Ultra-low level plutonium isotopes in the NIST SRM 4355A (Peruvian Soil-1)

High-precision measurement of U-Pu-Np-Am concentrations and isotope ratios in environmental reference materials by mass spectrometry

We report results of precise and sensitive mass spectrometric measurements of uranium, plutonium, neptunium, and americium concentrations and isotope ratios in a variety of environmental reference materials. Most of our work has been done on NIST SRM 4350b, River Sediment, but we also present results for NIST SRM 4354, Lake Sediment; NIST SRMs 4355 and 4355a, Peruvian Soil; NIST SRM 4357, Ocean Sediment; NIST SRM 1648a, Urban Particulate Material; NIST SRM 1649b, Urban Dust; IAEA CRM 385, Ocean Sediment; USGS BCR-2, Columbia River Basalt; and USGS BHVO-2, Hawaiian Volcanic Observatory Basalt. These materials reflect a wide range in long-lived actinide concentrations (e.g. 1E4 to 1E10 atoms ²³⁹Pu/g) and isotope ratios. Measurements were performed in a clean laboratory by isotope dilution, multi-collector thermal ionization and multi-collector inductively coupled plasma mass spectrometry. In general, our results are in agreement with, but lower the uncertainty of, literature or certificate values for these reference materials. Our uranium results for the basalts also confirm previously reported high-precision mass spectrometric results from our laboratory. In many cases our measurements of U-Pu-Np-Am nuclides appear to be novel. Extensive results for NIST SRM 4350b, River Sediment, indicate that this material is heterogeneous for Pu-Np-Am concentrations and isotope ratios at a sample size of 5 g or lower. Pu-Np isotope ratios and a²⁴¹Pu-²⁴¹Am model age of 1954 reflect a mix of plutonium production operations at the nearby Hanford, Washington site, and global atmospheric fallout from nuclear weapons testing. Results for the oceanic sediment materials (NIST SRM 4357 and IAEA 385) collected near Sellafield, U.K. vary but are also indicative of local anthropogenic sources of varying Pu isotopic composition and a mean ²⁴¹Pu-²⁴¹Am model age of 1964. Large environmental fractionation between Pu and Np is observed for the ocean, river, and lake sediment reference materials. Novel measurements for the two air particulate SRMs indicate high U, Pu and Np concentrations for these collections in 1976-1977 with an anomalous regional fallout Pu isotopic signature. Results for BHVO-2 and other Hawaiian basalts indicate that those which erupted before or during the period of abundant atmospheric nuclear weapons testing (1950-1970) contain significant levels of Pu (on the order of 1E7 atoms ²³⁹Pu/g) with a global fallout Pu isotopic composition, compared to more recent eruptions which incorporated less Pu. Hence, Hawaiian basalts may provide an integrated temporal record of anthropogenic actinide fallout deposition from the atmosphere since eruption.