The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
I D Hutcheon - One of the best experts on this subject based on the ideXlab platform.
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rate equation model of laser induced bias in Uranium Isotope ratios measured by resonance ionization mass spectrometry
Journal of Analytical Atomic Spectrometry, 2016Co-Authors: Brett H Isselhardt, S G Prussin, Michael R Savina, David Willingham, K B Knight, I D HutcheonAbstract:Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure Uranium Isotope abundances. In this approach, RIMS is used as an element-selective ionization process between Uranium atoms and potential isobars without the aid of chemical purification and separation. The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of the 235U/238U ratio to decrease laser-induced isotopic fractionation. In application, Isotope standards are used to identify and correct bias in measured Isotope ratios, but understanding laser-induced bias from first-principles can improve the precision and accuracy of experimental measurements. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variations in laser parameters on the measured Isotope ratio. The model uses atomic data and empirical descriptions of laser performance to estimate the laser-induced bias expected in experimental measurements of the 235U/238U ratio. Empirical corrections are also included to account for ionization processes that are difficult to calculate from first principles with the available atomic data. Development of this model has highlighted several important considerations for properly interpreting experimental results.
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improving precision in resonance ionization mass spectrometry influence of laser bandwidth in Uranium Isotope ratio measurements
Analytical Chemistry, 2011Co-Authors: Brett H Isselhardt, Michael R Savina, K B Knight, I D Hutcheon, Michael J Pellin, S G PrussinAbstract:The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of {sup 235}U/{sup 238}U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative Isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of Uranium oxide targets with essentially no sample preparation.
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natural variations in Uranium Isotope ratios of Uranium ore concentrates understanding the 238u 235u fractionation mechanism
Earth and Planetary Science Letters, 2010Co-Authors: Gregory A Brennecka, I D Hutcheon, L E Borg, Michael A Sharp, Ariel D. AnbarAbstract:Abstract Precise measurement of the 238 U/ 235 U ratio in geologic samples is now possible with modern techniques and mass spectrometers. Natural variations in this ratio have been shown in previous studies. In this study, data obtained from Uranium ore concentrates of mining facilities around the world show clear evidence that the depositional redox environment in which Uranium is precipitated is the primary factor affecting 238 U/ 235 U fractionation. Low-temperature Uranium deposits are, on average, isotopically ∼ 0.4‰ heavier than Uranium deposited at high temperatures or by non-redox processes. 238 U/ 235 U ratios coupled with 235 U/ 234 U ratios in the same sample provide evidence that the redox transition (U VI → U IV ) at low temperatures is the primary mechanism of 238 U/ 235 U fractionation and that aqueous alteration plays a very limited, if any, role in fractionation of the 238 U/ 235 U ratio. The isotopic variation of U is therefore a potential signature that can be used to trace the origin of Uranium ore concentrate.
S G Prussin - One of the best experts on this subject based on the ideXlab platform.
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rate equation model of laser induced bias in Uranium Isotope ratios measured by resonance ionization mass spectrometry
Journal of Analytical Atomic Spectrometry, 2016Co-Authors: Brett H Isselhardt, S G Prussin, Michael R Savina, David Willingham, K B Knight, I D HutcheonAbstract:Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure Uranium Isotope abundances. In this approach, RIMS is used as an element-selective ionization process between Uranium atoms and potential isobars without the aid of chemical purification and separation. The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of the 235U/238U ratio to decrease laser-induced isotopic fractionation. In application, Isotope standards are used to identify and correct bias in measured Isotope ratios, but understanding laser-induced bias from first-principles can improve the precision and accuracy of experimental measurements. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variations in laser parameters on the measured Isotope ratio. The model uses atomic data and empirical descriptions of laser performance to estimate the laser-induced bias expected in experimental measurements of the 235U/238U ratio. Empirical corrections are also included to account for ionization processes that are difficult to calculate from first principles with the available atomic data. Development of this model has highlighted several important considerations for properly interpreting experimental results.
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improving precision in resonance ionization mass spectrometry influence of laser bandwidth in Uranium Isotope ratio measurements
Analytical Chemistry, 2011Co-Authors: Brett H Isselhardt, Michael R Savina, K B Knight, I D Hutcheon, Michael J Pellin, S G PrussinAbstract:The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of {sup 235}U/{sup 238}U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative Isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of Uranium oxide targets with essentially no sample preparation.
Brett H Isselhardt - One of the best experts on this subject based on the ideXlab platform.
