Abstract
As analytical and microanalytical applications employing uranium isotope ratios increase, so does the need for reliable reference materials, particularly in the fields of geochemistry, geochronology, and nuclear forensics. We present working values for uranium isotopic data of NIST 610/611 glass, collected by multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS), multicollector thermal ionization mass spectrometry (MC-TIMS), and secondary ion mass spectrometry (SIMS). The presence of depleted U, and, in this case, measureable 236U, makes NIST 610/611 an ideal candidate for a uranium isotopic reference material for nuclear materials. We analyzed multiple chips of three different NIST 611 wafers and found no heterogeneity in 234U/238U, 235U/238U, and 236U/238U within or between the wafers, within analytical uncertainty. We determined working values and uncertainties (using a coverage factor of two) using data from this study and the literature for the following U isotope ratios: 234U/238U = 9.45 × 10−6 ± 5.0 × 10−8; 235U/238U = 2.38555 × 10−3 ± 4.7 × 10−7; and 236U/238U = 4.314 × 10−5 ± 4.0 × 10−8. SIMS data show 235U/238U is reproducible to within 1% (within analytical uncertainty) in a single wafer, at a scale of 25 μm. Multiple studies have demonstrated homogeneity between wafers of NIST 610 and NIST 611, thus the data reported here can be considered representative of NIST 610 as well.
Highlights
Several natural glass reference materials have well characterized uranium isotope compositions [15,16], but natural glasses do not contain measurable 236U and may not be ideally suited for the high precision and accuracy required for microanalytical assessment of nuclear materials
Results of individual MC-ICP-MS and multicollector thermal ionization mass spectrometry (MC-TIMS) measurements of U isotope ratios in 40–60 mg pieces of NIST SRM 611 and individual measurement uncertainties are shown in Figure 1
New high precision uranium isotope ratio measurements of NIST SRM 610/611 glass were collected by MC-ICP-MS and MC-TIMS, and secondary ion mass spectrometry (SIMS)
Summary
Uranium isotope ratios are useful signatures in U series geochemistry [1,2,3], radiochronometry/geochronology [4,5], nuclear forensics and nuclear nonproliferation studies [6,7]. The isotopes of U in glassy fallout debris from a historical nuclear test have been shown to be heterogeneously distributed [12,13] This spatially-resolved data may provide valuable information missed via bulk analysis methods. The NIST SRM 600 series of glass standards were prepared by the National Bureau of Standards ( National Institute of Standards and Technology, NIST) for use as calibration standards for in-situ trace element analysis. These glass standards are not certified with respect to U isotope composition, they can be useful to assess precision and accuracy of U-series laser ablation measurements [10,14]. We report new working values for the following U isotope ratios for NIST SRM 610/611: 234U/238U, 235U/238U, and 236U/238U
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