Jeroen J. M. de Goeij

Gamma-ray spectroscopy of voluminous sources Corrections for source geometry and self-attenuation

Abstract To facilitate gamma-ray spectroscopy of voluminous sources (with different sizes, shapes, orientations and distances to the detector, as well as different compositions) correction factors have been derived. These factors, which compare a voluminous source with a point source, describe to what extent the number of photons of a given energy finally recorded is affected by both the spatial distribution of the activity and the self-attenuation within the source. The method presented involves no a priori assumptions regarding source size, shape, orientation, source-to-detector distance and source matrix composition; the only assumption is that the radioactivity is homogeneously distributed within the source, while the composition and the density are uniform as well.

Electroless Silver Deposition in 100 nm Silicon Structures

A new and simple method is described to plate silicon structures with metallic silver for ultralarge-scale integration in dimensions down to 100 nm at an aspect ratio of 4.25. The silver deposition is initiated by an exchange reaction of silicon with silver ions, and the subsequent layer growth of the activated wafers occurs by electroless plating from supersaturated aqueous silver salt solutions at pH ∼ 11. No extra reducing agents are needed since silver ions are reduced at the catalytic silver surface by hydroxyl ions. The spontaneous ion-metal transition only proceeds at pH ∼ 11 and is likely mediated by the formation of subnanometer-sized [Ag 4 (OH) 2 ] 2+ clusters. The silver plating proceeds more easily in smaller structures and yields void-free, crystallized deposits.

In Vitro Exchangeable Erythrocytic Zinc

Exchangeable erythrocytic zinc is measured by65Zn uptake in and release from erythrocytes under standarized and near, physiological conditions: 7.6 μM zinc and 580 μM albumin in the medium. The intracellular exchangeable erythrocytic zinc pool in healthy volunteers amounts to 5 μmol zinc/L packed cells. The half-time of the exchange is 7 h, its activation energy 84 kJ/mol. The effects of the variation in temperature and the concentrations of albumin, as well as the effects of some zinc carriers, cell transport inhibitors, and stress hormones on the65Zn uptake are measured.

Uptake kinetics of 99Tc in common duckweed

The uptake of the nuclear waste product technetium-99 was studied in common duckweed (Lemna minor). In addition to measurements, a model involving two compartments in duckweed with different chemical forms of technetium was derived. The model was tested by chemical speciation, i.e. differentiating between reduced Tc-compounds and Tc(VII)O(4)(-). The TcO(4)(-) concentrations measured were in good agreement with those predicted by the model. Two processes determine technetium uptake: (1) transport of Tc(VII)O(4)(-) across the cell membrane, and (2) reduction of Tc(VII). The TcO(4)(-) concentration in duckweed reaches a steady state within 2 h while reduced Tc-compounds are stored, as a result of absence of release or re-oxidation processes. Bioaccumulation kinetic properties were derived by varying 99Tc concentration, temperature, nutrient concentrations, and light intensity. The reduction of technetium in duckweed was highly correlated with light intensity and temperature. At 25 degrees C the maximum reduction rate was observed at light intensities above 200 µmol m(-2) s(-1) while half of the maximum transformation rate was reached at 41 µmol m(-2) s(-1). Transport of TcO(4)(-) over the cell membrane requires about 9.4 kJ mol(-1), indicating an active transport mechanism. However, this mechanism behaved as first-order kinetics instead of Michaelis-Menten kinetics between 1x10(-14) and 2.5x10(-5) mol l(-1) TcO(4)(-). Tc uptake could not be inhibited by 10(-3) mol l(-1) nitrate, phosphate, sulphate or chloride.

Laboratory Assessment of Early Dietary, Subclinical Zinc Deficiency: A Model Study on Weaning Rats

ABSTRACT: Male weaning rats were pair-fed a low-zinc diet or a controt diet. After 10 d, the animals fed the low-zinc diet showed physiologic signs of deficiency; however, they showed no clinical symptoms. Their estimated whole body zinc was 25 μtmol versus 39 μmol for the controls. The 65Zn absorption increased 2-fold and the tissue distribution altered: muscle and erythrocytes contained more, small intestine and liver less activity at 0.5 h postdose. in vitro, the erythrocyte 65Zn uptake rate increased also. The 65Zn uptake experiments required small quantities of erythrocytes. The difference observed between the deficient and control cells was significant and showed little overlap. The increase of the 65Zn uptake from a medium was not affected when the animals underwent endotoxn exposure 24 h before, as was reported to occur in whole blood 65Zn uptake. Therefore, we suggest the in vitro erythrocyte 65Zn uptake, performed in a standardized, near physiologic medium, to detect early, subclinical zinc deficiency.

VOLTAMMETRY DETECTION OF COPPER IN HIGH SPECIFIC ACTIVITY 64CU

Abstract Voltammetry in cathodic stripping mode was used to measure ultratrace levels of copper in preparations of very high specific activity 64 Cu produced via the 64 Zn(n,p) 64 Cu reaction. With high-purity targets and reagents, strict acid-wash procedures and careful handling, specific activities of up to 4×10 16 Bq g −1 at 8 h after EOB could be attained when irradiating in a high flux reactor. During irradiation also 65 Cu is formed via the route 64 Zn(n,γ) 65 Zn→ 65 Cu, and this contribution to the total amount of copper in high specific activity 64 Cu preparations is not always negligible.

Uptake, biotransformation, and elimination of 99Tc in duckweed.

Aquatic plants may play an important role in the environmental fate of the long-lived radioactive waste product 99Tc. Aquatic plants show a strong accumulation and retention of Tc, even after they have died. This study focuses on possible bio-organic Tc compounds formed in the water dwelling plant duckweed to possibly explain the accumulation and retention. Moreover, a change in chemical speciation often implies a different fate and behaviour in the biosphere. A mild separation technique was used to distinguish between reduced Tc species and TcO(4)(-). Accumulation experiments suggested that reduction of Tc(VII)O(4)(-) and subsequent complexation are responsible for the accumulation of Tc in duckweed. A steady state concentration of TcO(4)(-) in duckweed was reached within 24 h, but the total concentration of Tc increased continuously. Only a small part (</=5%) of Tc was present as TcO(4)(-). Elimination experiments showed that TcO(4)(-) is the only mobile species. Other Tc species are responsible for the retention of Tc in duckweed. It is known that these species are not bio-available and only slowly re-oxidise to pertechnetate, resulting in a longer residence time in ecosystems.

Analysis of copper complex lability using 64Cu-equilibration techniques and free-ion selective radiotracer extraction

Abstract A novel analytical approach was applied to a conventional free-ion selective extraction technique in order to determine the lability of five copper complexes, viz. Cu–CDTA, Cu–EDTA, Cu–NTA, Cu–DTPA, and Cu–humic acid. The main purpose of this approach was to visualise dissociation kinetics of the copper complexes and to exclude reactions other than complex dissociation from the visualisation process. Therefore, three major modifications were applied to a conventional free-ion extraction with Chelex-100: (a) free-ligand concentrations were minimised, (b) the mobile phase as well as column material were provided with a surplus of free copper ions, and (c) the sample under study was equilibrated with a negligible mass of a high-specific activity 64 Cu radiotracer. The actual determination of copper complex lability was carried out in two different ways, either directly, by following dissociation reactions, or indirectly, by following association reactions. The two procedures were given different operationally defined detection windows, to maximise dissociation rate constant determinations. In a region where both detection windows overlapped, similar dissociation rate constants were found for the same copper species. Two species could be distinguished kinetically for Cu–CDTA, Cu–EDTA, as well as Cu–NTA. The dissociation rate constants of these two species differed about two orders of magnitude. The two main chemical forms of Cu–DTPA (viz. a 2:1 and 1:1 complex), predicted by speciation calculations, were found as well and equilibrium concentrations of both species were in accordance with theoretical values. Four species of the Cu–humic acid complex could be identified, with dissociation rate constants in a range of three orders of magnitude.

Photodegradation of Iron(III)-EDTA: Iron Speciation and Domino Effects on Cobalt Availability

Environmental Context. Aquatic life requires access to sufficient nutrients and trace metals in the surrounding waters. Measuring the speciation (in solution or precipitated, free ionic or complexed) of trace metals is a traditional procedure to assess the potential of waters for life. Iron, an important nutrient, is relatively insoluble, and metal–ligand complexes are required to keep the iron in solution and bioavailable. Sunlight often degrades these metal–ligand complexes, and the subsequently released iron can outcompete other (trace) metals for their ligands. A ‘domino’ effect on weaker metal–ligand complexes will occur which complicates the actual dynamic speciation and its measurements. Abstract. The effect of photodegradation of iron(iii)-EDTA on the chemical speciation of both iron and cobalt has been tested both in a simple medium and in a more complex algal growth medium. In both media, the photodegradation of iron(iii)-EDTA caused iron(iii) to be released as a free ionic species. Released iron(iii) could be modelled as engaged in precipitation and in competition with cobalt, initially also bound to EDTA. Cobalt appeared as free ionic species after a certain lag. The length of this lag phase was proportional to the amount of EDTA added to the media, which indicated that the surplus of EDTA was first targetted by iron(iii)-EDTA photodegradation, after which iron(iii) out-competed cobalt for the EDTA in the metal–EDTA pool. All data were fitted, using a model on speciation kinetics for cobalt-EDTA and iron(iii)-EDTA, taking into account photodegradation rates of iron(iii)-EDTA and precipitation of free iron(iii). The modelled rate of iron(iii)-EDTA photodegradation was compared to direct HPLC measurements of the disappearance of iron(iii)-EDTA due to photodegradation, the latter, however, with a different iron(iii)-EDTA concentration regime. The results suggest that photodegradation is a complex process which is greatly influenced by experimental conditions (e.g. light intensity/spectrum, iron(iii), and EDTA concentrations). Iron(iii)-desferrioxamine B (DFO-B) was suggested as a possible alternative for EDTA in experimental media: photostability was shown for prolonged experimental periods.