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rate equation model of laser induced bias in Uranium Isotope ratios measured by resonance ionization mass spectrometry
Journal of Analytical Atomic Spectrometry, 2016Co-Authors: Brett H Isselhardt, S G Prussin, Michael R Savina, David Willingham, K B Knight, I D HutcheonAbstract:Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure Uranium Isotope abundances. In this approach, RIMS is used as an element-selective ionization process between Uranium atoms and potential isobars without the aid of chemical purification and separation. The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of the 235U/238U ratio to decrease laser-induced isotopic fractionation. In application, Isotope standards are used to identify and correct bias in measured Isotope ratios, but understanding laser-induced bias from first-principles can improve the precision and accuracy of experimental measurements. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variations in laser parameters on the measured Isotope ratio. The model uses atomic data and empirical descriptions of laser performance to estimate the laser-induced bias expected in experimental measurements of the 235U/238U ratio. Empirical corrections are also included to account for ionization processes that are difficult to calculate from first principles with the available atomic data. Development of this model has highlighted several important considerations for properly interpreting experimental results.
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improving precision in resonance ionization mass spectrometry influence of laser bandwidth in Uranium Isotope ratio measurements
Analytical Chemistry, 2011Co-Authors: Brett H Isselhardt, Michael R Savina, K B Knight, I D Hutcheon, Michael J Pellin, S G PrussinAbstract:The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of {sup 235}U/{sup 238}U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative Isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of Uranium oxide targets with essentially no sample preparation.
Ariel D. Anbar - One of the best experts on this subject based on the ideXlab platform.
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Uranium Isotope fractionation 238u 235u during u vi uptake by freshwater plankton
Environmental Science & Technology, 2020Co-Authors: Xinming Chen, Wang Zheng, Ariel D. AnbarAbstract:Uranium contamination in the environment is a serious public health concern. Biotic U(VI) reduction and nonreductive U(VI) uptake by microorganisms (e.g., U(VI) biosorption by cyanobacteria) are ef...
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biological effects on Uranium Isotope fractionation 238u 235u in primary biogenic carbonates
Geochimica et Cosmochimica Acta, 2018Co-Authors: Xinming Chen, Stephen J. Romaniello, Achim D. Herrmann, Elias Samankassou, Ariel D. AnbarAbstract:Abstract Determining whether U Isotopes are fractionated during incorporation into biogenic carbonates could help to refine the application of 238U/235U in CaCO3 as a robust paleoredox proxy. Recent laboratory experiments have demonstrated that heavy Uranium (U) Isotopes were preferentially incorporated into abiotic aragonite, with an Isotope fractionation of ∼0.10‰ (238U/235U). In contrast, no detectable U Isotope fractionation has been observed in most natural primary biogenic carbonates, but the typical measurement precision of these studies was ±0.10‰ and so could not resolve a fractionation of the magnitude observed in the laboratory. To resolve this issue, we have developed a high precision 238U/235U method (±0.02‰, 2 SD) and utilized it to investigate 238U/235U in primary biogenic carbonates including scleractinian corals, calcareous green and red algae, echinoderms, and mollusks, as well as ooids from the Bahamas, Gulf of California, and French Polynesia. Our results reveal that many primary biogenic carbonates indeed fractionate U Isotopes during U incorporation, and that this fractionation is in the same direction as observed in abiotic CaCO3 coprecipitation experiments. However, the magnitude of Isotope fractionation in biogenic carbonates is often smaller than that predicted by abiotic CaCO3 coprecipitation experiments (0.00–0.09‰ vs. 0.11 ± 0.02‰), suggesting that one or more processes suppress U Isotope fractionation during U incorporation into biogenic carbonates. We propose that closed-system behavior due to the isolation of the local calcificiation sites from ambient seawater, and/or kinetic/disequilibrium Isotope fractionation caused by carbonate growth kinetics, explains this observation. Our results indicate that U Isotope fractionation between biogenic carbonates and seawater might help to constrain U partition coefficients, carbonate growth rates, or seawater chemistry during coprecipitation.
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Uranium Isotope variations in a dolomitized jurassic carbonate platform tithonian franconian alb southern germany
Chemical Geology, 2018Co-Authors: Achim D. Herrmann, Gwyneth W Gordon, Ariel D. AnbarAbstract:Abstract The Uranium (U) isotopic composition (δ238U) of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean. However, many limestones have undergone significant dolomitization during diagenesis. To assess the potential impact of diagenetic changes on the U Isotope composition of dolomitized rocks, we examined the Uranium isotopic composition of a Jurassic carbonate platform of the Franconian Alb in Southern Germany. This platform underwent dolomitization during shallow burial due to the supply of magnesium-rich fluids in the form of slightly modified seawater. This type of dolomitization is common in the geologic record. The carbonate platform of the Franconian Alb can therefore serve as an example for many dolomitized carbonate platforms. A positive correlation between the concentrations of redox sensitive elements (e.g., Re and V) and δ238U confirms that the authigenic uptake of U under reducing conditions preferentially incorporates U enriched in 238U. This positive correlation between redox sensitive elements and δ238U does not change within the dolomitized interval, suggesting that the δ238U is not altered during shallow burial dolomitization of limestones. Therefore, our results indicate that dolostones can retain information about secular variations in seawater δ238U. The diagenetic uptake of U shifts δ238U of bulk carbonate sediments to values heavier than contemporaneous seawater. The magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set. Samples with high Re and V concentrations and high Re/Mo ratios are the most enriched in 238U. This positive correlation could be used to constrain the amount of diagenetic offset of bulk δ238U values from seawater. This is particularly important in carbonate systems that had a primary calcitic mineralogy. Primary calcite precipitates have low U concentrations and so small amounts of 238U uptake during diagenesis can have a significant impact on the bulk composition. The samples with the most depleted δ238U values have high Mo concentrations and low Re/Mo ratios. As carbonate sediments are generally Mo poor, the high Mo concentrations suggests that the depleted δ238U values might be linked to a manganese cycle that operated across the sediment-water interface during the deposition of this Jurassic carbonate platform.
