAC) technique, the hyperfine interactions at hafnium sites in hafnium diethylenetriaminepentaacetate and to investigate their evolution as temperature increases. TDPAC results for KHfDTPA · 3H 2 O obtained by chemical synthesis yield a well defined and highly asymmetric interaction of quadrupole frequency ω Q  = 141 Mrad/s, which is consistent with the existence of a unique site for the metal in the crystal lattice. The thermal behaviour of the chelate is investigated by means of differential scanning calorimetry and thermogravimetrical analyses revealing that an endothermic dehydration of KHfDTPA · 3H 2 O takes place in one step between 80 ℃ and 180 ℃. The anhydrous KHfDTPA thus arising is characterized by a fully asymmetric and well defined interaction of quadrupole frequency ω Q  = 168 Mrad/s.