Evaluation of the Homogeneity of the Uranium Isotope Composition of NIST SRM 610/611 by MC-ICP-MS, MC-TIMS, and SIMS

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 U-236, 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 U-234/U-238, U-235/U-238, and U-236/U-238 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: U-234/U-238 = 9.45 x 10(-6) +/- 5.0 x 10(-8); U-235/U-238 = 2.38555 x 10(-3) +/- 4.7 x 10(-7); and U-236/U-238 = 4.314 x 10(-5) +/- 4.0 x 10(-8). SIMS data show U-235/U-238 is reproducible to within 1% (within analytical uncertainty) in a single wafer, at a scale of 25 mu m. 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.