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Diagenetic effects on Uranium Isotope fractionation in carbonate sediments from the Bahamas
Geochimica et Cosmochimica Acta, 2018Co-Authors: Xinming Chen, Stephen J. Romaniello, Achim D. Herrmann, Dalton S. Hardisty, Benjamin C. Gill, Ariel D. AnbarAbstract:Abstract Uranium Isotope variations (δ238U) recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time. However, the effects of diagenetic alteration on the U isotopic composition in carbonate sediments, which are crucial to understand the accurate reconstruction of marine δ238U, are currently poorly constrained. Here we examine the effects of the aragonite-to-calcite transition in the Pleistocene Key Largo Limestone of South Florida, and assess the effects of vadose meteoric, phreatic meteoric, and phreatic marine diagenesis on U Isotope fractionation in carbonate sediments from the Bahamas Transect, including the well-studied Clino, Unda, and ODP Site 1006 drill cores. Our results suggest that early diagenetic processes in Bahamas carbonate sediments fractionate U Isotopes by an average of 0.27 ± 0.14‰ (1 SD) heavier than contemporaneous seawater. Downcore variations of δ238U in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (δ13C, δ18O, Mn/Sr, Mg/Ca, Sr/Ca), enrichments of redox-sensitive elements, or rare earth elements anomalies. We propose two possible mechanisms to interpret the positive change in the δ238U during carbonate diagenesis: authigenic enrichment of isotopically positive U(IV) in carbonates and preferential incorporation of isotopically positive aqueous U(VI) species into carbonates. These processes likely operate during early (syndepositional) diagenesis on the banktop. Further diagenesis during deeper burial is limited by the low solubility of U(IV) under reducing pore water conditions. The early diagenetic behavior of U Isotopes in Bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past. We suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater δ238U from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment δ238U values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.
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Uranium Isotope fractionation during coprecipitation with aragonite and calcite
Geochimica et Cosmochimica Acta, 2016Co-Authors: Xinming Chen, Stephen J. Romaniello, Achim D. Herrmann, Laura E. Wasylenki, Ariel D. AnbarAbstract:Abstract Natural variations in 238U/235U of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ∼7.5 and ∼8.5 to study possible U Isotope fractionation during incorporation into these minerals. Small but significant U Isotope fractionation was observed in aragonite experiments at pH ∼8.5, with heavier U Isotopes preferentially enriched in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007 + 0.00002/−0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U Isotope fractionation was observed in an aragonite experiment at pH ∼7.5 or in calcite experiments at either pH. Equilibrium Isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U Isotope composition than most of the charged species. According to this hypothesis, the magnitude of U Isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U Isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism. These findings suggest that U Isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P CO 2 , Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (
K. J. Mathew - One of the best experts on this subject based on the ideXlab platform.
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erratum to comparison of mass spectrometric methods te mte and conventional for Uranium Isotope ratio measurements
Journal of Radioanalytical and Nuclear Chemistry, 2016Co-Authors: K. J. Mathew, Altug HasozbekAbstract:The Isotope ratio measurement techniques—total evaporation (TE), modified total evaporation (MTE), and conventional—used for characterization measurements of certified reference materials by thermal ionization mass spectrometer instruments are compared. The advantages of each method, the fractionation profiles resulting from the measurement techniques, and factors affecting systematic components of bias in the Isotope ratio measurements are discussed. The TE and MTE techniques yield major ratios and the conventional and MTE techniques yield minor ratios of comparable quality (in terms of precision and accuracy).
