A comparative study of234U/238U alpha-activity ratios determined by alpha-spectrometry and calculated from mass spectrometric data

²³⁴U/²³⁸U α-activity ratios determined by α-spectrometry (AS) and those calculated from the atom ratio data using the half-life values are compared in some of the isotopic reference materials of uranium and a few other uranium samples. For α-spectrometry, electrodeposited sources were prepared and a large area passivated ion implanted (IPE) detector (450 mm²) was used for recording the α-spectra. The isotopic composition of U was determined by thermal ionisation mass spectrometry (TIMS) and the recommended half-life values of²³⁴U and²³⁸U were used to calculate the α-activity ratio. It is observed that²³⁴U/²³⁸U α-activity ratios calculated from the atom ratio data are consistently high, with a mean difference of about 5%, when compared to the α-spectrometry results. This discrepancy warrants confirmation by a few more laboratories and suggests redetermination of the half-life values of²³⁴U and²³⁸U.     

Isotopic and ultratrace analysis of uranium by double-focusing sector field ICP mass spectrometry

An analytical method for the ultratrace and isotopic analysis of uranium in radioactive waste samples using a double-focusing sector field ICP mass spectrometer is described. In high-purity water a detection limit for uranium in the lowest fg/mL range has been achieved. Under optimum experimental conditions (²³⁵U/²³⁸U ≈ 1), the precision in ²³⁵U/²³⁸U isotopic ratio determinations has been determined as 0.07% RSD. With the isotopic standard U-020 (²³⁵U/²³⁸U = 0.0208) a precision of 0.23% RSD at the 100 pg/mL level using ultrasonic nebulization has been achieved. With ²³⁴U/²³⁸U isotopic ratios of down to 10^–5, the values obtained by double-focusing sector field ICP-MS and alpha spectrometry were in agreement. Received: 27 February 1997 / Revised: 10 Juni 1997 / Accepted: 12 June 1997     

Evaluation of a single collector, double focusing sector field inductively coupled plasma mass spectrometer for the determination of U and Pu concentrations and isotopic compositions at trace level

This work explores the performance of the “Axiom”, a double focusing sector field ICP-MS (ICP-SFMS) in the determination of actinide concentration and isotopic ratio at trace level. On the actinide mass range the performances observed are characterized by high sensitivity, around 2.8·10⁶ cps·μg⁻¹·l, and low background, below 0.3 cps. Therefore, the absolute instrumental detection limit is approximately 0.05 fg for Pu isotopes. Furthermore, the ²³⁵U/²³⁸U ratio for a 0.5 μg·l⁻¹ U500 isotopic standard could now be measured using the ICP-SFMS with a relative standard deviation less than 0.1%. Moreover, the accuracy of the measured ratio was demonstrated at low concentration with the target value remaining within experimental uncertainty limits. This revised version was published online in July 2006 with corrections to the Cover Date.     

<Superscript>234</Superscript>U/<Superscript>235</Superscript>U activity ratios as a probe for the <Superscript>238</Superscript>U/<Superscript>235</Superscript>U half-life ratio

High-resolution alpha-particle spectrometry was performed on three uranium materials enriched in ²³⁵U. Besides the ²³⁵U peaks, separate peaks belonging to impurity traces of ²³⁴U could be quantified. Relying on the isotopic composition of the uranium, as determined by mass spectrometry, the ratio of the half-lives of ²³⁸U and ²³⁵U was determined via the activity ratio of ²³⁴U and ²³⁵U in the materials. As an intermediate link, the ²³⁴U/²³⁸U half-life ratio was taken from published mass spectrometric analyses of ‘secular equilibrium’ uranium material. The resulting half-life ratio T _1/2(²³⁸U)/T _1/2(²³⁵U) = 6.351±0.031 is in agreement with the commonly adopted half-life values determined by Jaffey et al.     

