Band 88, Heft 7 -

The recoil properties of 14, 24, 26, 31, 21 and 20 radionuclides formed in photospallation reactions on nat V, nat Cu, 93 Nb, nat Ag, nat Ta, and 197 Au, respectively, induced by bremsstrahlung of end-point energies (E 0 ) from 600 to 1100 MeV, have been investigated using the thick-target thick-catcher method based on the two-step vector model. The measured forward-to-backward ratios (F/B) were found to be independent of the mass difference (ΔA) between a product (A p ) and a target (A t ) and also of A t . The recoil velocities ν from the first step and the mean kinetic energies T of the residual nuclei in the second step were deduced and discussed by comparing with the reported results on proton-induced reactions. A comparison with T values calculated by the PICA (Photon-Induced Intranuclear Cascade Analysis) code at E 0 = 400 MeV was also performed. It was found that although the code well reproduces the experimental results of nat V and nat Cu, the same calculation for heavier targets gives T values lower than the experimental results, indicating some nuclear-structure effect, such as a medium effect notably at A t ≥ 100. The parameter values of ε s = T/(ΔA/A t ), the average kinetic energy of the emitted particles in a random-walk process of both the photon- and proton-reactions, were found to increase along with an increase of A t , but change slopes to become constant above A t = 100.

The peak evaluation in alpha liquid scintillation is known to be easy, mostly due to the gaussian shape of the peaks. However, we often observed a high-energy tail in addition to a pure gaussian function. This effect is only detectable with a high resolution α liquid scintillation spectrometer such as the PERALS ® system. Indeed, its intrinsic resolution (180 keV for a 4 MeV α particle) is better than that obtained for conventional LSC spectrometers. The peak asymmetry was quantified using the Fisher´s coefficient γ1 (symmetry factor). We show that the main effect responsible for the asymmetry is internal conversion. Indeed, most of the even-even nuclides have low α intensity transitions leading to excited levels of their daughter nuclides. The internal conversion is almost equal to 100% and consequently produces a sum peak at higher energy. No generalization is possible for odd-even nuclides, but the knowledge of their disintegration scheme allows to explain the experimental values obtained and the differences observed (e.g. between 241 Am and 243 Am). The experimental determinations of γ1 are given for polonium, radon, radium, thorium, uranium, plutonium, americium, and curium isotopes. We show the necessity to take into account the L and M shell contributions for few nuclides like thorium isotopes to get a maximum accuracy in the activity measurements.

An electromigration device has been developed for the on-line measurement of absolute individual ion mobilities of radioactive species in free electrolytes. The device is composed of a migration cell, a γ-radiation scanning unit and a data processing system. The performance of two BGO-scintillators as scanning unit and the reliability of an on-line data processing system were studied with calibrated radioactive sources of given velocities. The system was found to work with a high reproducibility rate. The migration velocities were determined with an accuracy better than 1%.

The synergistic extraction of thorium(IV) and uranium(VI) has been studied using mixtures of thenoyltrifluoroacetone (Htta) and various imidazoles such as imidazole (Imz), 2-methylimidazole (2-Meimz), benzimidazole (Bimz) or 2-pyridylbenzimidazole (2-Pybimz). The extraction data have been analysed by both graphical and theoretical methods by taking into account chemical mass action principles. The results demonstrate that these metal ions are extracted into chloroform as Th(tta) 4 and UO 2 (tta) 2 with Htta alone and as Th(tta) 4 · nS and UO 2 (tta) 2 · nS in the presence of imidazoles (where n = 1 and 2 for thorium(IV) and n = 1 for uranium(VI); S represents imidazole). The equilibrium constants of the above extracted complexes have been deduced by non-linear regression analysis. The addition of an imidazole to the metal chelate system enhances the extraction efficiency of these metal ions. The complexation strength of these metal ions with various imidazoles follows the order: Imz > 2-Meimz > Bimz > 2-Pybimz.

