Validation of LG-SIMS analysis of plutonium-bearing particles using Pu-impregnated silica particles and its application to the Daigo Fukuryu-Maru sample.

Plutonium (Pu) isotope ratio analysis of individual particles is important for nuclear safeguards and environmental investigations. A large-geometry secondary ion mass spectrometry (LG-SIMS) methodology was validated for Pu isotopic analysis using Pu-impregnated porous silica particles as surrogate reference materials and subsequently applied to an iron rust sample collected from the Daigo Fukuryu-Maru (Lucky Dragon no. 5), a vessel exposed to Castle Bravo fallout in 1954. Although SEM-EDX screening failed to identify uranium (U)- and Pu-bearing particles, automated particle measurement (APM) using LG-SIMS successfully detected Pu-bearing particles, enabling isotopic characterisation of six particles. Five particles exhibited 240Pu/239Pu ratios of 0.281 ± 0.005-0.310 ± 0.002, whereas one particle exhibited a distinctly lower ratio of 0.198 ± 0.005. The measured 240Pu/239Pu and 242Pu/239Pu ratios were generally consistent with previously reported isotopic signatures of Marshall Islands fallout obtained using bulk analytical and activity measurement techniques. The distinctly lower 240Pu/239Pu ratio observed for one particle provides preliminary evidence for particle-to-particle isotopic heterogeneity that may be obscured by bulk analysis. These results demonstrate the applicability of LG-SIMS to ultra-trace environmental Pu-bearing particles and its potential for revealing particle-scale isotopic heterogeneity in environmental fallout samples.