Determination of uranium isotopic composition and 236U content of soil samples and hot particles using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The ²³⁶U isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. Nuclear track radiography was applied for the identification and extraction of hot radioactive particles from soil samples. A rapid and sensitive analytical procedure was developed for uranium isotopic ratio measurement in environmental samples based on double-focusing inductively coupled plasma mass spectrometry (DF–ICP–MS) with a MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN). The performance of the DF–ICP–MS with a quartz DIHEN and plasma shielded torch was studied. Overall detection efficiencies of 4×10^–4 and 10^–3 counts per atom were achieved for ²³⁸U in DF–ICP–QMS with the MicroMist nebulizer and DIHEN, respectively. The rate of formation of uranium hydride ions UH⁺/U⁺ was 1.2×10^–4 and 1.4×10^–4, respectively. The precision of short-term measurements of uranium isotopic ratios (n = 5) in 1 μg L^–1 NBS U-020 standard solution was 0.11% (²³⁸U/²³⁵U) and 1.4% (²³⁶U/²³⁸U) using a MicroMist nebulizer and 0.25% (²³⁵U/²³⁸U) and 1.9% (²³⁶U/²³⁸U) using a DIHEN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium; i.e. in these samples the ²³⁶U/²³⁸U ratio ranged from 10^–5 to 10^–3. Results obtained with ICP–MS, α- and γ-spectrometry showed differences in the migration properties of spent uranium, plutonium, and americium. The isotopic ratio of uranium was also measured in hot particles extracted from soil samples.     

Environmental monitoring as an important tool for safeguards of nuclear material and nuclear forensics

Summary The use of environmental monitoring as a technique to identify activities related to the nuclear fuel cycle has been proposed by international safeguards organizations. The elements specific for each kind of nuclear activity, or “nuclear signatures”, inserted in the ecosystem can be intercepted by different live organisms. This work demonstrates the technical viability of using pine needles as bioindicators of nuclear signatures associated with uranium enrichment activities. Additionally, it proposes the use of HR-ICP-MS to identify the signature corresponding to that kind of activities in the ecosystem. Nitric acid solutions, used to wash pine needles sampled near nuclear facilities and containing only 0.1 mg ^. kg^-1 of uranium, exhibit a n(²³⁵U)/n(²³⁸U) isotopic abundance ratio of 0.0092±0.0002, while solutions originated from samples collected at places located more than 200 km far from activities related to the nuclear fuel cycle exhibit a value of 0.0074±0.0002. Similar results were obtained for sample solutions prepared using the acid leaching process. The different values of n(²³⁵U)/n(²³⁸U) isotopic abundance ratio obtained permit to confirm the presence of anthropogenic uranium and demonstrate the viability of using the methodology proposed in this work.     

Simultaneous analysis of uranium and plutonium using thermal ionization mass spectrometry

Simultaneous isotopic analysis of uranium and plutonium using thermal ionization mass spectrometer coupled to a multi-collector detection assembly with 9 Faraday cups has been reported earlier. Subsequently investigations have been carried out (1) to understand the applicability of correction methodologies available to account for the contribution of²³⁸Pu at²³⁸U and (2) to evaluate the effectiveness of these methodologies on the accuracy of²³⁵U/²³⁸U atom ratio being determined, particularly when samples containing different U/Pu atom ratios. Isotopic fractionation for both U and Pu in the simultaneous isotopic analysis has been compared with the results of the individual analysis of these elements. The different isotopic fractionation factors observed for U were attributed to different conditions of analysis. There was no significant difference in the isotopic fractionation patterns for Pu. The consideration to extend this method to actual samples from our observations on synthetic samples with diferent U/Pu atom ratios containing U and Pu isotopic reference standards is described.     

