Andrew Austin Reinhard

The U.S. Department of Energy's Office of Scientific and Technical Information

Uranium isotopic composition can provide valuable information about the history and provenance of a nuclear material; therefore, uranium isotopic analyses are frequently made in the nuclear forensics, safeguards, and environmental monitoring communities. These measurements have always presented challenges due to the extreme variability in the relative abundance between the major (²³⁵U, ²³⁸U) and minor (²³³U, ²³⁴U, ²³⁶U) isotopes of uranium. The recently developed ATONA (Atto- to Nano-Amp) amplification system paired with Faraday cup detectors has a large dynamic range and low noise floor making it ideal for measuring uranium isotopic ratios in materials of both natural and anthropogenic origin. A wide variety of certified reference materials were analyzed to investigate the utility of the ATONA amplification system for determining uranium isotopic composition in samples ranging from depleted to highly enriched. The ATONA amplifiers provide nearly an order of magnitude improvement in external reproducibility over 10¹¹ Ω amplifiers when measuring the minor ²³⁴U/²³⁸U ratio in isotopically natural and depleted samples and when paired with a secondary electron multiplier can measure very low relative abundance uranium isotopes (i.e., ²³⁶U).

The U.S. Department of Energy's Office of Scientific and Technical Information

The U.S. Department of Energy's Office of Scientific and Technical Information

Rationale: Emerging research in the geological and nuclear forensics fields demands increasing analytical precision of isotope ratio measurements with decreasing sample sizes. Here we demonstrate the capability of a newly developed amplification technology to make precise neodymium (Nd) isotopic measurements on 100‐pg standard loads. Methods: The reference materials were analyzed as NdO ⁺ to increase the ionization efficiency of the small analyte loads. The Nd isotopic measurements were made using an IsotopX Isoprobe‐T thermal ionization mass spectrometer upgraded with the ATONA™ amplifier system. The ATONA™ amplifier system uses capacitance‐based amplification as opposed to traditional impedance‐based amplification. Results: The long‐term gains of the ATONA™ amplifiers are shown to have less than 1 ppm variability. Repeat measurements of the JNdi‐1 reference material demonstrate the ability of the ATONA™ amplification technology to make measurements of ¹⁴³ Nd/ ¹⁴⁴ Nd ratios with 23 ppm external reproducibility on 100‐pg loads. The effect of increasing integration time on analytical reproducibility is also displayed as increasing integration time from 10 to 30 s reduced the external measurement uncertainty from 37 to 23 ppm. Conclusions: These measurements represent an improvement of more than a factor of 3 in external measurement reproducibility relative to previously published ¹⁴³ Nd/ ¹⁴⁴ Nd measurements of 100‐pg loads. This new technology will allow for the measurement of smaller samples for precise isotope ratios and open new avenues of research in the geological and nuclear forensic communities.

In 2015 a geothermal exploration well was drilled on the island of Tutuila, American Samoa. The sample suite from the drill core provides 645 m of volcanic stratigraphy from a Samoan volcano, spanning 1.45 million years of volcanic history. In the Tutuila drill core, shield lavas with an EM2 (enriched mantle 2) signature are observed at depth, spanning 1.46 to 1.44 Ma. These are overlain by younger (1.35 to 1.17 Ma) shield lavas with a primordial “common” (focus zone) component interlayered with lavas that sample a depleted mantle component. Following ~1.15 Myr of volcanic quiescence, rejuvenated volcanism initiated at 24.3 ka and samples an EM1 (enriched mantle 1) component. The timing of the initiation of rejuvenated volcanism on Tutuila suggests that rejuvenated volcanism may be tectonically driven, as Samoan hotspot volcanoes approach the northern terminus of the Tonga Trench. This is consistent with a model where the timing of rejuvenated volcanism at Tutuila and at other Samoan volcanoes relates to their distance from the Tonga Trench. Notably, the Samoan rejuvenated lavas have EM1 isotopic compositions distinct from shield lavas that are geochemically similar to “petit spot” lavas erupted outboard of the Japan Trench and late stage lavas erupted at Christmas Island located outboard of the Sunda Trench. Therefore, like the Samoan rejuvenated lavas, petit spot volcanism in general appears to be related to tectonic uplift outboard of subduction zones, and existing geochemical data suggest that petit spots share similar EM1 isotopic signatures.