Proliferation of Interstellar Plutonium into the Solar System

Project Overview

The astrophysical site of production of actinides in the galaxy has long been debated. Recently, neutron star mergers have been shown to produce actinides, though their contribution to the total amount is unknown. A second site, Type II supernovae, likely also contributes. The relative frequencies and production totals of these two sources lead to two very different and distinguishable predictions for the contemporary concentration of actinides in the local interstellar medium. This project was aimed at measuring live cosmological plutonium-244 (²⁴⁴Pu) in lunar soil in order to infer the current interstellar concentration of Pu for comparison with the concentration inferred in the early solar system 4.5 billion years ago. This requires detecting Pu in lunar soil at a concentration of 10⁴ to 10⁵ atoms/gram. We developed Pu separation and detection methodologies that exceed the current state of the art by about 100 times and are now capable of making this extremely challenging measurement in principle. Based on that success we obtained a sample of lunar soil from the Apollo 17 mission. We have not yet analyzed it owing to difficulties in quantitatively and reproducibly transferring such minute quantities of Pu from the separation step to the analysis step. Once this technical issue is resolved we will be able to analyze the lunar soil.

Mission Impact

We have developed separation chemistry for extracting trace plutonium from soils, and a resonance ionization mass spectroscopy (RIMS) method to analyze it with high efficiency. The RIMS development has benefitted one of our current NNSA projects on Pu analysis. The combination of separation chemistry and RIMS should be useful anywhere ultra-trace detection of Pu is required, such as DOE Office of Science environmental radiochemistry or NNSA nuclear forensics research. For example, studies of Pu transport in the environment can be extended to include currently intractable samples that contain too little Pu to be analyzed with current separation and detection methods. The new methods, combined with the ability of RIMS to measure Pu isotopic composition, should be useful in enhancing current Pu forensic analysis.

Publications, Presentations, and Patents

Battino, U., et al. 2019. "NuGrid Stellar Data Set. III.: Updated Low-Mass AGB Models and S-Process Nucleosynthesis." Monthly Notices of the Royal Astronomical Society, 489, 1082-1098. LLNL-JRNL-765023.

Fryer, C., et al. 2019. "Core Collapse Supernovae and Multi-Messenger Astronomy." Bulletin of the American Astronomical Society, 51, 122. LLNL-JRNL-776835.

Harrison, L., et al. 2019. "Method Optimization to Extract 1,000,000 Pu Atoms from Lunar Soil." Goldschmidt Geochemical Conference 2019. LLNL-ABS-770547.

Trappitsch, R., et al. 2019. "Towards the 1,000 Atom Detection Limit for 244Pu." Lunar and Planetary Science Conference, 50, 2132. LLNL-ABS-765341.

Zingale, M., et al. 2019. "MMA SAG: Thermonuclear Supernovae." Bulletin of the American Astronomical Society, 51, 259. LLNL-JRNL-776836.