New methodology for uranium analysis in swipe samples for nuclear safeguards purposes

Environmental swipe sampling for safeguards purpose has been used by International Atomic Energy Agency since 1997, being a powerful tool to detect undeclared materials and activities. This work describes a new methodology for swipe samples analysis based on ultrasound-assisted acid leaching and compares it with traditional total digestion bulk analysis. The proposed method requires few preparation steps, decreasing the risk of contamination, reduced amounts of reagents and a good option to extract uranium from swipe sample. In a real case study, the swipe samples were collected in a conversion plant at IPEN/CNEN, Brazil. The measurements were carried out by ICP-MS and the results showed relative error lower than 0.96 % for uranium isotopic ratios for the certified reference material (NBS U200). The uncertainties were estimated by following the ISO GUM, with a confidence level of 95 %. The uncertainties percentage for n(²³⁵U)/n(²³⁸U) ratio of the samples ranged from 2.5 to 4.3 %. The values of uranium isotopic ratio obtained for each method demonstrate the viability of using the methodology proposed in this work.     

Development of a SIMS Method for Isotopic Measurements in Nuclear Forensic Applications

 Methodologies based on secondary ion mass spectrometry (SIMS) for isotopic measurements in nuclear forensic applications relevant to the age determination of Pu particles and isotopic composition of oxygen for geolocation assignment are described. For the age determination of Pu particles, a relative sensitivity factor (RSF) to correct for the different ionisation efficiencies of U and Pu, was obtained by analysing standard Pu materials with known ages. An RSF of 2.41±0.05 was obtained for PuO₂ from measurements on samples with different Pu/U ratios. In a sample of known origin, using this RSF value, the age calculated from the ²³⁸Pu/²³⁴U and ²⁴⁰Pu/²³⁶U ratios agreed well with the reported age of 2.3 years. For geolocation assignment, a new approach based on the measurement of differences in the natural abundance of ¹⁸O and ¹⁶O isotopes and their ratio was developed. The instrumental mass discrimination of the ¹⁸O/¹⁶O ratio was determined using three O-isotope samples of different chemical composition. The measured precision (the standard error of 100 cycles/analysis) obtained for the oxygen isotopic measurement on the samples was typically ±1.1‰.     

Improvement in precision of extremely low <Superscript>236</Superscript>U/<Superscript>238</Superscript>U isotope ratio determination by using mass spectrometer with narrow dynamic range

The precision in measurement of trace level uranium isotopic ratio, i.e., ²³⁶U/²³⁸U or ²³⁴U/²³⁸U, on single Faraday detector with narrow dynamic range is very hard to achieve. this is mainly due to the narrow dynamic range of a single detector systems. A significant improvement in mass spectrometric determination of ²³⁶U/²³⁸U ratio has been achieved by employing an alternate method using a single Faraday detector of narrow dynamic range. The method makes use of the precise measurements of the ²³⁶U/²³⁴U ratio, ²³⁴U/²³⁵U ratio and ²³⁵U/²³⁸U ratio, which are used to calculate the ²³⁶U/²³⁸U ratio using the equation ²³⁶U/²³⁸U=²³⁶U/²³⁴Uײ³⁴U/²³⁵Uײ³⁵U/²³⁸U. Despite the fact that correlation of the data tends to increase the uncertainty in the result, our results show a significant improvement, i.e., more than 8 times better precision in measuring the ²³⁶U/²³⁸U ratio with this method (σ=3.98×10⁻⁰⁸) as compared to direct measurement of ²³⁶U/²³⁸U (σ=3.104×10⁻⁰⁷). The method widens the applicability of the single collector system with narrow dynamic range and it will potentially be helpful to improve the precision in the case of the static multi-collector system also. The objective of the present study was to compare the results of the same sample analyzed with the present alternate method and the direct method for precision.     