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Uranium Isotope amount ratios in certified reference material 116 a Uranium enriched metal assay and isotopic standard
International Journal of Mass Spectrometry, 2014Co-Authors: K. J. Mathew, R M Essex, Altug Hasozbek, G Orlowicz, M SorianoAbstract:Abstract Certified reference material (CRM) 116-A, Uranium (enriched) metal assay and isotopic standard, was analyzed using TRITON and MAT261 thermal ionization mass spectrometer (TIMS) instruments to characterize the Uranium Isotope-amount ratios. The certified n(238U)/n(235U) “major” ratio in CRM 116-A was determined using a combination of two analytical techniques: total evaporation (TE) and modified total evaporation (MTE). The “minor” Isotope-amount ratios n(234U)/n(235U) and n(236U)/n(235U) in CRM 116-A were characterized using a combination of MTE and conventional analysis techniques. For the n(234U)/n(235U) and n(236U)/n(235U) ratios, both the MTE and conventional analysis routines incorporate an internal mass bias correction using the measured n(238U)/n(235U) ratio as well as corrections for peak tailing from 235U to 238U. The abundance of 233U, present in CRM 116-A at trace levels, was characterized using a conventional analysis technique that incorporates a secondary electron multiplier (SEM) equipped with an energy filter. CRM 116-A Isotope-amount ratios are traceable to the national measurement base and to the International System of Units (S.I.). The measurements leading to the certification of the Uranium isotopic abundances in CRM 116-A are discussed.
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Total evaporation method for Uranium Isotope-amount ratio measurements
Journal of Analytical Atomic Spectrometry, 2013Co-Authors: K. J. Mathew, G. O'connor, A. Hasozbek, M. KraiemAbstract:Total evaporation (TE) is an analysis technique for the measurement of Uranium isotopic abundance ratios using thermal ionization mass spectrometry (TIMS). A small mass dependent bias observed in this analytical technique is determined by an external correction factor using well characterized standards (most often certified reference materials, CRMs). The technique had been demonstrated to be highly precise and accurate for major Isotope-amount ratio measurements of Uranium and plutonium. We compare the performance of the TE analytical technique for Uranium Isotope ratio measurements on two TIMS instruments (TRITON and MAT261) using well characterized CRMs from NBL and investigate the dependence of the instrumental mass bias on the amount of sample analyzed. It is concluded that the mass bias during a TIMS Uranium isotopic analysis by TE is independent of the amount of material analyzed. Unlike the major ratio, minor Isotope ratio measurements by TE are biased high due to peak-tailing from the major Isotopes. The biases in the minor Isotope ratio data using TE are evaluated using well characterized NBL CRMs.
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Re-certification of the CRM 125-A UO2 fuel pellet standard for Uranium isotopic composition
International Journal of Mass Spectrometry, 2013Co-Authors: M. Kraiem, K. J. Mathew, R M Essex, G Orlowicz, M SorianoAbstract:Abstract The U.S. Department of Energy New Brunswick Laboratory (NBL) has been tasked to develop certified reference materials (CRMs) tailored for the demands of modern analytical methods in the field of nuclear forensics. As part of this effort, several existing Uranium CRMs are being characterized for additional attributes. One of these materials is the CRM 125-A Uranium oxide (UO2) pellet assay and isotopic standard (4% enriched in 235U), which is being developed as a CRM for U–Th age determinations. Reliable U isotopic composition data with relatively small uncertainties are essential for use as a Uranium age standard. Therefore, re-characterization of the Uranium isotopic composition of CRM 125-A was deemed necessary due to poorly constrained minor U abundances (i.e., 234U and 236U) resulting from instrumental limitations of the mass spectrometer used at the time of original certification (1996–1997). The analytical work presented in this study was undertaken with the purpose of reducing the uncertainties of certificate values for Uranium Isotope-amount ratios. Six randomly selected CRM 125-A pellets were characterized for the Uranium isotopic abundances by thermal ionization mass spectrometry (TIMS). The re-certified major ratio n(235U)/n(238U) of 0.042301(25) was determined using the total evaporation (TE) and modified total evaporation (MTE) methods. The re-certified minor Isotope ratios n(234U)/n(238U) and n(236U)/n(238U) of 0.00039130(38) and 0.0000040754(47), respectively, were measured by MTE and a conventional Faraday cup analysis method using internal normalization. Additionally, the material was checked for the presence of 233U using a secondary electron multiplier (SEM) equipped with a retarding potential quadrupole (RPQ). No 233U was observed within the instrumental detection limit. The homogeneity of CRM 125-A was confirmed by the absence of any statistically significant unit-to-unit variation in the Uranium Isotope amount ratios. The new values measured in this study represent a considerable refinement of the older data, in particular for the minor ratios, with uncertainties that are significantly smaller than those cited in the original certificate. The results of the characterization analyses are presented along with an explanation of the uncertainty estimates, which are compliant with the Guide to the Expression of Uncertainty in Measurement (GUM).
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Characterization of Uranium isotopic abundances in depleted Uranium metal assay standard 115
Journal of Radioanalytical and Nuclear Chemistry, 2012Co-Authors: K. J. Mathew, R M Essex, Altug Hasozbek, G Orlowicz, G. L. Singleton, M SorianoAbstract: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
