Uranium in thorium matrixes or in minerals and ores containing thorium is determined by epithermal neutron activation analysis
(ENAA). In some minerals and ores, such as monazite sands, the analysis can be carried out by purely instrumental means with
no chemical separation of uranium or thorium from the irradiated matrix. For thorium compound matrixes with very low uranium
contents, a rapid radiochemical separation method, based on the retention of uranyl ion on anion-exchange resins, is first
carried out, before counting the gamma-ray peaks for239U in multichannel analysers coupled to NaI(Tl) scintillators or to Ge(Li) detectors.
A program in “INSTRUMENT BASIC” language is proposed for analysis of gamma-ray spectra obtained with Ge(Li) detectors and
accumulated in multichannel analysers on-line with minicomputers. The program locates the peaks, evaluates the corresponding
energy values, the net peak areas and the standard deviations on the areas.
The distribution constant KD(HTTA) of thenoyltrifluoroacetone between 10–3M HNO3 and cyclohexane was determined by means of spectrophotometric measurements of HTTA concentration in the aqueous phase. The distribution ratio, D, of HTTA, when tri-n-octylphosphine (TOPO) is present, and the equilibrium constant,n
, of the reaction between HTTA and TOPO in the organic phase were also determined. By means of the known KD(HTTA) and D values, the equilibrium constant of the HTTA-TOPO interaction was calculated. Making use of KD(HTTA) andn
values and of the slope analysis method, the composition of the extracted lanthanide complexes was established. By considering the interaction reaction between the extractants, the species Ln(TTA)3 · TOPO and Ln(TTA)3 · 2(TOPO), for Ln=La and Yb, were identified in the organic phase. The equilibrium constants of the reactions that give rise to the species were also calculated.
The procedure for thorium determination in ammonium diuranate (ADU) and rocks, by neutron activation analysis after chemical separation of233Th, is presented. The separation of233Th from the interfering radioisotopes is based on the retention of233Th by a resin saturated with thorium (isotopic exchange) and on the elution of the interfering radioisotopes by a dilute solution of thorium in 0.5M HCl (ion exchange). The determination limit of thorium in rocks and ADU was found to be 0.56 and 9.3 g, respectively, when a 20% relative standard deviation was assumed as acceptable. The highest value obtained for the determination limit of thorium in uranium compounds, on account of the234Th activity present, is discussed.
The aim of the present work is to obtain the separation of233Th from the radioisotopes formed in the irradiation of Mn, U, Ba, Cs, Co and the lanthanide elements with thermal neutrons, because they may interfere in the neutron activation analysis of Th, when the activity of233Th is used. The experiments were performed with the resin Bio-Rad AG 50W X-4 and X-8 (100–200 mesh) in the thorium form. The separation of233Th from the interfering radioisotopes is based on the retention of233Th by the resin (isotope exchange) and the elution of the interfering radioisotopes with a dilute solution of Th in 0.5M HCl. Batch experiments were made in order to determine the equilibrium time for the isotopic ion exchange of thorium and also the distribution coefficients of the interfering elements between the solution and the resin. Column experiments were carried out with the purpose of establishing the conditions that allow the maximum isotope exchange of233Th and the minimum retention of the interfering radioisotopes in the resin. With this purpose, a statistical interpretation of a four variable experimental design is presented.