Use of uranium isotopes for the determination of river tributary flow rates: Case of the Var and the Vésubie

When a river flow rate is known on one point, it is possible to determine the one of upstream tributaries. This can be done by measuring the concentrations of a dissolved element (for example Ca²⁺), provided that no chemical precipitations have occurred in mixture waters. In this later case, the determination of flow rate is however possible by isotopic geochemistry, by knowing isotopic ratios of an element, for ex. ²³⁴U/²³⁸U, in each river considered and the ratio of the concentrations in dissolved ²³⁸U in two of them. A study of the Var and its tributary the Vésubie is performed: in January 31st, 2001, the flow rate D₃, downstream the confluence was 35 m³/s. The ratio of the ²³⁸U concentrations in the Vésubie river and in the Var upstream (²³⁸U₁/²³⁸U₂) was 5.5. The isotopic ratios ²³⁴U/²³⁸U measured in the Var, downstream the confluence were R₃ = 1.51, upstream R₂ = 1.75, and in the Vésubie R₁ = 1.08. We have calculated the Vésubie flow rate as D₁ = 3.2 m³/s.     

Isotopic analysis of uranium by neutron activation and high resolution gamma ray spectrometry

A method is described for the non-destructive and accurate determination of the isotopic composition of uranium by activation analysis. The high resolving power of Ge(Li) detectors permits easy identification on a single gamma spectrum of the gamma peaks of²³⁹Np formed from²³⁸U by activation and those of fission products formed from²³⁵U. The ratio of the peak intensities is proportional to the²³⁸U/²³⁵U ratio in the sample. A precision of ±0.6% has been obtained.     

The determination of235U/238U ratios by activation analysis utilizing high resolution γ-spectrometry

The application of high resolution γ-spectrometry to the direct determination of²³⁵U/²³⁸U isotopic ratios in small uranium samples after neutron activation is reported. The consistency obtained relative to mass spectrometrically determined isotopic ratios is better than 5% for isotopic ratios between 0.06885 and 1.095. The method is not subject to the main disadvantages of the known radiochemical methods for²³⁵U/²³⁸U isotopic ratio determinations, and should be appreciably more rapid compared to the solid-source mass spectrometric method giving the same precision.     

Characterization of uranium isotopic abundances in depleted uranium metal assay standard 115

Certified reference material (CRM) 115, Uranium (Depleted) Metal (Uranium Assay Standard), was analyzed using a TRITON Thermal Ionization Mass Spectrometer to characterize the uranium isotope-amount ratios. The certified ²³⁵U/²³⁸U “major” isotope-amount ratio of 0.0020337 (12) in CRM 115 was determined using the total evaporation (TE) and the modified total evaporation (MTE) analytical techniques. In the MTE method, the total evaporation process is interrupted on a regular basis to allow correction of background from peak tailing, internal calibration of the secondary electron multiplier detector versus the Faraday cups, peak-centering, and ion source re-focusing. For the “minor” ²³⁴U/²³⁸U and ²³⁶U/²³⁸U isotope-amount ratio measurements using MTE, precision and accuracy comparable to conventional analyses are achieved, without compromising the quality of the ²³⁵U/²³⁸U isotope-amount ratios. Characterized values of the ²³⁴U/²³⁸U and ²³⁶U/²³⁸U isotope-amount ratios in CRM 115 are 0.000007545 (10) and 0.000032213 (84), respectively. The ²³³U/²³⁸U isotope-amount ratio in CRM 115 is estimated to be <5 × 10^?9. The homogeneity of the CRM 115 materials is established through the absence of any statistically significant unit-to-unit variation in the uranium isotope-amount ratios. The measurements leading to the certification of uranium isotope-amount ratios are discussed.     

Evaluation of a new generation quadrupole-based ICP-MS for uranium isotopic measurements in environmental samples

We compare the analytical performance of a modern quadrupole-based ICP-MS (“X-Series”, Thermo-Electron, Winsford, UK) with a single-collector double-focusing sector-field ICP-MS (“Axiom”, VG Elemental, Winsford, UK) for uranium isotopic measurements in environmental samples. We focus on the precision and accuracy obtained with both instruments for the ²³⁵U/²³⁸U isotopic ratios and on the abundance sensitivity that is a key parameter for low ²³⁶U/²³⁸U isotopic ratios measurements. We observe that isotopic measurements are more precise accurate with the “X-Series” than with the “Axiom”. Besides, we demonstrate that the “Axiom’s” higher abundance sensitivity limits its capability to measure ²³⁶U/²³⁸U ratio below a few ppm.     

