Band 90, Heft 12 -

We measured the ( n , 2 n ) cross sections for neutron energies from 13.7 to 15.1 MeV on 239 Pu and 241 Am by α- and γ-spectroscopy measurements of the post–irradiation 238 Pu/ 239 Pu and 240 Am/ 241 Am ratios relative to the ( n , 2 n ) cross sections of 196 Au and other calibration reactions. Statistical errors on the measurements were not more than a few percent. Cross–section values ranged from 228 to 214 millibarns for the reaction on 239 Pu and from 260 to 222 millibarns for the 241 Am reaction for neutron energies from 13.7 to 15.1 MeV.

Excitation functions were measured by the stacked–foil technique for nat Sr( p , xn ) 88,87m,g Y reactions from threshold up to 25 MeV. From the measured cross sections integral yields of 88 Y, 87m Y and 87g Y were calculated. The optimum energy range for the production of 88 Y is E p = 14 → 9 MeV; the 88 Y yield amounts to 1.75 MBq (47.3 µCi/µA·h) and the 87 Y and 87m Y impurities to 2.0 and 4.4%, respectively. The isomeric cross section ratio for the pair 87m,g Y was determined as a function of incident proton energy and the results are discussed in terms of the spins of the two isomeric states involved. Medium scale production of both 86 Y and 88 Y has been carried out more than 15 times each using the ( p , n ) reaction. In the former case 96.3% enriched 86 SrCO 3 was used as target material, and in the latter nat SrCO 3 . The method of separation of radioyttrium has been improved. After coprecipitation with La(OH) 3 , cation–exchange HPLC was applied to isolate radioyttrium from the carrier lanthanum. The radionuclidic and chemical purities of the products were good. Batch yields of 86 Y and 88 Y amounted to about 3.5 GBq and 35 MBq, respectively.

The inner-sphere hydration environments of the Bk 3+ · n H 2 O and Bk 4+ · n ′H 2 O aquo ions in 1 M HClO 4 were determined and are viewed in the context of other 5 f (actinide) and 4 f (lanthanide) aquo ions. The formal potential of the Bk 4+ /Bk 3+ redox couple and the reorganization of the hydration sphere, in terms of the average Bk–OH 2 interatomic distances and number of coordinated water molecules ( n and n ′), were determined through in situ X–ray absorption spectroelectrochemistry. The participation of H 2 O in the electrochemical conversion and the rearrangement of the hydration environment upon reduction–oxidation are summarized in the equation: [Bk(OH 2 ) 8 ] 4+ + H 2 O + e − ⇆ [Bk(OH 2 ) 9 ] 3+ . The change of hydration about 249 Bk between the trivalent and tetravalent states is believed to reflect predominantly steric factors related to the difference of ionic radii, rather than electronic or bonding influences.

The solubility of PuO 2 · x H 2 O was measured at 25°C in a NaClO 4 solution containing Na 2 S 2 O 4 as a reductant. The experiment was carried out by an oversaturation method at I = 1.0 and by an undersaturation method at I = 0.5, 1.0 and 2.0. The concentration of Pu 3+ was measured by α-spectrometry and UV/Visible spectrophotometry, and the equilibrium constant of the reaction, PuO 2 · x H 2 O + e − = Pu 3+ + 4 OH − + (x–2) H 2 O, was obtained. From the obtained results, the solubility product ( K sp ) of PuO 2 · x H 2 O, for PuO 2 · x H 2 O = Pu 4+ + 4 OH − + (x − 2) H 2 O at I = 0.5, 1.0 and 2.0 was determined. By using the specific interaction theory (SIT), the solubility product of PuO 2 · x H 2 O at the standard state ( I = 0) was determined to be log K ° sp = −57.97 ± 0.24, and the ion interaction coefficient ε(Pu 4+ , ClO 4 -) was also evaluated to be 0.99 ± 0.20.