The structure of dioxouranium(VI) as a function of pH at different (CH 3 ) 4 N-OH concentrations has been investigated with the aid of U L III -edge EXAFS. Polynuclear hydroxo species were identified by an U-U interaction at 3.80 8 Å at pH = 4.1. The precipitate formed at pH = 7 has a schoepite like structure. In solution at high pH [0.5 M (CH 3 ) 4 N-OH], the EXAFS data are consistent with the formation of a monomeric four coordinated uranium(VI) hydroxide complex UO 2 (OH) 4 2- of octahedral geometry. The first shell contains two O atoms with a U=O distance of 1.83 0 Å, and four O atoms were identified at a U-O distance of 2.26 5 Å. In strong alkaline solutions [>1 M (CH 3 ) 4 N)-OH], 17 O-NMR spectra indicate the presence of two species, presumably UO 2 (OH) 4 2- and UO 2 (OH) 5 3- , the latter in low concentration, which are in rapid equilibrium with one another at 268 K in aqueous solution.

The uptake of Eu 3+ (a trivalent actinide homolog) by Aldrich humic acid covalently bonded to an inorganic support is examined. Two types of covalent linkages are used and the synthetic routes to produce the resins are discussed. The use of these resins excludes having to account for humic acid desorption from the surface, yields a well characterized system, and allows the experiment to focus on and account for the role of humic acid in the sorption of Eu. The proton exchange capacity of the resins is examined by titration and the differences observed are traced to the resin synthesis. Europium sorption experiments are performed at pH 4 and pH 6 in 0.1 M NaClO 4 . Kinetic experiments show equilibrium is reached in 24 hours. The kinetic data are used to evaluate the loading capacity, with results similar to equilibrium experiments. The complexation results are evaluated based on the metal ion charge neutralization model. For the resins an effect of pH and resin synthesis route on the Eu uptake is observed. The uptake increases with pH for both resins. The resin HA-Epo (Epoxy linkage) has a higher metal binding at pH 4, while the resin HA-HAB (2-hydroxylazobenzene linkage) had more proton exchange sites occupied by metal ions at pH 6. Overall, more Eu is bound to HA-Epo at pH 6 since its proton exchange capacity is higher. The evaluated stability constants vary slightly and show a dependence on the linkage group but are similar to literature values that examined complexation by aquatic humic acid analyzed with the same model. This result supports the utility of the metal ion charge neutralization model and the applicability of the resulting stability constants.

The oxidation of hydroxylamine by nitric acid in the presence of technetium ions at temperatures above ~60°C is an autocatalytic process comprising an induction period and then a catalysed reaction involving HNO 2 , which has accumulated in the solution. Tc ions have no appreciable effect on the reaction rate, which is governed only by the nitric and nitrous acid oxidation reactions of hydroxylamine, but the presence of Tc ions does extend the initial induction period. The rate of hydroxylamine oxidation by HNO 3 in the presence of HNO 2 , that is, after the induction period, was found to be: -d[NH 3 OH + ]/dt = k[NH 3 OH + ][HNO 2 ][HNO 3 ] 3.5 where k = 120±10 l 4.5 mol -4.5 min -1 at T = 80°C, μ = 2 and [H + ] ≤ 2 M. Under these conditions, the reaction apparently has a high activation energy of 160-180 kJ mol -1 . At low temperatures (20-40°C) hydroxylamine is effectively stable in solutions of HNO 3 up to concentrations of ~2 M, whether or not Tc(VII) ions are present. Tc(V) was also observed to form at least one complex on reduction with excess hydroxylamine with an absorption maximum between 467 and 480 nm dependent on the solution acidity.

Heavy ion irradiation on tantalum metal target with 57 MeV 11 B 4+ leads to the production of no-carrier-added radioisotopes of platinum, 187,188 Pt and iridium, 187,188 Ir in the matrix, which have been effectively separated from the bulk target and from each other with suitable anion exchangers employing liquid-liquid extraction (LLX). Also 188,189 Pt and 188,189 Ir radionuclides have been produced, from their short-lived precursors 188,189 Au, in 12 C 6+ irradiated tantalum matrix. Gamma-spectroscopy has been utilised to determine the production, extent of separation and purity of the radiotracers at different stages of the experiment.