Study of uranium isotopic composition in groundwater and deviation from secular equilibrium condition

Uranium concentration in groundwater reflect both redox conditions and uranium content in host rock. In the present study an attempt has been made to study the uranium concentration and activity ratios of uranium isotopes to present the geochemical conditions of the groundwater in Malwa region of Punjab state, India and the reason for high uranium levels and variation of activity ratios from secular equilibrium conditions. Uranium concentration in groundwater samples was found to be in the range of 13.9 ± 1.2 to 172.8 ± 12.3 μg/l with an average value of 72.9 μg/l which is higher than the national and international guideline values. On the basis of uranium concentration, the groundwater of the study region may be classified as oxidized aquifer on normal uranium content strata (20 %) or oxidized aquifer on enhanced uranium content strata (80 %). The ²³⁸U, ²³⁵U and ²³⁴U isotopic concentration in groundwater samples was found to be in the range of 89.2–1534.5, 4.4–68.5, and 76.4–1386.2 mBq/l, respectively. Activity ratios of ²³⁴U/²³⁸U varies from 0.94 to 1.85 with a mean value of 1.11 which is close to unity that shows secular equilibrium condition. High value of ²³⁴U isotope than ²³⁸U may be due to alpha recoil phenomenon. The plot of AR of ²³⁴U/²³⁸U against the total uranium content in log scale reveals that the groundwaters of the study region either belongs to stable accumulation or normal oxidized aquifer.     

Analysis of lanthanides and actinides in sea sediments from the Gulf of Tehuantepec

Seawater contains trace elements in solution, such as lanthanides and uranium, which can act as natural tracers when they flocculate to form the sediments of the seabed. Uranium is a special case because it is a radioactive element with isotopic ratio close to 1.14 (²³⁴U/²³⁸U) in sea water. Uranium is also present in all fractions of sediments, although the isotopic ratio in each fraction takes a specific value depending on its origin and surrounding environment. This work has been conducted in order to analyze lanthanides and uranium in a core of sea sediments. It is focused on separating the fractions forming the layers along the depth profile of the core sample so as to determine the ²³⁴U/²³⁸U activity ratios in fractions of organic matter, aquatic life skeletons and dissolved minerals. The Neutron Activation Analysis showed the presence of U, Lu, Hf, Eu, Ce and Yb ranging up to 50 ppm while alpha spectrometry gave a series of ²³⁴U/²³⁸U activity ratios which are discussed on the basis of statistical analysis.     

Reagent grade uranium salts: Isotopic composition by neutron activation

A rapid non-destructive neutron activation technique for the determination of the²³⁸U/²³⁵U ratio is described. Reagent grade uranium salts from commercial sources have a widely variable²³⁸U/²³⁵U ratio. The isotopic composition of uranium found in such salts is quite different from the natural value. This difference is largely due to the use of by-product uranium depleted in²³⁵U.     

Isotopic concentration of uranium from alpha spectrum of electrodeposited source on 4H-SiC detector at 500 °C

4H-SiC alpha detectors were fabricated with a 21-μm thick depletion depth and were packaged into a stainless-steel casing with a mineral insulation cable and a standard BNC connector. The packaged detectors had a resolution of 0.624% FWHM at 5.486 MeV prior to salt immersion. The detectors were then immersed in a LiCl–KCl–UCl₃ molten salt at 500 °C, from which a thin layer of depleted uranium was electrodeposited onto the detectors. Alpha particle emission spectra were collected from the electrodeposited source. The energy resolution of the surviving detector was 2.29% FWHM at 4.198 MeV and was sufficient to separate the ²³⁴U from ²³⁸U alpha emissions (577 keV difference). The ²³⁴U/²³⁸U activity ratio and the isotopic concentrations of ²³⁴U and ²³⁸U were determined and are representative of the uranium source used in the electrodeposition.