The effective diffusivities of neptunium and plutonium in Inada granite have been determined using the through–diffusion method. Experiments were performed under anaerobic conditions in the presence of carbonate. The actinides are expected to be present as carbonato or carbonatohydroxo complexes. Effective diffusivity (D e ) obtained were (4.0 ± 1.5) × 10 −13 m 2 s −1 for neptunium and (2.0 ± 1.6) × 10 −13 m 2 s −1 for plutonium. Effective diffusivities of neptunium and plutonium in granite were estimated based on the pore diffusion model and compared with the experimentally obtained values.

Fully relativistic molecular density–functional calculations of the electronic structures of hydrated, hydrolyzed and fluoride/chloride complexes have been performed for group–4 elements Zr, Hf, and element 104, Rf. Using the electronic density distribution data, relative values of the free energy change for hydrolysis and complex formation reactions were defined. The results show the following trend for the first hydrolysis step of the cationic species: Zr > Hf > Rf in agreement with experiments. For the complex formation in HF solutions, the trend to a decrease from Zr to Hf is continued with Rf, provided no hydrolysis takes place. At pH > 0, further fluorination of hydrolyzed species or fluoro–complexes has an inversed trend in the group Rf ≥ Zr > Hf, with the difference between the elements being very small. For the complex formation in HCl solutions, the trend is continued with Rf, so that Zr > Hf > Rf independently of pH. A decisive energetic factor in hydrolysis or complex formation processes proved to be a predominant electrostatic metal–ligand interaction. Trends in the K d (distribution coefficient) values for the group–4 elements are expected to follow those of the complex formation.

The redox–sensitive fission product technetium–99 has been investigated in systems containing different reducing solid phases (pyrite, magnetite, ironsulphide and Gorleben sand) on the one hand and Gorleben groundwater, which contains a high amount of humic substances, on the other hand. Initially, technetium–99 was added to these systems as pertechnetate (Tc(VII)), which was reduced in presence and absence of humic substances with the aid of the different reducing surfaces (neutral to alkaline pH). Both in absence and presence of humic substances, Tc concentrations were observed which exceeded the TcO 2 solubility limit, whereby the presence of humic substances had a 100 fold higher Tc concentration compared to its absence. Using the La–precipitation method, it was shown that Tc(IV) inorganic colloids and organic colloids were quantitatively precipitated. It was demonstrated for the first time by a combination of chemical speciation methods (La–precipitation method and gel permeation chromatography) and XANES spectroscopy of the humic substance containing supernatant solutions, that Tc(IV) species were formed in these systems, indicating an association between Tc(IV) and humic substances.

The vacancy transfer probabilities from L 3 to M shell (η L3M ) are obtained for Th and U. For these measurements, L 3 X–ray yields from targets excited by 5.96 and 16.898 and 17.781 keV incident photon, i.e. , below and above the L 3 edge of elements. For comparison with experimental results, we have made theoretical calculation using radiative and nonradiative transition rates. The radiative transition rates of these elements were taken from Dirac–Hartree–Slater model of Scofield [1] and the nonradiative transition rates were obtained from Dirac–Hartree–Slater model of Chen et al. [2]. The measured results are in good agreement with theoretically calculated values.

Rapid radiochemical neutron activation analysis (RNAA) procedures were developed and employed to the determination of 36 trace impurities in high purity titanium and tungsten. Anion–exchange column chromatography procedures were developed for the sequential group chemical separation of various elements, which helped in reducing the spectral interferences and improving the sensitivity of the method. The procedure is simple and requires a very short time to separate the elements in three groups for radiometric assay. To determine very low contents of uranium and thorium, 239 Np and 233 Pa as activation products were separated using anion–exchange and coprecipitation methods. The impurity contents were found to be low; therefore, their deleterious effects would be negligible.

Gas adsorption chromatography experiments were carried out with Es and Am in a temperature gradient tube using chlorinating carrier gas. From the observed deposition temperatures the adsorption enthalpies of AmCl 3 and EsCl 3 on the chromatographic surface (quartz glass surface, dynamically modified by the chlorinating carrier gases) were deduced for the first time. In good agreement, both the model of mobile adsorption and the Monte Carlo model resulted in: