Volume 88, Issue 5 -

The positron emitter 51 Mn (t 1/2 = 46.2 min, I β + = 97%) is of potential interest in positron emission tomography (PET). With a view to optimizing its production route, excitation functions for deuteron induced nuclear reactions on isotopically enriched (~95%) 50 Cr were determined radiochemically over the energy range of 3 to 13 MeV and for proton induced nuclear reactions on nat Cr in the range of 18 to 38 MeV. Cross sections are presented for the reactions: 50 Cr(d,2n) 50m Mn, 50 Cr(d,n) 51 Mn, 50 Cr(d,t) 49 Cr, 50 Cr(d,p) 51 Cr, 50 Cr(d,a) 48 V, 52 Cr(d,2p) 52 V, nat Cr(p,x) 51 Mn, nat Cr(p,x) 52 Mn, nat Cr(p,x) 52m Mn, nat Cr(p,x) 54 Mn, nat Cr(p,x) 48 Cr, nat Cr(p,x) 49 Cr, nat Cr(p,x) 51 Cr, nat Cr(p,x) 48 V and nat Cr(p,x) 45 Ti. In the case of proton induced reactions on nat Cr, the isomeric cross section ratio for the isomeric pair 52m,g Mn was also determined. The results are interpreted in terms of the spins of the two isomers concerned. Theoretical thick target yields were derived from the corresponding excitation functions. The maximum achievable thick target yield of 51 Mn is 1.1 GBq/μAh in the case of the 50 Cr(d,n) 51 Mn reaction at E d = 14 → 3 MeV, and 3.2 GBq/μAh in the case of the nat Cr(p,x) 51 Mn process for E p = 40 → 20 MeV. The 50 Cr(d,n) 51 Mn process is the favourable production route for 51 Mn, since the radioisotopic purity is high and a small-sized medical cyclotron is adequate to produce this positron emitter in sufficient quantities.

The fluoride complexation of the group-4 elements Zr, Hf and Rf, and of the pseudo-homolog Th, has been investigated in mixed HNO 3 /HF solutions by studying Kd values on both cation exchange resins (CIX) and anion exchange resins (AIX) using the automated rapid chemistry apparatus ARCA. On the CIX, the four elements are strongly retained as cations below 10 -3 M HF. For Zr and Hf, the decrease of the Kd values due to the formation of fluoride complexes occurs between 10 -3 M HF and 10 -2 M HF. For Rf and Th, this decrease is observed at one order of magnitude higher HF concentrations. On the AIX, for Zr and Hf, a rise of the Kd values due to the formation of anionic fluoride complexes is observed between 10 -3 M HF and 10 -2 M HF, i.e. in the same range of HF concentrations where the decrease of the Kd values on the CIX is observed, yielding a consistent picture. For Rf and Th, on the AIX, no rise of the Kd values is observed even if the HF concentration is increased up to 1 M. By varying the concentration of the counter ion NO 3 - which is competing for the binding sites on the AIX resin, it could be shown, nevertheless, that Rf does form anionic fluoride complexes. Apparently, there is a more specific competition of NO 3 - with respect to [RfF x ] (x-4)- than with [ZrF y ] (y-4)- and [HfF z ] (z-4)- .

We have studied the sorption of W on anion-exchange resins from HNO 3 /HF solutions under on-line conditions using continuous chromatography with the multi-column technique. Kd values and the charge of the W species were determined. In order to achieve an effective separation of mother (W) and daughter (Ta), also the sorption of Ta from HNO 3 /HF solutions on various anion exchange resins with different functional groups was studied. This investigation serves the purpose to select a suitable anion exchange resin for planned experiments with Sg.

Cation-cation complexes of NpO 2 + and PuO 2 + with UO 2 2+ and Th 4+ were studied using laser-induced photoacoustic spectroscopy (LIPAS). An equilibrium constant of 2.4±0.2 M -1 was measured at an ionic strength of 6.0 M for the NpO 2 + · UO 2 2+ complex which is in good agreement with previous measurements using conventional absorption spectroscopy and Raman spectroscopy. Equilibrium constants of 1.8±0.9 and 2.2±1.5 M -1 were measured for NpO 2 + · Th 4+ and PuO 2 + · UO 2 2+ complexes at an ionic strength of 6.0 M, respectively. This is the first time that LIPAS has been used to study cation-cation complexes and the first time that the PuO 2 + · UO 2 2+ cation-cation complex has been reported.

The careful control of actinide distribution between 30% tributylphosphate (TBP) in organic diluent and nitric acid is vital to the successful operation of nuclear fuel reprocessing plants. Uranium (VI) and tetravalent actinides are extracted into the organic phase as the nitrate complexes UO 2 (NO 3 ) 2 (TBP) 2 and An(NO 3 ) 4 (TBP) 2 respectively. The presence of dibutylphosphate (HDBP), a tributylphosphate decomposition product, affects this distribution due to the formation of strong, organic soluble, complexes between the phosphate and the actinide ions. This paper describes the investigation of U(VI), U(IV), Np(IV) and Pu(IV) complexation with HDBP in 30% TBP/organic diluent solutions. The distribution of U(VI) between nitric acid and 30% TBP/organic diluent solutions containing HDBP and subsequent analysis of the organic phases by 31 P nuclear magnetic resonance spectroscopy, absorption spectroscopy and extended X-ray absorption spectroscopy indicates that U(VI) can form a range of complexes with HDBP. At comparatively high HNO 3 loading in the organic phase, HDBP can displace TBP groups to form either UO 2 (NO 3 ) 2 (HDBP)(TBP) or UO 2 (NO3) 2 (HDBP) 2 . At lower HNO 3 loadings HDBP can also deprotonate and act as a chelate ligand displacing nitrates to form either UO 2 (DBP) 2 (HDBP) x or UO 2 (NO 3 )(DBP)(HDBP)x (where x = 1 or 2). Distribution data and absorption spectroscopy also indicates that Np(IV) forms at least two complexes in which nitrate groups are displaced by the DBP- anion. In contrast, for U(IV), it is almost certain that TBP groups are displaced by HDBP at increased HDBP loading forming U(NO 3 ) 4 (HDBP) (TBP). There is no evidence for the displacement of nitrates. Finally, Pu(IV) distribution data suggested that complex reactions were taking place in both phases ensuring that equilibrium extraction was difficult to ascertain. Nevertheless, HDBP does readily complex with Pu(IV) increasing extraction and displacing nitrates from the Pu(IV) species formed in the organic phase.

Optical and far-IR spectroscopy techniques have shown that the spectroscopic properties of Np(VI) and Pu(VI) compounds formed with different tetrahedral anions XO 4 2- (X = S, Se, Cr) are depending on the nature of the central atom X in the oxoanions. Changes in electronic transition energies within the ground state configuration are mainly due to crystal field perturbations induced by oxygen charge distribution variations in the actinide ion surrounding depending upon the nature of X. Changes may be related to covalence variation in An-O bonding or distortions in the ligand arrangement.

Data are extremely limited on the effects of temperature on crystallinity and the resulting changes in solubility products of thermally transformed thorium oxide phases. Such data are required to reliably predict thorium behavior in high-level waste repositories where higher than ambient temperatures are expected. Solubility studies were conducted as a function of pH and time and at 0.1 M NaCl for 1) ThO 2 (am) at 23°C, 2) ThO 2 (am→c), a thermally transformed amorphous thorium hydrous oxide at 90°C, and 3) ThO 2 (c) at 23°C and 90°C. Results show that when ThO 2 (am) is heated to 90°C, it transforms to a relatively insoluble and crystalline solid [ThO 2 (am→c)]. At a fixed pH, the observed solubility of ThO 2 (am) at 23°C is more than 11 orders of magnitude greater than those for ThO 2 (c) at 23°C or of ThO 2 (am→c) and ThO 2 (c) at 90°C. Solubility data were interpreted using the Pitzer ion-interaction model. The log of the solubility product for the thorium dioxide dissolution reaction [ThO 2 (s) + 2 H 2 O ↔ Th 4+ + 4 OH - ] was determined to be -44.9 for ThO 2 (am) at 23°C, ≥-56.9 for ThO 2 (c) at 23°C, and -51.4 for ThO 2 (c) at 90°C. At 90°C, a relatively less crystalline phase, ThO 2 (am→c), showed slightly higher solubility (log K sp = -49.2) than crystalline ThO 2 (c).

Alpha-emitting isotopes of uranium and thorium in surface marine sediments collected from the harbours at Port Sudan and Sawakin on the Sudanese coast of the Red Sea have been studied using radiochemical separation procedures and alpha-particle spectrometry. Activity concentrations of 238 U, 235 U, 234 U, 232 Th, 230 Th and 228 Th were measured. The ranges of activities observed for 238 U and 232 Th were 2.71-13.42 Bq/kg and 3.62-18.73 Bq/kg from Port Sudan and 3.87-24.79 Bq/kg and 1.13-10.04 Bq/kg dry weight from Sawakin, respectively. The behaviour of these decay-series nuclides was examined using daughter/parent and non-isotopic specific activity ratios in sediments. Simple regression analysis showed insignificant correlation between uranium and thorium activities and the organic matter content in sediments. The results are compared with those from coastal waters of other oceans.